Chapter
# 04
Disposal of Radioactive Waste
Disposal
“Disposal” means release of any material to the
environment in a manner leading to loss of control over the future disposition
of the radionuclide contained therein and includes emplacement of waste
materials in a repository;
Disposal
limits
“disposal limits” means the limits for disposal of
radioactive waste, prescribed from time
to time by the competent authority
Radioactive waste
Two basic nuclear reactions, namely fission of nuclei
like 235U, 239Pu and fusion of elements like hydrogen result in release of
enormous energy and radioactive elements. Controlled vast releases of energy
are possible in nuclear power plant reactors through the fission reaction. The
dream of controlled vast releases of energy through fusion reaction is still to
be realized. Uncontrolled vast releases of energy through both these reactions
have been possible in ‘atom’ and ‘hydrogen’ (thermonuclear) bombs. As in many
other industrial processes, in the nuclear industry also, one gets unusable and
unwanted waste products; the residues turn out to be hazardous. Waste,
by definition, is any mate
.rial (solid materials such as process residues as
well as liquid and gaseous effluents) that has been or will be discarded as
being of no further use. Note that what may be considered as one’s
waste may turn out to be another’s wealth. Reusable plastics and other
components in day-to-day household waste are good examples in this context.
This concept holds good for radioactive waste also, in some sense. Waste that
emits nuclear radiation is radioactive waste. (See Box 1 for basic concepts of
radioactivity.)
“radioactive waste” means any waste material
containing radionuclide in quantities or
concentrations as prescribed by the competent
Classification
of radioactive waste
Nuclear waste can be generally classified as either
‘lowlevel’ radioactive waste or ‘high level’ radioactive waste.
Low-level
radioactive waste
Basically all radioactive waste
that is not high-level radioactive waste or intermediate-level waste or
transuranic waste is classified as low-level radioactive waste. Volume-wise it
may be larger than that of highlevel radioactive waste or intermediate-level
radioactive waste or transuranic waste, but the radioactivity contained in the
low-level radioactive waste is significantly less and made up of isotopes
having much shorter halflives than most of the isotopes in high-level
radioactive waste or intermediate-level waste or transuranic waste. Large
amounts of waste contaminated with small amounts of radionuclides, such as
contaminated equipment (glove boxes, air filters, shielding materials and
laboratory equipment) protective clothing, cleaning rags, etc. constitute low-level
radioactive waste. Even components of decommissioned reactors may come under
this category (after part decontamination procedures). The level of
radioactivity and half-lives of radioactive isotopes in low-level waste are
relatively small. Storing the waste for a period of 10 to 50 years will allow
most of the radioactive isotopes in low-level waste to decay, at which point
the waste can be disposed of as normal refuse. It may come as a surprise that
several investigations have shown that exposure of mammals to low levels of
radiation may indeed be beneficial, including, ‘increased life span, greater
reproductive capacity, better disease resistance, increased growth rate,
greater resistance to higher radiation doses, better neurological function, better
wound healing and lower tumour induction and growth’ (Devaney, J. J., Phys.
Today, January 1998, p. 87). Beneficial effects on plants include
accelerated growth and development and increased harvests. Low-level
radioactive waste, therefore, seems to be benign.
High-level
radioactive waste
High-level radioactive waste is
conceptualized as the waste consisting of the spent fuel, the liquid effluents
arising from the reprocessing of spent fuel and the solids into which the
liquid waste is converted. It consists, generally, material from the core of a
nuclear reactor or a nuclear weapon. This waste includes uranium, plutonium and
other highly radioactive elements created during fission, made up of fission
fragments and transuranics. (Note that this definition does not specify the
radioactivity that must be present to categorize as high-level radioactive
waste.) These two componentshave different times to decay. The radioactive
fission fragments decay to different stable elements via different nuclear
reaction chains involving a, b
and g emissions
to innocuous levels of radioactivity, and this would take about 1000 years. On
the other hand, transuranics take nearly 500,000 years to reach such levels.
Heat output lasts over 200 years. Most of the radioactive isotopes in
high-level waste emit large amounts of radiation and have extremely long
half-lives (some longer than 100,000 years), creating long time-periods before
the waste will settle to safe levels of radioactivity. As a thumb-rule one may
note that ‘volumes of low level radioactive waste and intermediate-level waste
greatly exceed those of spent fuel or high-level radioactive waste’. In spite
of this ground reality, the public concerns regarding disposal of high-level
radioactive waste is worldwide and quite controversial.
Approaches
to radioactive waste disposal
Waste disposal is discarding
waste with no intention of retrieval. Waste management means the entire
sequence of operations starting with generation of waste and ending with
disposal.
Solid waste disposal, of waste such as municipal
garbage, is based on three well-known methods, namely landfills, incineration
and recycling. Sophisticated methods of landfills are adapted for radioactive
waste also. However, during incineration of ordinary waste, fly ash, noxious
gases and chemical contaminants are released into the air. If radioactive waste
is treated in this manner, the emissions would contain radioactive particulate
matter. Hence when adapted, one uses fine particulate filters and the gaseous
effluents are diluted and released. Recycling to some extent is feasible. We
have already dealt with the reprocessing approach, whereby useful radioactive
elements are recovered for cyclic use. But it still leaves some waste that is a
part of the high-level radioactive waste. Radioactive waste management involves
minimizing radioactive residues, handling waste-packing safely, storage and
safe disposal in addition to keeping sites of origin of radioactivity clean.
Poor practices lead to future problems. Hence choice of sites where
radioactivity is to be managed safely is equally important in addition to
technical expertise and finance, to result in safe and environmentally sound
solutions. The International Atomic Energy Agency (IAEA) is promoting
acceptance of some basic tenets by all countries for radioactive waste
management. These include:
(i)
securing acceptable level of protection of human
health;
(ii)
(ii) provision of an acceptable level of protection of
environment;
(iii)
(iii) while
envisaging (i) and (ii), assurance of negligible effects beyond national
boundaries;
(iv)
acceptable
impact on future generations; and
(v)
no undue burden on future generations. There are other
legal, control, generation, safety and management aspects also.
Next we review some approaches for radioactive waste
disposal. To begin with, the radioactive waste management approach is to
consider the nature of radioactive elements involved in terms of their
half-lives and then choose the appropriate method of handling. If the
concentrations of radioactive elements are largely short lived, then one would
resort to what is referred to as ‘delay and decay’ approach; that is, to hold
on to such a waste for a sufficiently long time that the radioactivity will die
in the meanwhile. A second approach is to ‘dilute and disperse’ so that the
hazard in the environment is minimized. But when the radioactivity is
long-lived, the only approach that is possible is to ‘concentrate and contain’
the activity. In order to carry out concentrating the waste (generally the
sludge), chemical precipitation, ion exchange, reverse osmosis and natural or
steam evaporation, centrifuging, etc. are resorted to. The resulting solids are
highly concentrated in radioactivity. In the following we shall discuss some of
the approaches that are being advocated or are currently in practice.
However, to the extent that the
mining operations result in ‘bringing the radioactivity to the surface and
change its chemical and physical form that may increase its mobility in the
environment’, they assume importance in radioactive waste management.
Long-lived isotopes like 230Th, 226Ra, the decay products of uranium are part
of the tailings and hence the tailings have to be contained. Low-level
radioactive waste and even transuranic waste is often buried in shallow
landfills. One has to pay attention to any groundwater contamination that may
result due to this. The highly radioactive liquid effluents are expected to be
ultimately solidified into a leach-resistant form such as borosilicate glass,
which is fairly robust in the sense that it is chemically durable, resistant to
radiolysis, relatively insensitive to fluctuations in waste composition and
easy to process remotely. (Immobilization in cement matrices or bituminization
or polymerization are also some of the other options that are practiced to some
extent.) However, it must be noted that plutonium does not bind strongly to the
matrix of the glass and ‘thus can be loaded only in trace amounts to prevent
the possibility of criticality or recovery for clandestine purposes’. This
glass in turn is placed in canisters made of specific alloys. Choice of the
canister material would depend on the ultimate site where the waste will be
disposed- off. For example, if the ultimate disposal is in the oceans, the
alloy chosen must have low corrosion rates under the environmental temperature,
pressure, oxygen concentration, etc. Studies have been carried out in this
respect. Forb example, it is found that in oxygenated sea water at 250oC, 7
mega Pascals pressure and 1750 ppm of dissolved oxygen, the corrosion rates of
1018 mild steel, copper, lead, 50 : 10 cupro-nickel, Inconel 600 and Ticode 12
are 11.0, 5.0, 1.0, 0.7, 0.1 and 0.06 mm/year, respectively.
One seeks to dispose-off the
high-level radioactive waste packages contained in multiple metal-barrier
canisters within natural or man-made barriers, to contain radioactivity for
periods as long as 10,000 to 100,000 years. ‘The barrier is a mechanism or
medium by which the movement of emplaced radioactive materials is stopped or
retarded significantly or access to the radioactive materials is restricted or
prevented’. It is obvious that recourse to multiple barriers may assure safety
of emplaced radioactivity over long periods of time. The man-made barriers,
namely the form to which waste is reduced, for example, in the glassy form, and
the canister along with overpackaging, go along with natural barriers. As far
as the choice of natural barriers is concerned, land-based mined depositories
over fairly stable geologic formations are preferred over disposal in them
oceans. However several social and environmental concerns have prevented the
land-route being adopted in counties like USA even after 50 years of
accumulation of radioactive waste. Therefore proposals have been made to take
to the ocean-route and there also the choice varies from just placement of the
canisters over the seabed to placement within the sub-seabed sediments and even
within the basement rocks. In the US, as spent fuels have reached levels of
radioactivity of the order of 50,000 MCi (excluding military sources), there is
dearth of space to store additional irradiated fuel removed from operating
reactors. Legally, the Department of Energy (DOE) is expected to take charge of
all commercial spent fuel. However, the DOE has run into a dead-end. On one
hand it is unable to use spent fuel and on the other, its attempts to develop a
permanent repository at Yucca Mountain in Nevada are met by social and State
challenges as well as lack of complete study of the site itself. Presidential
consent has not been forthcoming to any legislation in this connection.
Options
being aired for disposing radioactivity
Triet Nguyen, Department of
Nuclear Engineering, University of California, Berkeley, has written in an
article ‘High-level Nuclear Waste Disposal’, 14 November
1994 that ‘High-level nuclear waste from both
commercial reactors and defense industry presents a difficult problem to the
scientific community as well as the public. The solutions to this problem are
still debatable, both technically and ethically. There are many proposals for
disposing high-level nuclear wastes. However the most favored solution for the
disposal of these wastes is isolating radioactive waste from man and biosphere
for a period of time such that any possible subsequent release of radionuclide
from the waste repository will not result in undue radiation exposure. The
basic idea behind this is to use stable geological environments that have
retained their integrity for millions of years to provide a suitable isolation
capacity for the long time-periods required. The reason for relying on such
geological environments is based on the following main consideration:
‘Geological media is an entirely passive disposal system with no requirement
for continuing human involvement for its safety. It can be abandoned after
closure with no need for continuing surveillance or monitoring. ...The safety
of the system is based on multiple barriers, both engineered and natural, the
main one being the geological barrier itself.’ One way of disposing high-level
nuclear waste materials which meets the above condition is the concept of
disposing of these wastes by burial in suitable geologic media beneath the deep
ocean floor, which is called seabed disposal. The following options have been
aired sometime or the other. Each one of the options demands serious studies
and technical assessments:
· Deep geological
repositories
· Ocean dumping
· Seabed burial
· Sub-seabed disposal
· Subductive waste
disposal method
· Transforming
radioactive waste to non-radioactive stable waste
· Dispatching to the Sun.
Major problems due to legal, social, political and
financial reasons have arisen in execution due to
· Environmental
perceptions
· Lack of awareness and
education
· ‘Not-in-my-backyard’
syndrome
· ‘Not-in-the-ocean’
syndrome
· Lack of proven
technology.
Geologic
disposal
Geologic disposal in deep
geological formations whether under continental crust or under seabed as a
means of radioactive waste disposal has been recognized since 1957, for
handling long-lived waste. Quite often, contrary to views expressed by
environmentalists, it is ‘not chosen as a cheap and dirty option to get the
radioactive waste simply “out of site and out of mind”’. The deep geological
sites provide a natural isolation system that is stable over hundreds of
thousands of years to contain long-lived radioactive waste. In practice it is
noted that low-level radioactive waste is generally disposed in near-surface
facilities or old mines. High-level radioactive waste is disposed in host rocks
that are crystalline (granitic, gneiss) or argillaceous (clays) or salty or
tuff. Since, in most of the countries, there is not a big backlog of high-level
radioactive waste urgently awaiting disposal, interim storage facilities, which
allow cooling of the wastes over a few decades, are in place.
Ocean-dumping
For many years the
industrialized countries of the world (e.g. USA, France, Great Britain, etc.)
opted for the least expensive method for disposal of the wastes by dumping them
into the oceans. Before 1982, when the United States Senate declared a
moratorium on the dumping of radioactive wastes, the US dumped an estimated
112,000 drums at thirty different sites in the Atlantic and Pacific oceans.
Though this practice has been banned by most of the countries with nuclear
programmes, the problem still persists. Russia, which currently controls sixty
per cent of the world’s nuclear reactors, continues to dispose of its nuclear
wastes into the oceans. According to Russia’s Minister of Ecology, it will
continue to dump its wastes into the oceans because it has no other alternative
method. It will continue to do so until it receives enough international aid to
create proper storage facilities. In response, the United States has pledged
moneym to help Russia, but the problem continues. Although radioactive waste
has known negative effects on humans and other animals, no substantial
scientific proof of bad effects on the ocean and marine life has been found.
Hence some nations have argued that ocean-dumping should be continued. Others
argue that the practice should be banned until further proof of no harm is
available. Oceanic Disposal Management Inc., a British Virgin Islands company,
has also proposed disposing of nuclear and asbestos waste by means of Free-Fall
Penetrators. Essentially, waste-filled missiles, which when dropped through
4000 m of water, will embed themselves 60–80 m into the seabed’s clay
sediments. These penetrators are expected to survive for 700 to 1500 years.
Thereafter the waste will diffuse through the sediments. This was a method
considered by the Scientific Working Group (SWG) of the Nuclear Energy Agency
(NEA) during the eighties. Penetrator disposal is potentially both feasible and
safe, its implementation would depend on international acceptance and the
development of an appropriate international regulatory framework. Neither of
these exists, nor are they likely to in the foreseeable future. The penetrator
method has also been further constrained by a recent revision of the definition
of ‘dumping’, by the London Dumping Convention, to include ‘any deliberate
disposal or storage of wastes or other matter in the seabed and the subsoil
thereof’.
Sub-seabed
disposal
Seabed disposal is different
from sea-dumping which does not involve isolation of low-level radioactive
waste within a geological strata. The floor of deep oceans is a part of a large
tectonic plate situated some 5 km below the sea surface, covered by hundreds of
metres of thick sedimentary soft clay. These regions are desert-like,
supporting virtually no life. The Seabed Burial Proposal envisages drilling
these ‘mud-flats’ to depths of the order of hundreds of metres, such boreholes
being spaced apart several hundreds of metres. The high-level radioactive waste
contained in canisters, to which we have referred to earlier, would be lowered
into these holes and stacked vertically one above the other interspersed by 20
m or more of mud pumped in. The proposal to use basement-rock in oceans for radioactive
waste disposal is met with some problems: variability of the rock and high
local permeability. Oceanic water has a mixing time of the order of a few
thousand years which does not serve as a good barrier for long-lived
radionuclides. Since
experiments cannot be conducted to assure safety of seabed disposal on the
basis of actual canisters deposited in the seabed over periods of interest,
namely over hundreds of thousands of years, model calculations have been
performed to predict the capabilities of such a disposal option. The model
approach has started with selection of sites and acquisition of site-specific
data using marine geological methods. These sites are away from deep-sea
trenches, mid-oceanic ridges or formation zones where geological activities are
high. These sites are also far away from biologically productive areas in the
oceans. The sediments in chosen sites are fine-grained and are called ‘abyssal
red clay’. These sites are believed to have desirable barrier properties with
‘continuous stable and depositional histories’. Therefore these potential waste
repositories are geologically stable over periods of the order of 107 years and
are likely not to have human activities, as they are not resources of fishes or
hydrocarbons or minerals. Core samples from most Pacific and Atlantic sites
have been studied to investigate thermal, chemical and radiological effects. It
is found that when sea water and sample sediment mixtures are heated at 300oC
at high pressure, the solution pH changes from 8 to 3. Calculations suggest
that ‘less than 2 cubic metres of untreated sediment would be needed to
neutralize all the acid generated in the thermally perturbed region of about
5.5 m3’. The canister material has to be compatible with this type of environment
for periods of at least 500 years by which time fission fragment activity would
become acceptable. Similarly, other calculations have taken into account
sediment–nuclide interactions to determine ion concentration around a buried
source as a function of time.
Experimental work has already
established that clays have the property of holding on to several radioactive
elements, including plutonium; hence, seepage of these elements into saline
water is minimal. Rates of migration of these elements over hundreds of
thousands of years would be of the order of a few metres. Hence, during such
long times, radioactivity will diminish to levels below the natural
radioactivity in sea water due to natural radioactive decay. The clays also
have plastic- like behaviour to form natural sealing agents. Finally, the
mud-flats have rather low permeability to water; hence, leaching probability is
rather low. It may be noted that the method depends on standard deep-sea
drilling techniques routinely practised and sealing of the bore-holes. These
two aspects are welldeveloped, thanks to the petroleum industry and also
because of an international programme called the Ocean Drilling Programme. Core
samples from about half a dozen vastly separated sites in the Pacific and
Atlantic oceans have ‘showed an uninterrupted history of geological
tranquillity over the past 50–100 million years’. However there are questions
that remain to be answered:
· Whether migration of
radioactive elements through the ocean floor is at the same rate as that
already measured in the laboratories?
· What is the effect of
nuclear heat on the deep oceanic- clays?
· What is the import on
the deep oceanic fauna and waters above?
· In case the waste
reaches the seabed-surface, will the soluble species (for example, Cs, Tc,
etc.) be diluted to natural background levels? If so, at what rate?
· What happens to
insoluble species like plutonium?
· What is the likelihood
of radioactivity reaching all the way to the sea surface?
· In problems of
accidents in the process of seabed burial leading to, say, sinking ships, to
loss of canisters, etc. how does one recover the waste-load under such
scenarios?
· What is the likelihood
that the waste is hijacked from its buried location? Added to these technical
problems are others:
· International agreement
to consider seabed-burial as distinct from ‘ocean-dumping’.
· This method would be
expensive to implement, but its cost would be an impediment to any future
plutonium mining endeavour.
Although the world trend is
toward the option of land-based disposal, it is doubtful whether restricting
repositories to land-based sites really helps prevention of sea pollution. If
radionuclides from a land-based repository leached out to the surface, they
would be quickly transported to the sea by surface water. What is essential is
to isolate radionuclides from the biosphere as reliably as possible. If
sub-seabed disposal results in more reliable isolation, sub-seabed disposal is
the better safeguard against sea pollution. This method takes into
consideration technological feasibility, protection of marine environments, and
availability of international understanding. The United Nation’s Convention on
the Law of the Sea delineates that a coastal state is granted sovereign rights
to utilize all resources in water and under the seabed within its exclusive
economic zone (EEZ), which can extend from the coast line up to 200 nautical
miles (about 370 km) offshore. A repository is proposed to be constructed in
bedrock 2 km beneath the seabed. To utilize sub-seabed disposal within the EEZ,
it is also proposed that waste packages would be transported through a
submarine tunnel connecting land with them sub-seabed repository. Sea pollution
by an accident during disposal work would be improbable, because waste would
never go through sea water during the work. The proposed method is a variation
of geologic disposal. Long-term monitoring is also possible by maintaining the
access tunnel for some time after constructing artificial barriers. While sub-seabed
disposal of nuclear waste-filled canisters thrown from vessels apparently is
regulated by the London Convention, it is not prohibited or regulated by the
London Convention when accessed via landbased tunnels. Sweden has been
practising this method of sub-seabed disposal since 1988, when a repository for
reactor wastes was opened sixty metres below the Baltic seabed. This project
has been widely cited by politicians from other countries as a great example of
solving the nuclear waste problem. Because of Sweden’s initiative, nuclear
waste is already being deposited under the seabed. Other countries could follow
Sweden’s example and dispose-off nuclear waste under the seabed via land-based
tunnels.
Subductive
waste disposal method
This method is the state-of-the-art
in nuclear waste disposal technology. It is the single viable means of
disposing radioactive waste that ensures non return of the relegated material
to the biosphere. At the same time, it affords inaccessibility to eliminated
weapons material. The principle involved is the removal of the material from
the biosphere faster than it can return. It is considered that ‘the safest, the
most sensible, the most economical, the most stable long-term, the most
environmentally benign, themost utterly obvious places to get rid of nuclear
waste, high-level waste or lowlevel waste is in the deep oceans that cover 70%
of the planet’. Subduction is a process whereby one tectonic plate slides
beneath another and is eventually reabsorbed into the mantle. The subductive
waste disposal method forms a high-level radioactive waste repository in a
subducting plate, so that the waste will be carried beneath the Earth’s crust
where it will be diluted and dispersed through the mantle. The rate of
subduction of a
plate in one of the world’s slowest subduction zones
is 2.1 cm annually. This is faster than the rate (1 mm annually) of diffusion
of radionuclides through the turbidite sediments that would overlay a
repository constructed in accordance with this method. The subducting plate is
naturally predestined for consumption in the Earth’s mantle. The subducting
plate is constantly renewed at its originating oceanic ridge. The slow movement
of the plate would seal any vertical fractures over a repository at the
interface between the subducting plate and the overriding plate.
Transmutation
of high-level radioactive waste
This route of high-level
radioactive waste envisages that one may use transmutational devices,
consisting of a hybrid of a subcritical nuclear reactor and an accelerator of
charged particles to ‘destroy’ radioactivity by neutrons. Destroy’ may not be
the proper word; what is effected is that the fission fragments can be
transmuted by neutron capture and beta decay, to produce stable nuclides.
Transmutation of actinides involves several competing processes, namely
neutron-induced fission, neutron capture and radioactive decay. The large numbe
of neutrons produced in the spallation reaction by the accelerator are used for
‘destroying’ the radioactive material kept in the subcritical reactor. The
scheme has not yet been demonstrated to be practical and costeffective.
Solar
option
It is proposed that ‘surplus
weapons’ plutonium and other highly concentrated waste might be placed in then
Earth orbit and then accelerated so that waste would drop into the Sun.
Although theoretically possible, it involves vast technical development and
extremely high cost compared to other means of waste disposal. Robust
containment would be required to ensure that no waste would be released in the
event of failure of the ‘space transport system’.
Other
options and issues
In its 1994 report entitled ‘Management and
Disposition of Excess Weapons’ Plutonium’, the National Academy of Sciences set
forth two standards for managing the risks associated with surplus
weapons-usable fissile materials. First, the storage of weapons should not be
extended indefinitely because of non-proliferation risks and the negative
impact it would have on armsreduction objectives. Second, options for long-term
disposition of plutonium should seek to meet a ‘spent-fuel standard’ in which
the plutonium is made inaccessible for weapons use. One of the chosen options
of DOE is for dealing with surplus plutonium, its use as a Mixed Oxide Fuel
(MOX) to be burned in reactors such as the CANDU. The United States policy is
not to encourage the civil use of plutonium. The Nuclear Control Institute
regards the vitrification approach as posing fewer risks than the MOX approach
with regard to diversion or theft of warhead material, reversal of the
disarmament process, and other adverse effects on international arms control
and non-proliferation efforts. A decision to dispose-off warhead plutonium by
means of vitrification or other immobilization technology would be an essential
step toward achievement
of such a regime. Proponents of MOX disposition claim that vitrification
technology is immature, speculative and cannot be ready soon enough. On the
other hand, the MOX option, though it does not necessarily involve further
reprocessing, would clearly encourage civilian use of plutonium, which in some
countries like Japan even includes plans for reprocessing irradiated MOX fuel.
In the opinion of the Nuclear Control Institute, ‘the MOX option’ sends the
wrong signal in three ways. First, this option effectively declares that
plutonium has an asset value, and that the energy contained within it should be
viewed as a ‘national asset’ (as the US DOE expressed it) or even ‘national
treasure’ (as the Russians put it), when, in fact, plutonium fuel has been
shown to be an economic liability. Second, the MOX
option suggests that a commercial plutonium fuel cycle can be effectively
safeguarded, when, in fact, it is becoming obvious that large-throughput
plutonium plants face daunting safeguard problems. Third, the MOX option would
be portrayed as giving credibility to the claim that plutonium recycle in light
water reactors (LWRs) is essential to nuclear waste management, at a time when
direct disposal of spent fuel is looking increasingly attractive to utilities.
There are other arguments that relate to proliferation using high-level
radioactive waste. It is believed that the technologies of Laser Isotope
Separation and theLarge Volume Plasma Process may permit the mining of weapons
materials from any matrix. There are many international transporting-related
issues. It is not uncommon that reprocessing of one country’s spent fuel or
waste is taken up in a different country. Such movement is often via one or
more countries or over the international waters. Regulatory mechanisms, both
national and international, have to be in place to guarantee safety of the
waste under these conditions.
Radioactive
wastes are waste types containing radioactive
chemical
elements that do not have a practical purpose. They are sometimes the
products of nuclear processes, such as nuclear
fission. However, industries not directly connected to the nuclear industry
can produce large quantities of radioactive waste. It has been estimated, for
instance, that the past 20 years the oil-producing endeavors of the United
States have accumulated eight million tons of radioactive wastes.[1]
The majority of radioactive waste is "low-level
waste", meaning it contains low levels of radioactivity per mass or volume. This type
of waste often consists of used protective clothing, which is only slightly
contaminated but still dangerous in case of radioactive contamination of a human body
through ingestion,
inhalation,
absorption, or injection.
In
the United States alone, the Department of Energy states that
there are "millions of gallons of radioactive waste" as well as
"thousands of tons of spent nuclear fuel and material" and also
"huge quantities of contaminated soil and water".Despite these
copious quantities of waste, the DOE has a goal of cleaning all presently
contaminated sites successfully by 2025. The Fernald,
Ohio site for example had "31 million pounds of uranium product",
"2.5 billion pounds of waste", "2.75 million cubic yards of
contaminated soil and debris", and a "223 acre portion of the
underlying Great Miami Aquifer had uranium levels above drinking
standards".The United States currently has at least 108 sites it currently
designates as areas that are contaminated and unusable, sometimes many
thousands of acres.. The DOE wishes to try and clean or mitigate
many or all by 2025, however the task can be difficult and it acknowledges that
some will never be completely remediated, and just in one of these 108 larger
designations, Oak Ridge National Laboratory, there
were for example at least "167 known contaminant release sites" in
one of the three subdivisions of the 37,000-acre (150 km²)
site. Some of the U.S. sites were smaller in nature, however, and cleanup
issues were simpler to address, and the DOE has successfully completed cleanup,
or at least closure, of several sites.
The
issue of disposal methods for nuclear waste was one of the most pressing
current problems the international nuclear industry faced when trying to
establish a long term energy production plan, yet there was hope it could be
safely solved. In the United States, the DOE acknowledges much progress in
addressing the waste problems of the industry, and successful remediation of
some contaminated sites, yet also major uncertainties and sometimes
complications and setbacks in handling the issue properly, cost effectively,
and in the projected time frame. In other
countries with lower ability or will to maintain environmental integrity the
issue would be more problematic
The nature and
significance of radioactive waste
Radioactive
waste typically comprises a number of radioisotopes:
unstable configurations of elements that decay,
emitting ionizing radiation which can be harmful to
human health and to the environment. Those isotopes emit different types and
levels of radiation, which last for different periods of time.
The
radioactivity of all nuclear waste diminishes with time. All radioisotopes
contained in the waste have a half-life - the time it takes for any radionuclide to lose
half of its radioactivity and eventually all radioactive waste decays into
non-radioactive elements. Certain radioactive elements (such as plutonium-239)
in “spent” fuel will remain hazardous to humans and other living beings for
hundreds of thousands of years. Other radioisotopes will remain hazardous for
millions of years. Thus, these wastes must be shielded for centuries and
isolated from the living environment for hundreds of millennia.[4]
Some elements, such as Iodine-131, have a short half-life (around 8 days in this
case) and thus they will cease to be a problem much more quickly than other,
longer-lived, decay products but their activity is much greater initially. The
two tables show some of the major radioisotopes, their half-lives, and their radiation yield as a proportion of the
yield of fission of Uranium-235.
|
Long-lived fission
products
|
||||
|
Property: t½
Unit: (Ma) |
Yield
(%) |
Q *
(KeV) |
βγ
* |
|
|
99Tc
|
.211
|
6.0507
|
294
|
β
|
|
126Sn
|
.230
|
.0236
|
4050
|
βγ
|
|
79Se
|
.295
|
.0508
|
151
|
β
|
|
93Zr
|
1.53
|
6.2956
|
91
|
βγ
|
|
135Cs
|
2.3
|
6.3333
|
269
|
β
|
|
107Pd
|
6.5
|
.1629
|
33
|
β
|
|
129I
|
15.7
|
.6576
|
194
|
βγ
|
The faster a radioisotope
decays, the more radioactive it will be. The energy and the type of the ionizing radiation emitted by a pure
radioactive substance are important factors in deciding how dangerous it will
be. The chemical properties of the radioactive element
will determine how mobile the substance is and how likely it is to spread into
the environment and contaminate human bodies. This is further complicated by
the fact that many radioisotopes do not decay immediately to a stable state but
rather to a radioactive decay product
leading to decay chains.
|
Medium-lived fission
products
|
||||
|
Propertyt½Unit:
(a)
|
Yield (%)
|
Q
KeV)
|
βγ*
|
|
|
155Eu
|
4.76
|
.0330
|
252
|
βγ
|
|
85Kr
|
10.76
|
.2717
|
687
|
βγ
|
|
113mCd
|
14.1
|
.0003
|
316
|
β
|
|
90Sr
|
28.9
|
5.7518
|
2826
|
β
|
|
137Cs
|
30.23
|
6.0899
|
1176
|
βγ
|
|
121mSn
|
43.9
|
.00003
|
390
|
βγ
|
|
151Sm
|
90
|
.4203
|
77
|
β
|
The chemical
properties of the radioactive substance and the other substances found within
(and near) the waste store has a great effect upon the ability of the waste to
cause harm to humans or other organisms. For instance TcO4-
tends to adsorb
on the surfaces of steel objects which reduces its ability to move out of the
waste store in water.
Exposure to high
levels of radioactive waste may cause serious harm or death. Treatment of an adult animal with radiation
or some other mutation-causing
effect, such as a cytotoxic anti-cancer drug,
may cause cancer in the animal. In humans it has been calculated that a 1 sievert
dose has a 5% chance of causing cancer and a 1% chance of causing a mutation in
a gamete
(e.g. egg) which can be passed to the next
generation. If a developing organism such as an unborn child
is irradiated, then it is possible to induce a birth defect
but it is unlikely that this defect will be in a gamete or a gamete forming cell.
Depending
on the decay mode and the pharmacokinetics
of an element (how the body processes it and how quickly), the threat due to
exposure to a given activity of a radioisotope
will differ. For instance Iodine-131 is a short-lived beta
and gamma
emitter but because it concentrates in the thyroid
gland, it is more able to cause injury than caesium-137
which, being water soluble, is rapidly excreted in urine. In a similar way, the
alpha
emitting actinides and radium are considered very harmful as they tend to have long biological half-lives and their radiation
has a high linear energy transfer value. Because of such differences, the rules
determining biological injury differ widely according to the radioisotope, and
sometimes also the nature of the chemical compound which contains the
radioisotope.
The
main objective in managing and disposing of radioactive (or other) waste is to
protect people and the environment. This means isolating or diluting the waste
so that the rate or concentration of any radionuclides returned to the biosphere
is harmless. To achieve this the preferred technology to date has been deep and
secure burial for the more dangerous wastes; transmutation, long-term retrievable
storage, and removal to space have also been suggested: management options for
waste are discussed below.
Radioactivity
by definition reduces over time, so in principle the waste needs to be isolated
for a particular period of time until its components have decayed such that it
no longer poses a threat. In practice this can mean periods of hundreds of
thousands of years, depending on the nature of the waste involved.
Though
an affirmative answer is often taken for granted, the question as to whether or
not we should endeavour to avoid causing harm to remote future generations,
perhaps thousands upon thousands of years hence, is essentially one which must
be dealt with by philosophy.
Radioactive
waste comes from a number of sources. The majority originates from the nuclear
fuel cycle and nuclear weapon reprocessing, however other sources include
medical and industrial wastes, as well as naturally occurring radioactive
materials (NORM) that can be concentrated as a result of the processing or
consumption of coal, oil and gas, and some minerals.
Waste
from the front end of the nuclear fuel cycle is usually alpha emitting
waste from the extraction of uranium. It often contains radium and its
decay products.
Uranium
dioxide (UO2) concentrate from mining is not very
radioactive - only a thousand or so times as radioactive as the granite
used in buildings. It is refined from yellowcake
(U3O8), then converted to uranium hexafluoride gas (UF6).
As a gas, it undergoes enrichment to increase the U-235 content from 0.7% to
about 4.4% (LEU). It is then turned into a hard ceramic
oxide (UO2) for assembly as reactor fuel elements.
The
main by-product of enrichment is depleted
uranium (DU), principally the U-238 isotope, with a
U-235 content of ~0.3%. It is stored, either as UF6 or as U3O8.
Some is used in applications where its extremely high density makes it
valuable, such as the keels of yachts, and anti-tank
shells.
It is also used (with recycled plutonium) for making mixed oxide
fuel (MOX) and to dilute highly enriched uranium from weapons
stockpiles which is now being redirected to become reactor fuel. This dilution,
also called downblending, means that any nation or group
that acquired the finished fuel would have to repeat the (very expensive and
complex) enrichment process before assembling a weapon.
The
back end of the nuclear fuel cycle, mostly spent fuel rods,
contains fission products that emit beta and gamma
radiation, and actinides
that emit alpha particles, such as uranium-234,
neptunium-237,
plutonium-238
and americium-241,
and even sometimes some neutron emitters such as californium
(Cf). These isotopes are formed in nuclear
reactors.
It
is important to distinguish the processing of uranium to make fuel from the reprocessing of used fuel. Used fuel
contains the highly radioactive products of fission (see high level waste
below). Many of these are neutron absorbers, called neutron
poisons in this context. These eventually build up to a level where
they absorb so many neutrons that the chain reaction stops, even with the
control rods completely removed. At that point the fuel has to be replaced in
the reactor with fresh fuel, even though there is still a substantial quantity
of uranium-235
and plutonium
present. In the United States, this used fuel is stored, while in countries
such as the United Kingdom, France, and Japan, the fuel is reprocessed
to remove the fission products, and the fuel can then be re-used. This
reprocessing involves handling highly radioactive materials, and the fission
products removed from the fuel are a concentrated form of high-level waste as
are the chemicals used in the process.
When
dealing with uranium and plutonium, the possibility that they may be used to
build nuclear weapons is often a concern. Active
nuclear reactors and nuclear weapons stockpiles are very carefully safeguarded
and controlled. However, high-level waste from nuclear reactors may contain
plutonium. Ordinarily, this plutonium is reactor-grade
plutonium, containing a mixture of plutonium-239
(highly suitable for building nuclear weapons), plutonium-240
(an undesirable contaminant and highly radioactive), plutonium-241,
and plutonium-238;
these isotopes are difficult to separate. Moreover, high-level waste is full of
highly radioactive fission products. However, most fission
products are relatively short-lived. This is a concern since if the waste is
stored, perhaps in deep geological storage, over many years
the fission products decay, decreasing the radioactivity of the waste and
making the plutonium easier to access. Moreover, the undesirable contaminant
Pu-240 decays faster than the Pu-239, and thus the quality of the bomb material
increases with time (although its quantity decreases during that time as well).
Thus, some have argued, as time passes, these deep storage areas have the
potential to become "plutonium mines", from which material for
nuclear weapons can be acquired with relatively little difficulty. Critics of
the latter idea point out that the half-life of Pu-240 is 6,560 years and
Pu-239 is 24,110 years, and thus the relative enrichment of one isotope to the
other with time occurs with a half-life of 9,000 years (that is, it takes 9000
years for the fraction of Pu-240 in a sample of mixed plutonium
isotopes, to spontaneously decrease by half-- a typical enrichment needed to
turn reactor-grade into weapons-grade Pu). Thus "weapons grade plutonium
mines" would be a problem for the very far future (>9,000 years from now),
so that there remains a great deal of time for technology to advance to solve
this problem, before it becomes acute.
Pu-239
decays to U-235
which is suitable for weapons and which has a very long half life (roughly 109
years). Thus plutonium may decay and leave uranium-235. However, modern
reactors are only moderately enriched with U-235 relative to U-238, so the
U-238 continues to serve as denaturation agent for any U-235 produced by
plutonium decay.
One
solution to this problem is to recycle the plutonium and use it as a fuel e.g.
in fast reactors.
But in the minds of some, the very existence of the nuclear fuel reprocessing plant
needed to separate the plutonium from the other elements represents a
proliferation concern. In pyrometallurgical fast reactors, the waste
generated is an actinide compound that cannot be used for nuclear weapons.
Waste
from nuclear weapons reprocessing (as opposed to
production, which requires primary processing from reactor fuel) is unlikely to
contain much beta or gamma activity other than tritium
and americium.
It is more likely to contain alpha emitting actinides such as Pu-239 which is a
fissile material used in bombs, plus some material with much higher specific
activities, such as Pu-238 or Po.
In
the past the neutron trigger for a bomb tended to be beryllium
and a high activity alpha emitter such as polonium;
an alternative to polonium is Pu-238. For reasons of national security, details of the
design of modern bombs are normally not released to the open literature. It is
likely however that a D-T fusion reaction in either an electrically
driven device or a D-T fusion reaction driven by the chemical explosives would
be used to start up a modern device.
Some
designs might well contain a radioisotope thermoelectric generator
using Pu-238 to provide a long lasting source of electrical power for the
electronics in the device.
It
is likely that the fissile material of an old bomb which is due for refitting
will contain decay products of the plutonium isotopes used in it, these are
likely to include alpha-emitting Np-236 from Pu-240 impurities, plus some
U-235 from decay of the Pu-239; however, due to the relatively long half-life
of these Pu isotopes, these wastes from radioactive decay of bomb core material
would be very small, and in any case, far less dangerous (even in terms of
simple radioactivity) than the Pu-239 itself.
The
beta decay of Pu-241
forms Am-241;
the in-growth of americium is likely to be a greater problem than the decay of
Pu-239 and Pu-240 as the americium is a gamma emitter (increasing
external-exposure to workers) and is an alpha emitter which can cause the
generation of heat.
The plutonium could be separated from the americium by several different
processes; these would include pyrochemical processes and aqueous/organic
solvent extraction. A truncated PUREX type extraction
process would be one possible method of making the separation.
Radioactive medical waste tends to contain beta particle
and gamma ray
emitters. It can be divided into two main classes. In diagnostic nuclear
medicine a number of short-lived gamma emitters such as technetium-99m
are used. Many of these can be disposed of by leaving it to decay for a short
time before disposal as normal trash. Other isotopes used in medicine, with
half-lives in parentheses:
- Y-90, used for treating lymphoma
(2.7 days)
- I-131, used for thyroid
function tests and for treating thyroid
cancer (8.0 days)
- Sr-89, used for treating bone
cancer, intravenous injection (52 days)
- Ir-192, used for brachytherapy
(74 days)
- Co-60, used for brachytherapy and external
radiotherapy (5.3 years)
- Cs-137, used for brachytherapy, external
radiotherapy (30 years)
Industrial
source waste can contain alpha, beta,
neutron
or gamma emitters. Gamma emitters are used in radiography
while neutron emitting sources are used in a range of applications, such as oil well
logging.
Processing of
substances containing natural radioactivity; this is often known as NORM. A lot
of this waste is alpha particle-emitting matter from the decay
chains of uranium
and thorium.
The main source of radiation in the human body is potassium-40
(40K).
Coal contains a small
amount of radioactive uranium, barium and thorium, around or slightly more than
the average concentration of those elements in the Earth's crust.
They become more concentrated in the fly ash
because they do not burn well. However, the radioactivity of fly ash is still
very low. It is about the same as black shale and is less than phosphate
rocks, but is more of a concern because a small amount of the fly ash ends up
in the atmosphere where it can be inhaled.
Residues from
the oil
and gas
industry often contain radium and its daughters. The sulphate scale from an oil well
can be very radium rich, while the water, oil and gas from a well often
contains radon.
The radon decays to form solid radioisotopes which form coatings on the inside
of pipework. In an oil processing plant the area of the plant where propane
is processed is often one of the more contaminated areas of the plant as radon
has a similar boiling point as propane.
Removal of very
low-level waste
Although not
significantly radioactive, uranium mill tailings are waste. They are
byproduct material from the rough processing of uranium-bearing ore. They are
sometimes referred to as 11(e)2 wastes, from the section of the U.S. Atomic
Energy Act that defines them. Uranium mill tailings typically also contain
chemically-hazardous heavy metals such as lead and arsenic.
Vast mounds of uranium mill tailings are left at many old mining sites,
especially in Colorado,
New Mexico,
and Utah.
Low level
waste (LLW) is generated from hospitals and industry, as
well as the nuclear fuel cycle. It comprises paper, rags,
tools, clothing, filters, etc., which contain small amounts of mostly
short-lived radioactivity. Commonly, LLW is designated as such as a
precautionary measure if it originated from any region of an 'Active Area',
which frequently includes offices with only a remote possibility of being
contaminated with radioactive materials. Such LLW typically exhibits no higher
radioactivity than one would expect from the same material disposed of in a
non-active area, such as a normal office block. Some high activity LLW requires
shielding during handling and transport but most LLW is suitable for shallow
land burial. To reduce its volume, it is often compacted or incinerated before
disposal. Low level waste is divided into four classes, class A, B, C and GTCC,
which means "Greater Than Class C".
Intermediate
level waste (ILW) contains higher amounts of radioactivity and in some
cases requires shielding. ILW includes resins, chemical sludge and
metal reactor fuel cladding, as well as contaminated
materials from reactor decommissioning. It may be solidified in concrete or
bitumen for disposal. As a general rule, short-lived waste (mainly non-fuel
materials from reactors) is buried in shallow repositories, while long-lived
waste (from fuel and fuel-reprocessing) is deposited in deep underground facilities. U.S.
regulations do not define this category of waste; the term is used in Europe
and elsewhere.
High Level Waste
flasks are transported by train in the United Kingdom. Each flask is
constructed of 3ft thick solid steel and weighs in excess of 50 tons
High level
waste (HLW) is produced by nuclear
reactors. It contains fission
products and transuranic elements generated in the reactor core.
It is highly radioactive and often thermally hot. HLW accounts for over 95% of
the total radioactivity produced in the process of nuclear electricity generation. The amount of HLW
worldwide is currently increasing by about 12,000 metric tons every year, which
is the equival to about 100 double-decker busses or a two-story structure built
on top of a basketball court.[9]
Transuranic
waste (TRUW) as defined by U.S. regulations is, without regard to form or
origin, waste that is contaminated with alpha-emitting transuranic
radionuclides with half-lives greater than 20 years, and concentrations greater
than 100 nCi/g
(3.7 MBq/kg),
excluding High Level Waste. Elements that have an atomic number
greater than uranium are called transuranic ("beyond uranium").
Because of their long half-lives, TRUW is disposed more cautiously than either
low level or intermediate level waste. In the U.S. it arises mainly from
weapons production, and consists of clothing, tools, rags, residues, debris and
other items contaminated with small amounts of radioactive elements (mainly
plutonium).
Under U.S. law,
TRUW is further categorized into "contact-handled" (CH) and
"remote-handled" (RH) on the basis of radiation dose measured at the
surface of the waste container. CH TRUW has a surface dose rate not greater
than 200 mrem per hour (2 mSv/h),
whereas RH TRUW has a surface dose rate of 200 mrem per hour (2 mSv/h) or greater. CH
TRUW does not have the very high radioactivity of high level waste, nor its
high heat generation, but RH TRUW can be highly radioactive, with surface dose
rates up to 1000000 mrem per hour (10000 mSv/h). The United
States currently permanently disposes of TRUW generated from nuclear power
plants and military facilities at the Waste Isolation Pilot Plant
Management of waste
Nuclear waste
locations in USA
Nuclear waste
requires sophisticated treatment and management in order to successfully isolate
it from interacting with the biosphere. This usually necessitates treatment, followed by a
long-term management strategy involving storage, disposal or transformation of
the waste into a non-toxic form.
Long-term
storage of radioactive waste requires the stabilization of the waste into a
form which will not react, nor degrade, for extended periods of time. One way
to do this is through vitrification. Currently at Sellafield,
England the high-level waste (PUREX first cycle raffinate)
is mixed with sugar
and then calcined. Calcination involves passing the waste through
a heated, rotating tube. The purposes of calcination are to evaporate the water
from the waste, and de-nitrate the fission products to assist the stability of
the glass produced.
The 'calcine'
generated is fed continuously into an induction heated furnace with fragmented glass[2].
The resulting glass is a new substance in which the waste products are bonded
into the glass matrix when it solidifies. This product, as a molten fluid, is
poured into stainless steel cylindrical containers
("cylinders") in a batch process. When cooled, the fluid solidifies
("vitrifies") into the glass. Such glass, after being formed, is very
highly resistant to water.According to the ITU, it will require about 1 million
years for 10% of such glass to dissolve in water.
After
filling a cylinder, a seal is welded onto the cylinder. The cylinder is then washed. After
being inspected for external contamination, the steel cylinder is stored,
usually in an underground repository. In this form, the waste products are
expected to be immobilized for a very long period of time (many thousands of
years).
The
glass inside a cylinder is usually a black glossy substance. All this work (in
the United
Kingdom) is done using hot cell
systems. The sugar is added to control the ruthenium
chemistry and to stop the formation of the volatile RuO4 containing radio ruthenium.
In the west, the glass is normally a borosilicate glass (similar to Pyrex {NB Pyrex is
a trade name}), while in the former Soviet bloc it
is normal to use a phosphate glass. The amount of fission products in the glass
must be limited because some (palladium, the other Pt group metals, and tellurium)
tend to form metallic phases which separate from the glass. In Germany a
vitrification plant is in use; this is treating the waste from a small
demonstration reprocessing plant which has since been closed down.
It is common for
medium active wastes in the nuclear industry to be treated with ion exchange
or other means to concentrate the radioactivity into a small volume. The much
less radioactive bulk (after treatment) is often then discharged. For instance,
it is possible to use a ferric hydroxide floc to remove radioactive metals from aqueous mixtures After
the radioisotopes are absorbed onto the ferric hydroxide, the resulting sludge
can be placed in a metal drum before being mixed with cement to form a solid
waste form. In order to get better long-term performance (mechanical stability)
from such forms, they may be made from a mixture of fly ash,
or blast furnace
slag, and portland
cement, instead of normal concrete
(made with portland cement, gravel and sand).
The Australian Synroc
(synthetic rock) is a more sophisticated way to immobilize such waste, and this
process may eventually come into commercial use for civil wastes (it is
currently being developed for U.S. military wastes). Synroc was invented by the
late Prof Ted Ringwood (a geochemist) at the Australian National University.[13]
The Synroc contains pyrochlore and cryptomelane type
minerals. The original form of Synroc (Synroc C) was designed for the liquid
high level waste (PUREX raffinate) from a light water reactor. The main minerals in this
Synroc are hollandite (BaAl2Ti6O16),
zirconolite
(CaZrTi2O7) and perovskite
(CaTiO3). The zirconolite and perovskite are hosts for the actinides.
The strontium
and barium
will be fixed in the perovskite. The caesium
will be fixed in the hollandite.
High-level
radioactive waste is stored temporarily in spent fuel
pools and in dry cask storage facilities. This allows the
shorter-lived isotopes to decay before further handling.
In 1997, in the
20 countries which account for most of the world's nuclear power generation,
spent fuel storage capacity at the reactors was 148,000 tonnes, with 59% of
this utilized. However, a number of nuclear power plants in countries that do
not reprocess had nearly filled their spent fuel pools, and resorted to
Away-from-reactor storage (AFRS). AFRS capacity in 1997 was 78,000 tonnes, with
44% utilized, and annual additions of about 12,000 tonnes. AFRS cannot be
expanded forever, and the lead times for final disposal sites have proven to be
unpredictable (see below).
In 1989 and
1992, France commissioned commercial plants to vitrify
HLW left over from reprocessing oxide fuel, although there are adequate
facilities elsewhere, notably in the United Kingdom and Belgium.
The capacity of these western European plants is 2,500 canisters (1000 t) a
year, and some have been operating for 18 years.
The process of
selecting appropriate deep final repositories for high level
waste and spent fuel is now under way in several countries with the first
expected to be commissioned some time after 2010. However, many people remain
uncomfortable with the immediate stewardship cessation of
this management system. In Switzerland, the Grimsel Test Site is an international
research facility investigating the open questions in radioactive waste
disposal ([5]).
Sweden
is well advanced with plans for direct disposal of spent fuel, since its
Parliament decided that this is acceptably safe, using the KBS-3 technology. In Germany,
there is a political discussion about the search for an Endlager (final
repository) for radioactive waste, accompanied by loud protests especially in
the Gorleben
village in the Wendland area, which was seen ideal for the
final repository until 1990 because of its location next to the border to the
former German Democratic Republic. Gorleben is
presently being used to store radioactive waste non-permanently, with a
decision on final disposal to be made at some future time. The U.S. has opted
for a final repository at Yucca Mountain in Nevada, but this project is
widely opposed and is a hotly debated topic, with some of the main concerns
being the long distance transportation of the waste from across the United
States to this area, and the possibility of accidents over time that could
occur. The Waste Isolation Pilot Plant in the United
States is the world's first underground repository for transuranic waste. There
is also a proposal for an international HLW repository in optimum geology, with
Australia or Russia as possible locations, although the proposal for a global
repository for Australia has raised fierce domestic political objections.
The Canadian
government, for example, is seriously considering this method of disposal,
known as the Deep Geological Disposal concept. Under the current plan, a
vault is to be dug 500 to 1000 meters below ground, under the Canadian
Shield, one of the most stable landforms on the planet. The vaults
are to be dug inside geological formations known as batholiths,
formed about a billion years ago. The used fuel bundles will be encased in a
corrosion-resistant container, and further surrounded by a layer of buffer
material, possibly of a special kind of clay (bentonite
clay). The case itself is designed to last for thousands of years,
while the clay would further slow the corrosion rates of the container. The
batholiths themselves are chosen for their low ground-water movement rates,
geological stability, and low economic value.
The Finnish
government has already started building a vault to store nuclear waste 500 to
1000 meters below ground, not far from the Olkiluoto Nuclear Power Plant.
Storing
high level nuclear waste above ground for a century or so is considered
appropriate by many scientists. This allows for the material to be more easily
observed and any problems detected and managed, while the decay over this time
period significantly reduces the level of radioactivity and the associated
harmful effects to the container material. It is also considered likely that over
the next century newer materials will be developed which will not break down as
quickly when exposed to a high neutron flux thus increasing the longevity of
the container once it is permanently buried.
Sea-based
options for disposal of radioactive waste include burial beneath a stable abyssal plain,
burial in a subduction
zone that would slowly carry the waste downward into the Earth's mantle, and burial beneath a remote
natural or human-made island. While these approaches all have merit and would
facilitate an international solution to the vexing problem of disposal of
radioactive waste, they are currently not being seriously considered because of
the legal barrier of the Law of the Sea
and because in North America and Europe
sea-based burial has become taboo from fear that such a repository could leak
and cause widespread damage. Dumping of radioactive waste from ships has
reinforced this concern, as has contamination of islands in the Pacific.
However, sea-based approaches might come under consideration in the future by
individual countries or groups of countries that cannot find other acceptable
solutions.
Another
approach termed Remix & Return would blend high-level waste with uranium mine
and mill tailings down to the level of the original radioactivity of the uranium ore,
then replace it in empty uranium mines. This approach has the merits of totally
eliminating the problem of high-level waste, of providing jobs for miners who
would double as disposal staff, and of facilitating a cradle-to-grave cycle for
all radioactive materials.
There have been
proposals for reactors that consume nuclear waste and transmute it to other,
less-harmful nuclear waste. In particular, the Integral Fast Reactor was a proposed
nuclear reactor with a nuclear fuel cycle that produced no transuranic
waste and in fact, could consume transuranic waste. It proceeded as far as
large-scale tests but was then canceled by the U.S. Government. Another
approach, considered safer but requiring more development, is to dedicate subcritical reactors to the transmutation of the left-over transuranic
elements.
While
transmutation has been banned in the US since 1977 due to the danger of plutonium proliferation [19],
work on the method continues in the EU. This has resulted in a practical nuclear research reactor
called Myrrha
in which transmutation is possible. Additionally, a new research program called
ACTINET has been started
in the EU
to make transmutation possible on a large, industrial scale.
There
have also been theoretical studies involving the use of fusion
reactors as so called "actinide burners" where a fusion
reactor plasma such as in a tokamak,
could be "doped" with a small amount of the "minor"
transuranic atoms which would be transmuted (meaning fissioned in the actinide
case) to lighter elements upon their successive bombardment by the very high
energy neutrons produced by the fusion of deuterium
and tritium
in the reactor. It was recently found by a study done at MIT, that only 2 or 3
fusion reactors with parameters similar to that of the International
Thermonuclear Experimental Reactor (ITER) could transmute the entire
annual minor actinide production from all of the light water reactors presently operating in the
United States fleet while simultaneously
generating approximately 1 gigawatt of power from each reactor.
Another option
is to find applications of the isotopes in nuclear waste so as to reuse them. . Already, caesium-137,
strontium-90
and a few other isotopes are extracted for certain industrial applications such
as food irradiation and radioisotope thermoelectric
generators. While re-use does not eliminate the need to manage
radioisotopes, it may reduce the quantity of waste produced.
Space disposal
is an attractive notion because it permanently removes nuclear waste from the
environment. However, it has significant disadvantages, not least of which is
the potential for catastrophic failure of a launch
vehicle. Furthermore, the high number of launches that would be required
— due to the fact that no individual rocket would be able to carry very much of
the material relative to the material needed to be disposed of—makes the
proposal impractical (for both economic and risk-based reasons). To further
complicate matters, international agreements on the regulation of such a
program would need to be established.
It has been
suggested that through the use of a stationary launch system many of the risks
of catastrophic launch failure could be avoided. A promising concept is the use
of high power lasers to launch "indestructible" containers from the
ground into space. Such a system would require no rocket propellant, with the
launch vehicle's payload making up a near entirety of the vehicle's mass.
Without the use of rocket fuel on board there would be little chance of the
vehicle exploding.
Another form of
safe removal would possibly be the space
elevator. Encasing the waste in glassified form inside a steel shell
9 inches thick, which in turn is tiled with shuttle tile to its exterior. So if
the launch vehicle fails just before reaching orbit, the waste ball will safely
re-enter the earth's atmosphere. The steel shell would deform on impact, but
does not rupture due to the density of the shell. Also, this would allow the
waste to be potentially shot into the Sun.
Accidents involving
radioactive waste
A number of
incidents have occurred when radioactive material was disposed of improperly,
shielding during transport was defective, or when it was simply abandoned or
even stolen from a waste store. In the former Soviet Union, waste stored in Lake Karachay
was blown over the area during a dust storm after the lake had partly dried
out.In other cases lakes or ponds with radioactive waste accidentally
overflowed into the rivers during exceptional storms.[citation needed]
Scavenging of
abandoned radioactive material has been the cause of several other cases of radiation exposure, mostly in developing
nations, which may have less regulation of dangerous substances (and
sometimes less general education about radioactivity and its hazards) and a
market for scavenged goods and scrap metal. The scavengers and those who buy
the material are almost always unaware that the material is radioactive and it
is selected for its aesthetics or scrap value Irresponsibility on the part of the
radioactive material's owners, usually a hospital, university or military, and
the absence of regulation concerning radioactive waste, or a lack of
enforcement of such regulations, have been significant factors in radiation
exposures. For an example of an accident involving radioactive scrap
originating from a hospital see the Goiânia accident.
Transportation
accidents involving spent nuclear fuel from power plants are unlikely to have
serious consequences due to the strength of the spent nuclear fuel shipping casks.
In fiction,
radioactive waste is often cited as the reason for gaining super-human
powers and abilities. An example of this fictional scenario is the 1981 movie
"Modern Problems" in which actor Chevy Chase
portrays a jealous, harried air traffic controller Max Fiedler; Max Fiedler,
recently dumped by his girlfriend, comes into contact with nuclear waste and is
granted the power of telekinesis, which he uses to not only win her back, but
to gain a little revenge.
In reality, of
course, exposure to radioactive waste instead would lead to illness and/or
death.
In the science
fiction television series, "Space: 1999,"
a massive nuclear waste dump on the Moon explodes, hurtling the Moon, and the inhabitants
of "Moonbase Alpha" out of the Solar System
at interstellar speeds
Radioactive Contamination
Radioactive
contamination is the uncontrolled distribution of radioactive
material in a given environment The amount of radioactive material released in
an accident is called the source term.
Sources of
contamination
Radioactive
contamination is typically the result of a spill or accident during the
production or use of radionuclides (radioisotopes), an unstable nucleus which
has excessive energy.Contamination may occur from radioactive gases, liquids or
particles. For example, if a radionuclide used in nuclear
medicine is accidentally spilled, the material could be spread by people as
they walk around. Radioactive contamination may also be an inevitable result of
certain processes, such as the release of radioactive xenon in nuclear fuel reprocessing. In cases that
radioactive material cannot be contained, it may be diluted to safe
concentrations. Nuclear fallout is the distribution of radioactive
contamination by a nuclear explosion. For a discussion of
environmental contamination by alpha
emitters please see actinides in the environment.
Containment is what differentiates radioactive material from radioactive
contamination. Therefore, radioactive material in sealed and designated
containers is not properly referred to as contamination, although the units of
measurement might be the same.
Measurement
Radioactive contamination may exist on surfaces or in volumes
of material or air. In a nuclear power plant, detection and measurement
of radioactivity and contamination is often the job of a Certified Health Physicist.
Surface contamination
Surface contamination is usually expressed in units of
radioactivity per unit of area. For SI, this is becquerels per square
meter (or Bq/m²). Other units such as pico Curies per 100 cm² or disintegrations
per minute per square centimeter (1 dpm/cm² = 166 2/3 Bq/m²) may be used.
Surface contamination may either be fixed or removable. In the case of fixed
contamination, the radioactive material cannot by definition be spread, but it
is still measurable.
Hazards
In practice there is no such thing as zero radioactivity. Not
only is the entire world constantly bombarded by cosmic rays,
but every living creature on earth contains significant quantities of carbon-14 and
most (including humans) contain significant quantities of potassium-40.
These tiny levels of radiation are not any more harmful than sunlight, but just
as excessive quantities of sunlight can be dangerous, so too can excessive
levels of radiation.
Low level contamination
The hazards to people and the environment from radioactive
contamination depend on the nature of the radioactive contaminant, the level of
contamination, and the extent of the spread of contamination. Low levels of
radioactive contamination pose little risk, but can still be detected by
radiation instrumentation. In the case of low-level contamination by isotopes
with a short half-life, the best course of action may be to simply allow the
material to naturally decay. Longer-lived isotopes should be cleaned up
and properly disposed off, because even a very low level of radiation can be
life-threatening when in long exposure to it. Therefore, whenever there's any
radiation in an area, many people take extreme caution when approaching.
Unintentionally
released radiation can reach humans by a variety of means.
High level contamination
High levels of contamination may pose major risks to people
and the environment. People can be exposed to potentially lethal radiation
levels, both externally and internally, from the spread of contamination
following an accident (or a deliberate
initiation) involving large quantities of radioactive material. The biological effects of external exposure to
radioactive contamination are generally the same as those from an external
radiation source not involving radioactive materials, such as x-ray machines, and
are dependent on the absorbed dose.
Biological effects
The biological effects of internally deposited radionuclides
depend greatly on the activity and the biodistribution and removal rates of the
radionuclide, which in turn depends on its chemical form. The biological
effects may also depend on the chemical toxicity of the
deposited material, independent of its radioactivity. Some radionuclides may be
generally distributed throughout the body and rapidly removed, as is the case
with tritiated
water. Some radionuclides may target specific organs and have much lower
removal rates. For instance, the thyroid gland takes up a large percentage of any iodine that enters
the body. If large quantities of radioactive iodine are inhaled or ingested,
the thyroid may be impaired or destroyed, while other tissues are affected to a
lesser extent. Radioactive iodine is a common fission
product; it was a major component of the radiation released from the Chernobyl disaster, leading to many cases of
pediatric thyroid cancer and hypothyroidism.
On the other hand, radioactive iodine is used in the diagnosis and treatment of
many diseases of the thyroid precisely because of the thyroid's selective
uptake of iodine.
Means
of contamination
Radioactive contamination can enter the body through ingestion, inhalation,
absorption,
or injection. For this reason, it is important to
use personal protective equipment when
working with radioactive materials. Radioactive contamination may also be
ingested as the result of eating contaminated plants and animals or drinking
contaminated water or milk from exposed animals. Following a major
contamination incident, all potential pathways of internal exposure should be considered.
Method for disposing naturally
occurring radioactive material within a subterranean formation
A
method for disposing of radioactive material in a disposal area, which is
accessible from the surface through a wellbore which contains a wellbore fluid.
The method comprises the steps of: encapsulating the radioactive material
within a container, placing the container in the wellbore, passing the
container through the wellbore to the disposal area, and equalizing the
differential pressure between the wellbore pressure and internal pressure
within the container as the capsule descends within the wellbore, prior to the
differential pressure reaching a magnitude which would exceed a design collapse
rating of the container.
Explanation
While
this invention is susceptible of embodiment in many different forms, there is
shown in the drawings, and will herein be described in detail, specific
embodiments of the invention. It should be understood, however, that the
present disclosure is to be considered an exemplification of the principles of
the invention and is not intended to limit the invention to the specific
embodiments illustrated.
Briefly,
the present invention utilizes a lightweight capsule for transporting NORM to a
downhole disposal site. The capsule utilizes equalization means, such as a
pressure equalization valve, which allows the pressure within the capsule to
equalize with the wellbore pressure as the capsule descends within a wellbore
to the downhole disposal site. This will facilitate the use of a capsule having
a design collapse rating which is less than the forces which will be exerted on
the outside surface of the capsule, as a result of the increasing wellbore
pressure, as the capsule descends within the wellbore.
The
equalization means is designed so that as the capsule containing NORM descends
within the wellbore and the external pressure exerted on the capsule increases,
the equalization means allows wellbore fluid to enter the capsule and equalize
the pressure between the interior of the capsule and the wellbore. The
equalization means does not have to ensure complete equalization of pressure
between the inside of the capsule and the wellbore; it should, however, be
designed so that the differential pressure between the inside of the capsule
and the wellbore is not large enough to create forces which will exceed the
design collapse rating of the capsule. This will assist in maintaining the
integrity of the capsule as it descends within the wellbore. Additionally,
preferably the equalization means allows fluid to enter the interior of the
capsule, but under normal operating conditions does not allow fluid to flow out
of the capsule. This reduces the chance of NORM seeping from the capsule to the
wellbore during its descent to the wellbore disposal area.
Turning now to the drawings, FIG. 1 illustrates a capsule 21
as used in the current invention; the capsule 21 has a cylindrical hollow body
23 in which is placed NORM. The capsule 21 is preferably filled with NORM, but
there will usually be a small amount of void space within the capsule near the
ends and between the particles of radioactive material. Over each end of the
body 23 is located a cap 27 and 29 respectively. The body 23 and caps 27 and 29
are preferably constructed of a lightweight durable material, such as schedule
40 polyvinyl chloride. The body 23 and the caps 27 and 29 are typically welded
together using a polyvinyl chloride cement.
Polyvinyl
chloride is the preferred material to be used for several reasons, including:
1) it is characterized by a relatively high resistance to corrosion and
chemical attack by acid and other chemicals; 2) it is lightweight; 3) it is
easy to work with when cutting, drilling and applying fittings; and 4) it is
readily available and relatively inexpensive when compared to other materials
such as metal tubing.
FIGS.
2, 3, and 4 are expanded views of the cap 29 which show in detail an example of
a typical equalization means used by the current invention. The equalization
means comprises pressure equalization valves 31 and 33 which are self-threaded
into holes 35 and 36 located in cap 29. Valves 31 and 33 are preferably one-way
check valves that allow fluid to flow from the wellbore into the interior of
capsule 21, but not in the opposite direction. A sealing material 37,
preferably a silicon based sealer, covers the portion of valves 31 and 33 which
extend into the interior of the capsule 21. The sealing material 37 ensures the
NORM remains sealed within the capsule 21 if the valves 31 and 33 fail during
storage and/or transport of the capsule 21.
Capsule
Design
The
capsule design must satisfy storage and handling requirements for the NORM.
Preferably, the capsule should be small enough to be manipulated and
repositioned by hand. This will greatly reduce the handling machinery and
equipment required, thereby reducing the cost of disposing of the NORM. Also, a
smaller size will minimize concerns over storage of capsules and allow capsules
to be stored temporarily in available racking systems. When determining what
size of capsule to use, the advantages of a smaller size, such as ease of
handling and storage, must be balanced against the increased number of capsules
necessary to dispose of a given quantity of NORM when smaller capsules are
utilized.
In
addition to the above considerations, the capsule should be designed to
maintain its structural integrity during the descent to the selected downhole
disposal site. By maintaining the structural integrity of the capsule, leakage
of NORM is minimized. Also, if the integrity of the capsule were breached, it
would be more likely that the capsule could deform in shape and cause an
obstruction of the wellbore.
As
discussed earlier, to ensure that the structural integrity of the capsule is
maintained, the capsule should be designed so that the equalization means
allows fluid to enter the interior of the capsule before the differential
pressure between the wellbore and the interior of the capsule is large enough
to create forces which will exceed the design design collapse rating of the
capsule. It has been determined that if the capsule is designed to withstand
the pressure and pressure changes that occur during its descent within a
wellbore, it will also maintain its structural integrity with the given
temperature changes that occur during its descent to the disposal site.
Therefore,
in order to determine the design requirements for a capsule, the pressure and pressure
changes which act on the capsule during its descent must be determined. This
will facilitate the determination of a size of equalization means to utilize in
a given capsule.
The
pressure within a wellbore changes as a function of the depth within the
wellbore according to: P wellbore =Depth×Density Brine ×(0.052)(1)
where:
P
wellbore is the pressure in the wellbore at a given depth measured
in pounds per square inch;
Depth
is measured in feet;
Density
Brine is measured in pounds per gallon; and
0.052
is a correction factor to relate foot pounds per gallon to pounds per square
inch.
As
can be seen from Equation 1, the change in pressure within the wellbore with
respect to depth is linear. For a typical wellbore filled with about 9.0 to
about 9.5 pound per gallon (ppg) brine, the pressure changes approximately 500
psi per 1000 ft. FIG. 5 displays the calculated pressure as a function of depth
within a wellbore for a typical well.
To
determine the rate of change of the pressure acting on the capsule as it
descends within a wellbore, equation (1) should be used together with an
estimate of the rate of descent of the capsule within the wellbore. An estimate
of the typical rate of descent of a capsule can be developed from a free body
diagram of the forces acting on a capsule within the wellbore.
The
free body diagram of a capsule in a wellbore results in the following
expression for the acceleration of the capsule within the wellbore. m c a
c =W-B-D (2)
where:
m
c is the mass of the capsule;
a
c is the acceleration of the capsule;
W
is the weight of the capsule with the NORM enclosed;
B
is the buoyancy of the capsule; and
D
is the drag forces acting on the capsule.
It
should be noted that the free body diagram used to develop equation (2) does not
take into account the friction between the wellbore and the capsule. This may
result in a value for the velocity of the capsule which is greater than the
actual velocity, and therefore may result in a higher estimated pressure change
acting on the capsule than actually occurs. By using the estimated pressure
change, a capsule design will result which is more conservative than if the
actual pressure changes were used. As the difference in size between the
diameter of the wellbore and the diameter of the capsule becomes greater, the
friction between the wellbore and the capsule becomes less significant and the
difference between the actual velocity and the calculated velocity becomes
smaller.
The
necessary parameters can be used with equation (2) to estimate the acceleration
and velocity of a capsule within a wellbore as it descends toward a disposal
site. For a typical capsule constructed of schedule 40 PVC, which is four
inches in diameter by six feet long, with a five inch diameter cap and which is
filled with NORM: the weight of the capsule and NORM is about 58.7 lbf; the
buoyancy of the capsule is typically about 44.6 lbf for a wellbore containing
about 9.0 pounds per gallon fluid; and the drag force is determined to be about
0.117 V c 2 , where V c 2 is the
square of the velocity of the capsule within the wellbore. This drag force
takes into account the pressure and viscous shear forces exerted on the
capsule. But, it does not take into account the shear forces created by the
capsule's travel through a conduit, such as a wellbore.
FIG.
6 displays the velocity and acceleration of a capsule having the parameters
listed above. As can be seen from FIG. 6, the capsule will reach a terminal
velocity of about 14.9 ft/sec after approximately 3.5 seconds. Using the
calculated terminal velocity of 14.9 ft/sec and Equations 1, the maximum rate
of pressure increase acting on the capsule is calculated to be about 420 p.s.i.
per minute. For the typical capsule described above, the equalizing means is
designed to open at about 40 p.s.i. and allow wellbore fluid to flow into the
capsule. The equalization means is also designed so that the differential
pressure between the inside of the capsule and the wellbore does not exceed the
design collapse rating of the capsule while the wellbore pressure is increasing
at about 420 p.s.i. per minute. This will help ensure the structural integrity
of the capsule is maintained as the capsule descends to the down hole disposal
site.
Disposal
Area
The
NORM is disposed of in a down hole disposal area. Preferably, the disposal area
comprises a wellbore casing located in a stable geological formation. The
bottom end of the wellbore casing is sealed off from the formation, preferably
using concrete. The capsules containing NORM are placed within the wellbore
casing to the desired level and then the top of the wellbore casing is sealed
off from the formation, preferably using concrete.
Specifically,
turning now to the drawings, FIG. 7 illustrates a typical downhole disposal
area for NORM as utilized in the current invention. Typically, a well is
utilized which was previously drilled for oil and/or gas production. The well
has surface casing 39 that extends from the surface to a desired depth below
the surface. The surface casing 39 was originally used to anchor blowout
preventors and prevent the pollution of near surface, fresh water aquifers by
recovered hydrocarbons and/or drilling mud. The surface casing 39 terminates in
a surface casing shoe 41, which anchors the surface casing to the subterranean
formation surrounding the surface casing. The surface casing shoe 41 is
preferably located at least 100 feet below the lowermost formation which
contains drinking water.
Extending beneath the surface casing 39 is production casing
43. The production casing is smaller in diameter than the surface casing 39,
and rides within the surface casing 39 for a short distance.
An
annulus 45 is formed where the surface casing 39 and the production casing 43
overlap. The production casing 43 may penetrate a formation that contains
hydrocarbons. The production casing 43 may have perforations in the region
penetrating a hydrocarbon containing formation.
A
lower cement plug 47, within the production casing 43, which extends across the
entire hydrocarbon containing region forms the lower boundary of the NORM
disposal area 46. The plug 47 should preferably extend at least an additional
100 feet above any casing perforation.
The
region of the production casing 43, which is located above the plug 47, will
normally form the inner boundary of the NORM disposal area 46. If corrosion of
the production casing 43 is a serious concern, an inner liner may be placed
within the production casing 43 to line the inside of the NORM disposal area
46. The production casing 43 prevents fluid from entering the disposal area 46
and also maintains the shape and structural integrity of the disposal area. The
disposal area 46 extends upward to a desired depth below the surface.
Referring
to FIG. 8, the capsules 21 filled with NORM descend within the wellbore to the
disposal area 46. Once the disposal area is filled to the desired level, the
top is sealed to isolate the disposal area 46 from the wellbore. The top of the
disposal area is preferrably sealed with an upper cement plug 49, which is
placed across the top of production casing 43.
The upper cement plug 49 preferably fills the
production casing 43 for at least fifty feet above and fifty feet below the
casing shoe 41. Also, the annulus 45, between the production casing 43 and
surface casing 39, is preferably filled with cement for a distance of at least
50 feet above the bottom of the casing shoe 41. In the most preferred
embodiment of the invention an additional surface plug 51 is utilized to
further isolate the disposal area 46 from the surface.
Referring
to FIGS. 1 through 8, the disposal of NORM is carried out in the following
manner:
1.
With the lower cement plug 47 in place perform a pressure test of the plug 47
and the production casing 43 above the plug. Typically, the test is carried out
at 1000 p.s.i.g. and a satisfactory test is obtained when the pressure drops
100 p.s.i.g. or less over a thirty minute period.
2.
After a satisfactory pressure test is obtained, check the wellbore casing for
restrictions which could impede the descent of capsule 21 to the disposal area
46. Preferably, the wellbore casing will have no restrictions which result in
the casing being less than about 90% of its rated internal diameter.
3.
Before placing the capsules 21 in the wellbore ensure that the wellbore is
filled with the desired fluid. Preferably, the wellbore is filled with at least
about 9.0 pound per gallon fluid. This will allow allow the same fluid which
provided wellbore control during placement of the lower cement plug 47 to be
utilized during the disposal of the NORM. Also, a wellbore containing at least
9.0 pound per gallon brine fluid will increase the buoyancy of the capsule and
therefore slow its descent to the disposal area.
4.
Calculate the number of capsules 21 that will fill the production casing 43 to
the desired level. For the majority of applications, the capsules 21 should
preferably fill the production casing 43 to a level no higher than 500 feet
below the surface casing shoe 41.
5.
Insert the capsules 21 filled with NORM into the wellbore and allow them to
descend to the downhole disposal area 46. It may be advantageous to check the
level of the capsules 21 within the production casing 43 after one half of the
capsules 21 have been inserted and again when a sufficient number of capsules
21 have been inserted for the capsules to reach a level approximately 1000 feet
below the bottom of the surface casing shoe 41.
6.
Once the desired number of capsules 21 have been placed in the wellbore the
disposal area 46, the disposal area 46 should be sealed on the top by placing
an upper cement plug 49 in the wellbore above the disposal area. Preferably the
upper plug 49 fills at least 50 feet of the annulus 45 formed between the
production casing 43 and the surface casing 39. Also, preferably the upper plug
49 extends within the production casing 43 to the level at least fifty feet
below the surface casing shoe 41. The upper cement plug 49 should be pressure
tested in a manner similar to that used to pressure test the lower cement plug
47. A surface cement plug can be utilized to further isolate the downhole
disposal area from the surface.
From
the foregoing description, it will be observed that numerous variations,
alternatives and modifications will be apparent to those skilled in the art.
Accordingly, this description is to be construed as illustrative only and is
for the purpose of teaching those skilled in the art the manner of carrying out
the invention. Various changes may be made, materials substituted and features
of the invention may be utilized. For example, a concrete crypt or lined cavern
may be used instead of the cased wellbore to receive the NORM filled capsules.
The ends of the capsule may be sealed with end plates instead of caps and if
caps are utilized they may be screwed onto the body of the capsule instead of
being welded in place. Additionally, the invention may be utilized to dispose
of other types of waste besides radioactive material, including hazardous
waste.
- Franz N. D. Kurie, J.
R. Richardson, H. C. Paxton (March 1936). "The Radiations Emitted
from Artificially Produced Radioactive Substances. I. The Upper Limits and
Shapes of the β-Ray Spectra from Several Elements". Physical
Review 49 (5): 368-381.
F. N. D. Kurie (May 1948). "On the Use of the Kurie
Plot". Physical Review 73 (10): 1207. doi:10.1103/PhysRev.73.1207.
John Ayto "20th Century Words" (1999) Cambridge
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