Nuclear Waste Managemnt
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Transcript of Nuclear Waste Managemnt
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Department of CE, GEC, Thrissur
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Nuclear waste management
1. INTRODUCTION
Nuclear waste, like other wastes, may be composed of materials varying inorigin, chemical composition, and physical state. However, what differentiates
nuclear waste from other waste forms is that it contains components that are
unstable due to radioactive decay. Managing nuclear waste requires different
approaches to ensure the protection of both humans and the environment from the
radiation.
In general, three options exist for managing nuclear waste: (1) concentrate and
contain (concentrate and isolate the wastes in an appropriate environment); (2)
dilute and disperse (dilute to regulatory-acceptable levels and then discharge to
the environment); and; (3) delay to decay (allow the radioactive constituents to
decay to an acceptable or background level). The first two options are common to
managing non- radioactive waste but the third is unique to nuclear waste.
Eventually all nuclear wastes become benign because they decay to stable
elements while non-radioactive, hazardous waste remains hazardous forever or
until their chemical speciation is changed.
By far the largest source of radioactive waste from the civilian sector results
from the generation of power in nuclear reactors. Much smaller quantities of
civilian radioactive waste result from use of radio nuclides for scientific research
as well as from industrial sources such as medical isotope production for
diagnostic and therapeutic use and from X-ray and neutron sources. The other
significant sources of radioactive waste are due to defence related activities that
support the production and manufacture of nuclear weapons.
2. SOURCES OF WASTE
Radioactive waste comes from a number of sources. The majority of waste
originates from the nuclear fuel cycle and nuclear weapons 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
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processing or consumption of coal, oil and gas, and some minerals, as discussed
below.
2.1 Nuclear fuel cycle
The major steps generating radioactive waste in the uranium fuel cycle are:
2.1.1 Mining and Milling
This waste results from the production of uranium. It contains low
concentrations of uranium and is contaminated principally by its daughter
products, e.g. thorium, radium and radon.
2.1.2 Fuel supply
This waste may result from purification, conversion and enrichment of uranium
and the fabrication of fuel elements. It includes contaminated trapping materials
from off-gas systems, lightly contaminated trash, and residues from recycle or
recovery operations. This radioactive waste generally contains uranium and, in the
case of mixed oxide fuel, also plutonium.
2.1.3 Reactor operations/power generation
This waste results from treatment of cooling water and storage ponds,
equipment decontamination, and routine facility maintenance. Reactor waste is
normally contaminated with fission products and activation products. Radioactive
waste generated from routine operations includes contaminated clothing, floor
sweepings, paper and concrete. Radioactive waste from treatment of the primary
coolant systems and off-gas system includes spent resins and filters as well as
some contaminated equipment. Radioactive waste may also be generated from
replacement of activated core components such as control rods or neutron sources.
2.1.4 Management of spent fuel
In addition to the radioactive waste described above, reactor operations
generate spent nuclear fuel. This material contains uranium, fission products and
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actinides. It generates significant heat when freshly removed from the reactor.
Spent fuel is either considered a waste or waste is generated from reprocessing
operations. Reprocessing operations generate solid and liquid radioactive waste
streams. Solid radioactive waste such as fuel element cladding hulls, hardware,
and other insoluble residues are generated during fuel dissolution. They may
contain activation products, as well as some undissolved fission products, uranium
and plutonium. The principal liquid radioactive waste stream, however, is the
nitric acid solution which contains both high activity fission products and
actinides in high concentrations.
2.2 Nuclear weapons decommissioning
Waste from nuclear weapons decommissioning 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.
Some designs might contain a radioisotope thermoelectric generatorusing Pu-
238 to provide a long lasting source of electrical power for the electronics in the
device.
2.3 Medical
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 waste.
2.4 Industrial
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.
http://en.wikipedia.org/wiki/Tritiumhttp://en.wikipedia.org/wiki/Americiumhttp://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generatorhttp://en.wikipedia.org/wiki/Medical_wastehttp://en.wikipedia.org/wiki/Beta_particlehttp://en.wikipedia.org/wiki/Gamma_rayhttp://en.wikipedia.org/wiki/Gamma_rayhttp://en.wikipedia.org/wiki/Nuclear_medicinehttp://en.wikipedia.org/wiki/Nuclear_medicinehttp://en.wikipedia.org/wiki/Technetium-99mhttp://en.wikipedia.org/wiki/Industryhttp://en.wikipedia.org/wiki/Alpha_decayhttp://en.wikipedia.org/wiki/Beta_decayhttp://en.wikipedia.org/wiki/Neutron_emissionhttp://en.wikipedia.org/wiki/Radiographyhttp://en.wikipedia.org/wiki/Oil_wellhttp://en.wikipedia.org/wiki/Oil_wellhttp://en.wikipedia.org/wiki/Radiographyhttp://en.wikipedia.org/wiki/Neutron_emissionhttp://en.wikipedia.org/wiki/Beta_decayhttp://en.wikipedia.org/wiki/Alpha_decayhttp://en.wikipedia.org/wiki/Industryhttp://en.wikipedia.org/wiki/Technetium-99mhttp://en.wikipedia.org/wiki/Nuclear_medicinehttp://en.wikipedia.org/wiki/Nuclear_medicinehttp://en.wikipedia.org/wiki/Gamma_rayhttp://en.wikipedia.org/wiki/Gamma_rayhttp://en.wikipedia.org/wiki/Beta_particlehttp://en.wikipedia.org/wiki/Medical_wastehttp://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generatorhttp://en.wikipedia.org/wiki/Americiumhttp://en.wikipedia.org/wiki/Tritium -
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2.5 Naturally occurring radioactive material (NORM)
Radioactive materials which occur naturally and where human activities
increase the exposure of people to ionising radiation are known by the acronym
'NORM'. NORM results from activities such as burning coal, making and usingfertilisers, oil and gas production.
2.6 Oil and gas
Residues from the oil and gas industry often contain radium and its daughters.
The sulfate scale from an oil well can be very radium rich, while the water, oil and
gas from a well often contain radon. The radon decays to form solid radioisotopes
which form coatings on the inside of pipework.
2.7 Coal
Coal contains a small amount of radioactive uranium, barium, thorium and
potassium, but, in the case of pure coal, this is significantly less than the average
concentration of those elements in the Earth's crust. The more active ash minerals
become concentrated in the fly ash precisely because they do not burn well. The
radioactivity of fly ash 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.
3. CLASSIFICATION OF NUCLEAR WASTE
The classification of radioactive wastes varies from country to country, the
following groupings are generally accepted internationally.
3.1 Exempt waste & very low level waste
Exempt waste and very low level waste (VLLW) contains radioactive materials
at a level which is not considered harmful to people or the surrounding
environment. It consists mainly of demolished material (such as concrete, plaster,
bricks, metal, valves, piping etc) produced during rehabilitation or dismantling
operations on nuclear industrial sites. Other industries, such as food processing,
chemical, steel etc also produce VLLW as a result of the concentration of natural
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radioactivity present in certain minerals used in their manufacturing processes.
The waste is therefore disposed of with domestic refuse, although countries such
as France are currently developing facilities to store VLLW in specifically
designed VLLW disposal facilities.
3.2 Low Level Waste (LLW)
All civilian and defense-related facilities that use or handle radioactive
materials generate some LLW. These include research laboratories, hospitals
using radionuclides for diagnostic and therapeutic procedures, as well as nuclear
power plants. LLW includes materials that become contaminated by exposure to
radiation or by contact with radioactive materials. Items such as paper, rags, tools,
protective clothing, filters and other lightly contaminated materials that contain
small amounts of short-lived nuclides are usually classified as LLW. By its nature,
LLW does not require shielding during normal handling and transportation and
both principles of "delay to decay" and "dilute and disperse" can be employed for
disposal depending on the exact nature of the waste. Often, it is advantageous to
reduce the volume of LLW by compaction or incineration before disposal.Worldwide it constitutes ~90% of the volume but only ~1% of the radioactivity
associated with all radioactive waste. However, wastes containing small amounts
of long-lived radionuclides can be included under the LLW classification. The
disposal options for this class of waste are near-surface burial or no restrictions
depending on level of radioactivity.
3.3 Intermediate-level Waste (ILW)
ILW contains lower amounts of radioactivity than HLW but still requires use of
special shielding to assure worker safety. Reactor components, contaminated
materials from reactor decommissioning, sludge from spent fuel cooling and
storage areas, and materials used to clean coolant systems such as resins and
filters are generally classified as ILW. The most common management option is
"delay to decay" for short-lived solid waste, but for the long-lived waste, the
"concentrate and contain" principle (solidification for deep geologic disposal) is
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required. ILW comprises about 7% of the volume and, roughly, 4% of the
radioactivity of all radioactive wastes. The disposal options for this class of waste
are burial in a deep geologic repository for the long-lived radionuclides and near-
surface burial for the short-lived ones. Intermediate-level waste (ILW) - requires
shielding. If it has more than 4000 Bq/g of long-lived (over 30 year half-life)
alpha emitters it is categorised as "long-lived" and requires moresophisticated
handling and disposal.
3.4 High Level Waste (HLW)
HLW generally refers to the radioactive nuclides at high levels from nuclear
power generation, (i.e. reprocessing waste streams or unprocessed spent fuel) or
from the isolation of fissile radio nuclides from irradiated materials associated
with nuclear weapons production. High-level waste (HLW) - sufficiently
radioactive to require both shielding and cooling, generates >2 kW/m3 of heat and
has a high level of long-lived alpha-emitting isotopes.HLW is highly radioactive,
generates a significant amount of heat, and contains long-lived radio nuclides.
Typically these aqueous waste streams are treated by the principle of "concentrate
and contain," as the HLW is normally further processed and solidified into either a
glass (vitrification) or a ceramic matrix waste form. Spent nuclear fuel not
reprocessed is also considered as HLW. Because of the highly radioactive fission
products contained within the spent fuel, it must be stored for "cooling" for many
years before final disposal by isolation from the environment. HWL constitutes
only a small fraction (a few percent). However, the vast majority of the
radioactivity (> 95%) resides in the HLW. The only disposal option for this class
of waste is burial in a deep geologic repository.
4. EFFECTS OF RADIATION
Every inhabitant on this planet is constantly exposed to naturally occurring
ionizing radiation called background radiation. Sources of background radiation
include cosmic rays from the Sun and stars, naturally occurring radioactive
materials in rocks and soil, radionuclides normally incorporated into our bodys
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tissues, and radon and its products, which we inhale. We are also exposed to
ionizing radiation from man-made sources, mostly through medical procedures
like X-ray diagnostics. Radiation therapy is usually targeted only to the affected
tissues.
Much information of the effects of large doses of radiation comes from
survivors of the atomic bombs dropped on Hiroshima and Nagasaki in 1945 and
from other people who received large doses of radiation, usually for treatment.
Only about 12% of all the cancers that have developed among those survivors are
estimated to be related to radiation. Ionizing radiation can cause important
changes in our cells by breaking the electron bonds that hold molecules together.
Radiation can damage our genetic material (DNA). But the cells also have several
mechanisms to repair the damage done to DNA by radiation. Potential biological
effects depend on how much and how fast a radiation dose is received. An acute
radiation dosage (a large dose delivered during a short period of time) may result
in effects which are observable within a period of hours to weeks. A chronic dose
is a relatively small amount of radiation received over a long period of time. The
body is better equipped to tolerate a chronic dose than an acute dose as the cells
need time to repair themselves.
Radiation effects are also classified in two others ways, namely somatic and
genetic effects. Somatic effects appears in the exposed person. The delayed
somatic effects have a potential for the development of cancer and cataracts.
Acute somatic effects of radiation include skin burns, vomiting, hair loss,
temporary sterility or sub-fertility in men, and blood changes. Chronic somatic
effects include the development of eye cataracts and cancers. The second class of
effects, namely genetic or heritable effects appears in the future generations of the
exposed person as a result of radiation damage to the reproductive cells, but risks
from genetic effects in humans are seen to be considerably smaller than the risks
for somatic effects.
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5. FUNDAMENTAL PRINCIPLES OF RADIOACTIVE WASTE
MANAGEMENT
Responsible radioactive waste management requires the implementation of
measures that will afford protection of human health and the environment since
improperly managed radioactive waste could result in adverse effects to human
health or the environment now and in the future.
The timely creation of an effective national legal framework and an associated
organizational infrastructure provides the basis for appropriate management of
radioactive waste. The individual steps in radioactive waste management may be
dependent on each other, and thus require co-ordination. Taking this
interdependence into account will help to ensure safety in all radioactive waste
management steps.
Observance of the principles of radioactive waste management will ensure that
the above considerations are addressed, and thus contribute to achieving the
objective of radioactive waste management. The principles and their supporting
text should be considered as an entity and are presented in the following text.
i. Protection of human healthRadioactive waste shall be managed in such a way as to secure an acceptable level
of protection for human health.
ii. Protection of the environmentRadioactive waste shall be managed in such a way as to provide an acceptable
level of protection of the environment.
iii. Protection beyond national borders
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Radioactive waste shall be managed in such a way as to assure that possible
effects on human health and the environment beyond national borders will be
taken into account.
iv. Protection of future generationsRadioactive waste shall be managed in such a way that predicted impacts on the
health of future generations will not be greater than relevant levels of impact that
are acceptable today.
v. Burdens on future generationsRadioactive waste shall be managed in such a way that will not impose undue
burdens on future generations.
vi. National legal frameworkRadioactive waste shall be managed within an appropriate national legal
framework including clear allocation of responsibilities and provision for
independent regulatory functions.
vii. Radioactive waste generation and management interdependenciesInterdependencies among all steps in radioactive waste generation and
management shall be appropriately taken into account.
viii. Safety of facilitiesThe safety of facilities for radioactive waste management shall be appropriately
assured during their lifetime.
6. STEPS IN NUCLEAR WASTE MANAGEMENT
Once created, radioactive waste will undergo some of the following stages
depending on the type of waste and the strategy for its management
6.1 Treatment and Conditioning of Nuclear Wastes
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Treatment and conditioning processes are used to convert radioactive waste
materials into a form that is suitable for its subsequent management, such as
transportation, storage and final disposal. The principal aims are to:
i. Minimise the volume of waste requiring managementvia treatment processes.
ii. Reduce the potential hazard of the waste by conditioning it into a stablesolid form that immobilises it and provides containment to ensure that the
waste can be safely handled during transportation, storage and final
disposal.
6.1.1 Incineration
Incineration of combustible wastes can be applied to both radioactive and other
wastes. In the case of radioactive waste, it has been used for the treatment of low-
level waste from nuclear power plants, fuel production facilities, research centres
(such as biomedical research), medical sector and waste treatment facilities.
Following the segregation of combustible waste from non-combustible
constituents, the waste is incinerated in a specially engineered kiln up to around
1000oC. Any gases produced during incineration are treated and filtered prior to
emission into the atmosphere and must conform to international standards and
national emissions regulations. Following incineration, the resulting ash, which
contains the radionuclides, may require further conditioning prior to disposal such
as cementation or bituminisation. Compaction technology may also be used to
further reduce the volume, if this is cost-effective.
6.1.2 Compaction
Compaction is a mature, well-developed and reliable volume reduction
technology that is used for processing mainly solid man-made low-level waste
(LLW). Some countries (Germany, UK and USA) also use the technology for the
volume reduction of man-made intermediate-level/transuranic waste. Compactors
can range from low-force compaction systems (~5 tonnes or more) through to
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presses with a compaction force over 1000 tonnes, referred to as supercompactors.
Volume reduction factors are typically between 3 and 10, depending on the waste
material being treated.
Figure 6.1.2 Compaction apparatus
Low-force compaction utilises a hydraulic or pneumatic press to compress
waste into a suitable container, such as a 200-litre drum. In the case of a
supercompactor, a large hydraulic press crushes the drum itself or other receptacle
containing various forms of solid low- or intermediate-level waste (LLW or ILW).
The drum or container is held in a mold during the compaction stroke of the
supercompactor, which minimises the drum or container outer dimensions.. Two
or more crushed drums, also referred to as pellets, are then sealed inside an
overpack container for interim storage and/or final disposal.
6.1.3 Cementation
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Figure 6.1.3 Cementation of nuclear wastes
Cementation through the use of specially formulated grouts provides the means
to immobilise radioactive material that is on solids and in various forms of sludges
and precipitates/gels (flocks) or activated materials.
In general the solid wastes are placed into containers. The grout is then added into
this container and allowed to set. The container with the monolithic block of
concrete/waste is then suitable for storage and disposal.
.
6.1.4 Vitrification
Figure 6.1.4 Vitrification experiment for the study of nuclear waste disposal
The immobilisation of high-level waste (HLW) requires the formation of an
insoluble, solid waste form that will remain stable for many thousands of years. In
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general borosilicate glass has been chosen as the medium for dealing with HLW.
The stability of ancient glass for thousands of years highlights the suitability of
borosilicate glass as a matrix material.
This type of process, referred to as vitrification, has also been extended for
lower level wastes where the type of waste or the economics have been
appropriate.
Most high-level wastes other than spent fuel itself arise in a liquid form from
the reprocessing of spent fuel. To allow incorporation into the glass matrix this
waste is initially calcined (dried) which turns it into a solid form. This product is
then incorporated into molten glass in a stainless container and allowed to cool,giving a solid matrix. The containers are then welded closed and are ready for
storage and final disposal.
6.2 Transport of Nuclear Waste
The nuclear waste should be properly transported to the sites where it is treated
for future disposal or to the respective sites for disposal.
6.2.1 Transport of LLW and ILW
Low-level and intermediate-level wastes (LLW and ILW) are generated
throughout the nuclear fuel cycle and from the production of radioisotopes used in
medicine, industry and other areas. The transport of these wastes is commonplace
and they are safely transported to waste treatment facilities and storage sites.
Low-level radioactive wastes are a variety of materials that emit low levels ofradiation, slightly above normal background levels. They often consist of solid
materials, such as clothing, tools, or contaminated soil. Low-level waste is
transported from its origin to waste treatment sites, or to an intermediate or final
storage facility.
Low-level wastes are transported in drums, often after being compacted in order
to reduce the total volume of waste. The drums commonly used contain up to 200
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litres of material. Typically, 36 standard, 200 litre drums go into a 6-metre
transport container. Low-level wastes are moved by road, rail, and
internationally, by sea. However, most low-level waste is only transported within
the country where it is produced.
The composition of intermediate-level wastes is broad, but they require
shielding. Much ILW comes from nuclear power plants and reprocessing facilities
Intermediate-level wastes are taken from their source to an interim storage site,
a final storage site or a waste treatment facility. They are transported by road, rail
and sea.
The radioactivity level of intermediate-level waste is higher than low-level
wastes. The classification of radioactive wastes is decided for disposal purposes,
not on transport grounds. The transport of intermediate-level wastes take into
account any specific properties of the material, and requires shielding.
6.2.2 Transport of used nuclear fuel
When used fuel is unloaded from a nuclear power reactor, it contains: 96%
uranium, 1% plutonium and 3% of fission products (from the nuclear reaction)
and transuranics.
Used fuel will emit high levels of both radiation and heat and so is stored in
water pools adjacent to the reactor to allow the initial heat and radiation levels to
decrease. Typically, used fuel is stored on site for at least five months before it
can be transported, although it may be stored there long-term.
From the reactor site, used fuel is transported by road, rail or sea to either an
interim storage site or a reprocessing plant where it will be reprocessed.
Used fuel assemblies are shipped in casks which are shielded with steel, or a
combination of steel and lead, and can weigh up to 110 tonnes when empty. A
typical transport cask holds up to 6 tonnes of used fuel..
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6.2.3 Transport of plutonium
Plutonium is separated during the reprocessing of used fuel. It is normally then
made into mixed oxide (MOX) fuel.
Plutonium is transported, following reprocessing, as an oxide powder since this
is its most stable form. It is insoluble in water and only harmful to humans if it
enters the lungs.
Plutonium oxide is transported in several different types of sealed packages and
each can contain several kilograms of material. Criticality is prevented by the
design of the package, limitations on the amount of material contained within the
package, and on the number of packages carried on a transport vessel. Special
physical protection measures apply to plutonium consignments.
6.2.4Transport of vitrified waste
The highly radioactive wastes (especially fission products) created in the
nuclear reactor are segregated and recovered during the reprocessing operation.
These wastes are incorporated in a glass matrix by a process known as
'vitrification', which stabilises the radioactive material.
The molten glass is then poured into a stainless steel canister where it cools and
solidifies. A lid is welded into place to seal the canister. The canisters are then
placed inside a cask, similar to those used for the transport of used fuel.
The quantity per shipment depends upon the capacity of the transport cask.
Typically a vitrified waste transport cask contains up to 28 canisters of glass.
6.3Storage and Disposal Options
Most low-level radioactive waste (LLW) is typically sent to land-based disposal
immediately following its packaging for long-term management. This means that
for the majority (~90% by volume) of all of the waste types, a satisfactory
disposal means has been developed and is being implemented around the world.
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Concentrating on intermediate-level waste (ILW) and high-level waste (HLW),
many long-term waste management options have been investigated worldwide
which seek to provide publicly acceptable, safe and environmentally sound
solutions to the management of radioactive waste.
6.3.1 Near-surface disposal
The International Atomic Energy Agency (IAEA) definition of this option is the
disposal of waste, with or without engineered barriers, in:
i. Near-surface disposal facilities at ground level. These facilities are on orbelow the surface where the protective covering is of the order of a few
metres thick. Waste containers are placed in constructed vaults and when
full the vaults are backfilled. Eventually they will be covered and capped
with an impermeable membrane and topsoil. These facilities may
incorporate some form of drainage and possibly a gas venting system.
ii. Near-surface disposal facilities in caverns below ground level. Unlikenear-surface disposal at ground level where the excavations are conducted
from the surface, shallow disposal requires underground excavation of
caverns but the facility is at a depth of several tens of metres below the
Earth's surface and accessed through a drift.
These facilities will be affected by long-term climate changes (such as
glaciation) and this effect must be taken into account when considering safety as
such changes could cause disruption of these facilities. This type of facility is
therefore typically used for LLW and ILW with a radionuclide content of short
half-life (up to about 30 years).
6.3.2 Deep geological disposal
The long timescales over which some of the waste remains radioactive led to
the idea of deep geological disposal in underground repositories in stable
geological formations. Isolation is provided by a combination of engineered and
natural barriers (rock, salt, clay) and no obligation to actively maintain the facility
is passed on to future generations. This is often termed a multi-barrier concept ,
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with the waste packaging, the engineered repository and the geology all providing
barriers to prevent the radionuclides from reaching humans and the environment.
A repository is comprised of mined tunnels or caverns into which packaged
waste would be placed. In some cases (e.g. wet rock) the waste containers are then
surrounded by a material such as cement or clay (usually bentonite) to provide
another barrier (called buffer and/or backfill). The choice of waste container
materials and design and buffer/backfill material varies depending on the type of
waste to be contained and the nature of the host rock-type available.
6.3.3 Multinational repositories
Not all countries are adequately equipped to store or dispose of their own
radioactive waste. Some countries are limited in area, or have unfavourable
geology and therefore siting a repository and demonstrating its safety could be
challenging. Some smaller countries may not have the resources to take the proper
measures on their own to assure adequate safety and security, or they may not
have enough radioactive waste to make construction and operation of their own
repositories economically feasible.
It has been suggested that there could be multinational or regional repositories
located in a willing host country that would accept waste from several countries.
They could include, for example use by others of a national repository operating
within a host country, or a fully international facility owned by a private company
operated by a consortium of nations or even an international organisation.
However, for the time being, many countries would not accept nuclear waste from
other countries under their national laws.
6.3.4 Interim waste storage
Specially designed interim surface or sub surface storage waste facilities are
currently used in many countries to ensure the safe storage of radioactive waste
pending the availability of a long-term management/disposal option. It must be
noted that interim storage, whether short-term or long-term, is not a final solution
- something will still remain to be done with the waste. Interim storage facilities
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are generally used for intermediate-level waste (ILW) and high-level waste
(HLW), although some countries, namely Finland, Sweden and the USA, now
have disposal facilities for ILW in operation. Similar arrangements exist for the
storage of used nuclear fuel from reactors.
The multi-layer approach to containment is designed to ensure that the most
penetrating forms of radiation cannot enter the outer environment. Recognising
that long-term management options, specifically for ILW and HLW, may require
significant time to be achieved, interim storage arrangements may need to be
extended beyond the time periods originally envisaged.
6.3.5 Long-term above ground storage
Figure 6.3.5 Steel Canisters for Radioactive Waste Storage
Above ground storage is normally considered an interim measure for the
management of radioactive waste. Long-term above ground storage involves
specially constructed facilities at the earth's surface that would be neither
backfilled nor permanently sealed. Hence, this option would allow monitoring and
retrieval at any time without excessive expenditure.
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Suggestions for long-term above ground storage broadly fall into two categories:
i. Conventional stores of the type currently used for interim storage, whichwould require replacement and repackaging of waste every 200 years or
so.
ii. Permanent stores that would be expected to remain intact for tens ofthousands of years. These structures are often referred to as 'Monolith'
stores or 'Mausoleums'.
The latter category of store is derived from the principle of 'guardianship',
where future generations continue to monitor and supervise the waste.
Both suggestions would require information to be passed on to future
generations, leading to the question of whether the stability of future societies
could be ensured to the extent necessary to continue the required monitoring and
supervision.
6.3.6 Deep boreholes
For the deep borehole option, solid packaged wastes would be placed in deep
boreholes drilled from the surface to depths of several kilometres with diameters
of typically less than 1 metre. The waste containers would be separated from each
other by a layer of bentonite or cement. The borehole would not be completely
filled with wastes. The top two kilometres would be sealed with materials such as
bentonite, asphalt or concrete.
Boreholes can be readily drilled offshore as well as onshore in host rocks both
crystalline and sedimentary. This capability significantly expands the range of
locations that can be considered for the disposal of radioactive waste.
6.3.7 Rock melting
The deep rock melting option involves the melting of wastes in the adjacent
rock. The idea is to either produce a stable, solid mass that incorporates the waste
or encases the waste in a diluted form (i.e. dispersed throughout a large volume of
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rock) that cannot easily be leached and transported back to the surface. This
technique has been mainly suggested for heat generating wastes such as vitrified
HLW and host rocks with suitable characteristics to reduce heat loss.
The HLW in liquid or solid form could be placed in an excavated cavity or a
deep borehole. The heat generated by the wastes would then accumulate resulting
in temperatures great enough to melt the surrounding rock and dissolve the
radionuclides in a growing sphere of molten material. As the rock cools it would
crystallise and incorporate the radionuclides in the rock matrix, thus dispersing the
waste throughout a larger volume of rock. There are some variations of this option
in which the heat-generating waste would be placed in containers and the rock
around the container melted. Alternatively, if insufficient heat is generated the
waste would be immobilised in the rock matrix by conventional or nuclear
explosion.
6.3.8 Disposal at a subduction zone
Subduction zones are areas where one denser section of the Earth's crust is
moving towards and underneath another lighter section. The movement of one
section of the Earth's crust below another is marked by an offshore trench, and
earthquakes occur adjacent to the inclined contact between the two plates. The
edge of the overriding plate is crumpled and uplifted to form a mountain chain
parallel to the trench. Deep sea sediments may be scraped off the descending slab
and incorporated into the adjacent. As the oceanic plate descends into the hot
mantle, parts of it may begin to melt. The magma thus formed migrates upwards,
some of it reaching the surface as lava erupting from volcanic vents. The idea for
this option would be to dispose of wastes in the trench region such that they would
be drawn deep into the Earth.
Although subduction zones are present at a number of locations across the
Earth's surface they are geographically very restricted. Not every waste-producing
country would be able to consider disposal to deep-sea trenches, unless
international solutions were sought. However, this option has not been
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implemented anywhere and, as it is a form of sea disposal, it is therefore not
permitted by international agreements.
6.3.9 Disposal at sea
Disposal at sea involves radioactive waste being shipped out to sea and dropped
into the sea in packaging designed to either: implode at depth, resulting in direct
release and dispersion of radioactive material into the sea; or sink to the seabed
intact. Over time the physical containment of containers would fail, and
radionuclides would be dispersed and diluted in the sea. Further dilution would
occur as the radionuclides migrated from the disposal site, carried by currents. The
amount of radionuclides remaining in the sea water would be further reduced bothby natural radioactive decay, and by the removal of radionuclides to seabed
sediments by the process of sorption. This method is not permitted by a number of
international agreements. This option has not been implemented for HLW.
6.3.10 Sub seabed disposal
For the sub seabed disposal option radioactive waste containers would be buried
in a suitable geological setting beneath the deep ocean floor. This option has been
suggested for LLW, ILW and HLW. Variations of this option include:
i. A repository located beneath the seabed. The repository would be accessedfrom land, a small uninhabited island or from an offshore structure.
ii. Burial of radioactive waste in deep ocean sediments.
Sub seabed disposal has not been implemented anywhere and is not permitted
by international agreements.
6.3.11 Disposal in ice sheets
For this option containers of heat-generating waste would be placed in stable ice
sheets such as those found in Greenland and Antarctica. The containers would
melt the surrounding ice and be drawn deep into the ice sheet, where the ice would
refreeze above the wastes creating a thick barrier. Although disposal in ice sheets
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could be technically considered for all types of radioactive wastes, it has only
been seriously investigated for HLW, where the heat generated by the wastes
could be used to advantage to self-bury the wastes within the ice by melting.
The option of disposal in ice sheets has not been implemented anywhere. It has
been rejected by countries that have signed the 1959 Antarctic Treaty or have
committed to providing a solution to their radioactive waste management within
their national boundaries. Since 1980 there has been no significant consideration
of this option.
6.3.12 Direct injection
This approach involves the injection of liquid radioactive waste directly into a
layer of rock deep underground that has been chosen because of its suitable
characteristics to trap the waste (i.e. minimise any further movement following
injection).
In order to achieve this there are two geological prerequisites. There must be a
layer of rock (injection layer) with sufficient porosity to accommodate the waste
and with sufficient permeability to allow easy injection (i.e. act like a sponge).
Above and below the injection layer there must be impermeable layers that act as
a natural seal. Additional benefits could be provided from geological features that
limit horizontal or vertical migration. For example, injection into layers of rock
containing natural brine groundwater. This is because the high density of brine
(salt water) would reduce the potential for upward movement.
Direct injection could in principle be used on any type of radioactive waste
provided that it could be transformed into a solution or slurry (very fine particles
in water). Slurries containing a cement grout that would set as a solid when
underground could also be used to help minimise movement of radioactive waste.
Direct injection has been implemented in Russia and the USA.
6.3.13 Transmutation of high-level radioactive waste
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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 number 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 cost- effective.
6.3.14 Disposal in outer space
The objective of this option is to remove the radioactive waste from the Earth,
for all time, by ejecting it into outer space. The waste would be packaged so that it
would be likely to remain intact under most conceivable accident scenarios. A
rocket or space shuttle would be used to launch the packaged waste into space.
There are several ultimate destinations for the waste which have been considered,
including directing it into the Sun. It is proposed that 'surplus weapons' plutonium
and other highly concentrated waste might be placed in the 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'.
7. NUCLEAR WASTE MANAGEMENT IN INDIA
Sixteen nuclear reactors produce about 3% of Indias electricity, and seven
more are under construction. Radioactive waste management has been an integral
part of the entire nuclear fuel cycle in India. Low-level radioactive waste and
intermediate-level waste arise from operations of reactors and fuel reprocessing
facilities. The low-level radioactive waste liquid is retained as sludge after
chemical treatment, resulting in decontamination factors ranging from 10 to 1000.
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Solid radioactive waste is compacted, bailed or incinerated depending upon the
nature of the waste. Solar evaporation of liquid waste, reverse osmosis and
immobilization using cement matrix are adopted depending on the form of waste.
Underground engineered trenches in near-surface disposal facilities are utilized
for disposal of solid waste; these disposal sites are under continuous surveillance
and monitoring. High efficiency particulate air (HEPA) filters are used to
minimize air-borne radioactivity. Over the past four decades radioactive waste
management facilities have been set up at Trombay, Tarapore, Rawatbhata,
Kalpakkam, Narora, Kakrapara, Hyderabad and Jaduguda, along with the growth
of nuclear power and fuel-reprocessing plants. Multiple- barrier approach is
followed in handling solid waste.
After the commissioning of the fast breeder test reactor at Kalpakkam, one is
required to reprocess the burnt carbide fuel from this reactor. As the burn-up of
this fuel is likely to be of the order of 100 MWD/kg, nearly an order of magnitude
more than that of thermal reactors and due to short cooling-time before
reprocessing, specific activity to be handled will be greatly enhanced. The use of
carbide fuel would result in new forms of chemicals in the reprocessing cycle.
These provide new challenges for fast-reactor fuel reprocessing.
As a national policy, each nuclear facility in India has its own Near Surface
Disposal Facility (NSDF). There are seven NSDFs currently operational within
the country. These NSDFs in India have to address widely varied geological and
climatological conditions. The performance of these NSDFs is continuously
evaluated to enhance the understanding of migration, if any and to adopt measures
for upgrading the predictability over a long period of time. Performance
assessment and service life prediction of Reinforced Concrete Trenches:
Performance assessment of Reinforced Concrete Trench (RCT) is systematically
undertaken through field investigations and predictive modeling. NDT
investigations on operating RCTs and laboratory studies on NSDF materials have
demonstrated that RCTs are in sound condition even after an operational period of
three to four decades. Mathematical models have been developed to predict the
probability of failure as a function of target lives for various safety indices such as
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concrete cover thicknesses, climatic factors, maintenance period for the structure,
water to cement ratio, water proofing etc. Modeling studies for a typical RC
trench under limiting conditions have predicted a minimum service life of nearly
240 years.
8. CONCLUSION
Many people quite reasonably feel that the nuclear industry shouldn't continue
operation without having a solution for the disposal of its radioactive waste.
However, the industry has in fact developed the necessary technologies and
implemented most of them - the remaining issue is to ensure that the proposed
solutions are acceptable to the public.
Today, safe management practices are implemented or planned for all
categories of radioactive waste. Low-level waste (LLW) and most intermediate-
level waste (ILW), which make up most of the volume of waste produced (97%),
are being disposed of securely in near-surface repositories in many countries so as
to cause no harm or risk in the long-term. This practice has been carried out for
many years in many countries as a matter of routine.
High-level waste (HLW) is currently safely contained and managed in interim
storage facilities. The amount of HLW produced is in fact small in relation to
other industry sectors. The use of interim storage facilities currently provides an
appropriate environment in which to contain and manage this amount of waste. In
the long-term however, appropriate disposal arrangements are required for HLW,
due to its prolonged radioactivity. Disposal solutions are currently being
developed for HLW that are safe, environmentally sound and publicly acceptable.
The solution that is widely accepted as feasible is deep geological disposal, and
repository projects are well advanced in some countries, such as Finland, Sweden
and the USA.
With the availability of technologies and the continued progress being made to
develop publicly acceptable sites, it is logical that construction of new nuclear
facilities can continue. Nuclear energy has distinct environmental advantages over
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fossil fuels. As well as containing and managing virtually all its wastes, nuclear
power stations do not cause any pollution. The fuel for nuclear power is virtually
unlimited, considering both geological and technological aspects. There is plenty
of uranium in the Earth's crust and furthermore, well-proven (but not yet fully
economic) technology means that we can extract about 60 times as much energy
from it as we do today. The safety record of nuclear energy is better than for any
major industrial technology. All these benefits should be taken into account when
considering the construction of new facilities.
REFERENCES
1. Current Science, 1534 Vol. 81, No. 12, 25 December 2001
2. The Principles of Radioactive Waste Management, Safety Series No. 111-F, a
publication within the RADWASS programme, IAEA (1995).
3. Radiochemistry and Nuclear chemistry Nuclear Waste Management and the
Nuclear Fuel Cycle- Patricia. A. Baisden, Gregory R. Choppin.
4. The principles of radioactive waste management. Vienna : International
Atomic Energy Agency, 1995.
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