Waste Management in the Nuclear Fuel Cycle

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Scorie Nucleari
Adriano Duatti
Laboratorio di MedicinaNucleare, Departimento di Scienze C/A e Radiologiche, Università di Ferrara,
Via L. Borsari, 46, 44100 Ferrara, Italy (email: [email protected])
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
processing or consumption of coal, oil and gas, and some minerals.
Classification of radioactive wastes
 Low-level waste (LLW): contains enough radioactive material to
require action for the protection of people, but not so much that it
requires shielding in handling or storage.
 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 more sophisticated handling
and disposal.
 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.
 Very low level waste or exempt waste: these categories contain
negligible amount amounts of radioactivity and may be disposed of
with domestic refuse.
Low-level Waste
Comprises the bulk of waste from the nuclear fuel cycle. It comprises
paper, rags, tools, clothing, filters etc which contain small amounts of
mostly short-lived radioactivity. It does not require shielding during
handling and transport and is suitable for shallow land burial. To reduce
its volume, these wastes are often compacted or incinerated before
disposal. Disposal sites for low level waste are in operation in many
countries. Worldwide they make up 90% of the volume but have only 1%
of the total radioactivity of all radioactive wastes.
Intermediate-level Waste
Contains higher amounts of radioactivity and normally requires
shielding. Shielding can be barriers of lead, concrete or water to give
protection from penetrating radiation such as gamma rays.
Intermediate-level wastes typically comprise resins, chemical sludges
and metal fuel cladding, as well as contaminated materials from reactor
decommissioning. It may be solidified in concrete or bitumen for
disposal. Generally short-lived waste (mainly from reactors) is buried,
but long-lived waste (from fuel reprocessing) will be disposed of
underground.
High-level Waste
Contains the fission products and transuranic elements
generated in the reactor core which are highly radioactive
and hot. High-level waste accounts for over 95% of the
total radioactivity produced though the actual amount of
material is low, 25-30 tonnes of spent fuel or three cubic
metres per year of vitrified waste for a typical large nuclear
reactor (1000 MWe, light-water type).
Radioactive medical source
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. 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)
Other low-level sources
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.
Naturally occurring radioactive material (NORM) contains alpha particleemitting matter from the decay chains of uranium and thorium. The main
source of radiation in the human body is potassium-40 (40K). There is a natural
background radioactivity that life systems are built to resist. Most rocks, due to
their components, have a certain, but low level, of radioactivity.
Coal contains a small amount of radioactive uranium, barium, thorium and
potassium. However, a coal power plant releases 100 times as much radiation
as a nuclear power plant of the same wattage. It is estimated that during 1982,
US coal burning released 155 times as much radioactivity into the atmosphere
as the Three Mile Island accident.
The Nuclear Fuel Cycle
 The nuclear fuel cycle is the series of industrial processes which involve
the production of electricity from uranium in nuclear power reactors.
 Uranium is a relatively common element that is found throughout the
world. It is mined in a number of countries and must be processed before
it can be used as fuel for a nuclear reactor.
 Fuel removed from a reactor, after it has reached the end of its useful
life, can be reprocessed to produce new fuel.
The Nuclear Fuel Cycle
Fuel Production
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.
Uranium fuel
Uraninite
(Pitchblende)
Yellowcake
U3O8
UO2
Uranium fuel
UF6
Fuel rods (UO2)
Reaction in standard UO2 fuel
Fission products
Fission product yields by mass for thermal neutron fission of U-235, Pu-239, and
U-233 used in the thorium cycle
Fission products
About 3% of the mass consists of fission products of 235U and
239Pu and products in the decay chain. The fission products
include every element from zinc through to the lanthanides,
much of the fission yield is concentrated in two peaks, one in the
second transition row (Zr, Mo, Tc, Ru, Rh, Pd, Ag) while the other
is later in the periodic table (I, Xe, Cs, Ba, La, Ce, Nd). Many of
the fission products are either non radioactive or only shortly
lived radioisotopes. But a considerable number are medium to
long lived radioisotopes such as 90Sr, 137Cs, 99Tc and 129I.
Medium-lived fission products
Nuclide
T1/2, y
Yield ,%
E, keV

155Eu
4.76
0.0803
252

58Kr
10.76
0.2180
687

113mCd
14.1
0.0008
316

90Sr
28.9
4.505
2826

137Cs
30.23
6.337
1176

121mSn
43.9
0.00005
390

151Sm
90
0.5314
77

Long-lived fission products
Nuclide
T1/2, My
Yield ,%
E, keV

99mTc
0.211
6.1385
294

126Sn
0.230
0.1084
4050

79Se
0.295
0.0447
151

93Zr
1.53
5.4575
91

135Cs
2. 3
6.9110
269

107Pd
6.5
1.2499
33

129I
15.7
0.8410
194

‘Front End’ Waste
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.
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.
‘Back End’ Waste
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.
Annual operation of a 1000 MWe nuclear power reactor
Mining
20,000 tonnes of 1% uranium ore
Milling
230 tonnes of uranium oxide concentrate (which
contains 195 tonnes of uranium)
Conversion
288 tonnes uranium hexafluoride, UF6 (with 195 t U)
Enrichment
35 tonnes enriched UF6 (containing 24 t enriched U)
Fuel
fabrication
27 tonnes UO2 (with 24 t enriched U)
Reactor
operation
8640 million kWh (8.64 TWh) of electricity at full output
Used fuel
27 tonnes containing 240 kg plutonium, 23 t uranium (0.8%
U-235), 720kg fission products, also transuranics.
A typical reactor generates about 27 tonnes of
spent fuel or 3 m3 per year of vitrified waste
Decay in radioactivity of fission
fuel in one tonne of spent fuel
Storage in ponds at reactor sites
There are about 270,000 tonnes of used fuel in storage, much of it at reactor sites. About 90% of this
is in storage ponds, the balance in dry storage. Annual arisings of used fuel are about 12,000 tonnes,
and 3,000 tonnes of this goes for reprocessing. Final disposal is not urgent in any logistical sense
Waste Management in the Nuclear Fuel Cycle
 Minimise the volume of waste requiring management via
treatment processes.
 Reduce the potential hazard of the waste by conditioning it into a
stable solid form that immobilises it and provides containment to
ensure that the waste can be safely handled during transportation,
storage and final disposal.
 Identifying a suitable matrix material - such as cement, bitumen,
polymers or borosilicate glass - that will ensure stability of the
radioactive materials for the period necessary. The type of waste
being conditioned determines the choice of matrix material and
packaging.
 Immobilising the waste through mixing with the matrix material.
 Packaging the immobilised waste in, for example, metallic drums,
metallic or concrete boxes or containers, copper canisters.
Waste Management in the Nuclear Fuel Cycle: LLW and ILW
The intermediate-level waste (ILW) along with the low-level
waste represent some 90% of the total volume of radioactive
waste generated during the lifetime of a nuclear power plant.
This relatively large volume of long-lived and short-lived ILW
contains only about 1% of the total radioactivity. Only a small
proportion of the intermediate-level waste remains significantly
radioactive for years, but all ILW requires shielding when it is
handled.
Waste Management in the Nuclear Fuel Cycle: LLW
Incineration
This technique can be applied to both radioactive and other wastes. In
the case of radioactive waste, it has been used for the treatment of lowlevel 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 1000 °C. 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. Volume
reduction factors of up to around 100 are achieved, depending on the
density of the waste.
Waste Management in the Nuclear Fuel Cycle: ILW
Compaction and Cementation
Cementation
Compaction
Waste Management in the Nuclear Fuel Cycle: LLW
Disposal
Near-surface disposal facilities at ground level. These facilities are on
or below 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.
Near-surface disposal facilities in caverns below ground level.
Unlike near-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.
Waste Management in the Nuclear Fuel Cycle: HLW
 Immobilise waste in an insoluble matrix such as borosilicate
glass or synthetic rock (fuel pellets are already a very stable
ceramic: UO2).
 Seal it inside a corrosion-resistant container, such as stainless
steel.
 Locate it deep underground in a stable rock structure.
 Surround containers with an impermeable backfill such as
bentonite clay if the repository is wet.
Waste Management in the Nuclear Fuel Cycle: HLW
Vitrification
The first step in the waste vitrification process involves 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[27]. 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. A typical reactor
generates about 27 tonnes of spent fuel or 3 m3 per year of vitrified
waste.
Vitrification
Typical Storage Container for Spent Fuel
Waste Management in the Nuclear Fuel Cycle: HLW
The Synroc method
Synroc is composed of three titanate minerals – hollandite,
zirconite and perovskite – plus rutile and a small amount of
metal alloy. These are combined into a slurry to which is
added a portion of high-level liquid nuclear waste. The
mixture is dried, calcined at 750 °C to produce a powder, and
then compressed in a bellows-like stainless steel container at
temperatures of between 1150 and 1200 °C. The result is a
cylinder of hard, dense, black synthetic rock. Unlike
borosilicate glass, which is amorphous, Synroc is a ceramic
that incorporates the radioactive waste into its crystal
structure
Waste Management in the Nuclear Fuel Cycle: HLW
Disposal
Geological repositories are planned in stable rock formations in the main
countries utilising nuclear energy. It is the responsibility of each country
to dispose of its wastes. Typically a repository will be 500 metres down
in rock, clay or salt. The disposing strategy is a multiple barrier concept:
 The waste, either as a ceramic oxide (e.g. the spent fuel itself) or
through vitrification (separated HLW from reprocessing) is immobilised.
 It is then sealed in a corrosion resistant canister such as stainless
steel or copper.
 Finally it is buried in a solid rock formation.
Waste Management in the Nuclear Fuel Cycle: HLW
 Disposal in strong fractured rocks
 Disposal in clay
 Disposal in natural rock salt
 Disposal in outer space
 Disposal at a subduction zone
 Disposal at sea
 Sub seabed disposal
 Disposal in ice sheets
Waste Management in the Nuclear Fuel Cycle: HLW
The Oklo natural reactor
A natural nuclear reactor was discovered in 1972 at the Oklo uranium
mine in the West African republic of Gabon. The deposit of ore, which
contained about 3% U-235, began a self-sustaining chain reaction
millions of years ago. Like all reactors, this one created its own high-level
waste, up to 5000 kg of fission products and transuranic elements which
today are found only in used fuel. The Oklo chain reaction occurred
intermittently for more than 500,000 years. Despite its location in a wet,
tropical climate, Oklo's uranium deposit and high-level waste has
remained securely locked in this natural repository for the past 2 million
years. Many of the waste products stayed where they were created or
moved only a few centimetres before decaying into harmless products.
Reprocessing and Recycling
Fresh uranium oxide fuel contains up to 5% U-235. When the fuel reaches
the end of its useful life, it is removed from the reactor. At this point it
typically contains about 95% U-238, 3% fission products (the residues of
the fission reactions) and transuranic isotopes, 1% plutonium and 1% U235. The plutonium is produced by the neutron irradiation of U-238.
Spent fuel still contains about a quarter of the original fissile U-235 as well
as much of the plutonium which has been formed in the reactor.
Reprocessing separates out this uranium and plutonium. The wastes left
after reprocessing can then be disposed of, while the uranium and
plutonium may be recycled for use in a nuclear reactor as mixed oxide
(MOX) fuel.
Decay in radioactivity of high-level waste
after recycling one tonne of spent fuel
Reprocessing and Recycling
Plutonium is recycled through a special fuel fabrication plant to
produce mixed oxide (MOX) fuel. MOX fuel is a mixture of plutonium
and uranium oxides (formed from natural, depleted or reprocessed
uranium). MOX fuel containing 5 to 7% plutonium has characteristics
that are similar to uranium oxide based fuel and used as part of a
reactor's fuel loading. It should be noted that plutonium arising from
the civil nuclear fuel cycle is not suitable for bombs because it contains
far too much of the Pu-240 isotope, due to the length of time the fuel
has been in the reactor.
Uranium from reprocessing, sometimes referred to as Rep-U, must
usually be enriched, and to facilitate this it must first be converted to
UF6.
Reprocessing: the PUREX method
PUREX is an acronym standing for Plutonium and Uranium Recovery by
EXtraction. Essentially, it is a liquid-liquid extraction ion-exchange method.
The irradiated fuel is first dissolved into nitric acid. An organic solvent
composed of 30% tributyl phosphate (TBP) in odorless kerosene (or
hydrogenated propylene trimer) is used to recover the uranium and
plutonium; the fission products remain in the aqueous nitric phase. Once
separated from the fission products, further processing allows separation of
the heavier plutonium from the uranium.
Reprocessing and Recycling
MOX
This type of nuclear fuel can be made by grinding together
reprocessed uranium oxide (UO2) and plutonium oxide (PuO2)
before the mixed oxide is pressed into pellets. MOX fuel, consisting
of 7% plutonium mixed with depleted uranium, is equivalent to
uranium oxide fuel enriched to about 4.5% U-235, assuming that
the plutonium has about 60- 65% Pu-239.
Reaction in MOX fuel
International organisations and safety standards
In 1997, 'The Joint Convention on the Safety of Spent Fuel Management
and the Safety of Radioactive Waste Management' was adopted by a
diplomatic conference of the International Atomic Energy Agency
(IAEA).
IAEA is the international organisation that oversees the peaceful uses
of atomic energy. It is an agency of the United Nations, that is based in
Vienna, Austria and was founded in 1957. The IAEA develops safety
standards, guidelines and recommendations and provides technical
guidance to member states regarding radiological practices and
protection. The IAEA'S Waste Safety Section works to co-ordinate the
development of internationally agreed standards on the safety of
radioactive waste.
The OECD/NEA (Nuclear Energy Agency of the Organisation for
Economic Co-operation and Development) is based in Paris, France.
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