Waste Management: Overview
updated December 2012
- Like all industries, the
thermal generation of electricity produces wastes. Whatever fuel is
used, these wastes must be managed in ways which safeguard human
health and minimise their impact on the environment.
- Nuclear power is the only energy industry which takes full responsibility for all its wastes, and costs this into the product.
Nuclear power is characterised by the very large amount of energy available from a very small amount of fuel. The amount of waste is correspondingly very small. However, much of the waste is radioactive and therefore must be carefully managed as hazardous waste.
Since the radioactive wastes are essentially created in a nuclear power reactor, it is accepted that they are the responsibility of the country which uses uranium to generate power. There is no moral or legal basis for the responsibility to be elsewhere.
Radioactive wastes comprise a variety of materials requiring different types of management to protect people and the environment. They are normally classified as low-level, medium-level or high-level wastes, according to the amount and types of radioactivity in them.
Another factor in managing wastes is the time that they are likely to remain hazardous. This depends on the kinds of radioactive isotopes in them, and particularly the half-lives characteristic of each of those isotopes. (The half-life is the time it takes for a given radioactive isotope to lose half of its radioactivity. After four half lives the level of radioactivity is 1/16th of the original and after eight half lives 1/256th, and so on.)
The various radioactive isotopes have half-lives ranging from fractions of a second to minutes, hours or days, through to billions of years. Radioactivity decreases with time as these isotopes decay into stable, non-radioactive ones.
The rate of decay of an isotope is inversely proportional to its half-life; a short half life means that it decays rapidly. Hence, for each kind of radiation, the higher the intensity of radioactivity in a given amount of material, the shorter the half lives involved.
Three general principles are employed in the management of radioactive wastes:
The first two are also used in the management of non-radioactive wastes. The waste is either concentrated and then isolated, or it is diluted to acceptable levels and then discharged to the environment. Delay-and-decay however is unique to radioactive waste management; it means that the waste is stored and its radioactivity is allowed to decrease naturally through decay of the radioisotopes in it.
Radioactivity arises naturally from the decay of particular forms of some elements, called isotopes. Some isotopes are radioactive, most are not, though here the focus is on the former.There are three kinds of radiation to consider: alpha, beta and gamma. A fourth kind, neutron radiation, generally only occurs inside a nuclear reactor.
Different types of radiation require different forms of protection:
- Alpha radiation cannot penetrate the skin and can be blocked out by a sheet of paper, but is dangerous in the lung.
- Beta radiation can penetrate into the body surface but can be blocked out by a sheet of aluminium foil.
- Gamma radiation can go deeply into the body and requires several centimetres of lead or concrete, or a metre or so of water, to block it.
All of these kinds of radiation are, at low levels, naturally part of our environment, where we are all naturally exposed to them at low levels. Any or all of them may be present in any classification of radioactive waste
radioactive waste (radwaste)
Low-level Waste is generated
from hospitals, laboratories 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. It
is not dangerous to handle, but must be disposed of more carefully
than normal garbage. Usually it is buried in shallow landfill
sites. To reduce its volume, it is often compacted or incinerated
(in a closed container) before disposal. Worldwide it comprises 90%
of the volume but only 1% of the radioactivity of all radwaste.
contains higher amounts of radioactivity and may require special
shielding. It typically comprises resins, chemical sludges and
reactor components, as well as contaminated materials from reactor
decommissioning. Worldwide it makes up 7% of the volume and has 4%
of the radioactivity of all radwaste. It may be solidified in
concrete or bitumen for disposal. Generally short-lived waste
(mainly from reactors) is buried, but long-lived waste (from
reprocessing nuclear fuel) is disposed of deep underground.
High-level Waste may be the
used fuel itself, or the principal waste separated from
reprocessing this. While only 3% of the volume of all radwaste, it
holds 95% of the radioactivity. It contains the highly-radioactive
fission products and some heavy elements with long-lived
radioactivity. It generates a considerable amount of heat and
requires cooling, as well as special shielding during handling and
transport. If the used fuel is reprocessed, the separated waste is
vitrified by incorporating it into borosilicate (Pyrex) glass which
is sealed inside stainless steel canisters for eventual disposal
On the other hand, if used reactor
fuel is not reprocessed, all the highly-radioactive isotopes remain
in it, and so the whole fuel assemblies are treated as high-level
waste. This used fuel takes up about nine times the volume of
equivalent vitrified high-level waste which is separated in
reprocessing. Used fuel treated as waste must be encapsulated ready
Both high-level waste and used fuel
are very radioactive and people handling them must be shielded from
their radiation. Such materials are shipped in special containers
which shield the radiation and which will not rupture in an
Whether used fuel is reprocessed or
not, the volume of high-level waste is modest, - about 3 cubic
metres per year of vitrified waste, or 25-30 tonnes of used fuel
for a typical large nuclear reactor. The relatively small amount
involved allows it to be effectively and economically isolated.
Radioactive materials in the
materials are widespread throughout the environment, although
concentrations are very low and they are not normally harmful.
However, human activity may concentrate these so that they need
careful handling- eg in coal ash and gas well residues.
Soil naturally contains a variety of
radioactive materials - uranium, thorium, radium and the
radioactive gas radon which is continually escaping to the
atmosphere. Many parts of the Earth's crust are more radioactive
than the low-level waste described above. Radiation is not
something which arises just from using uranium to produce
electricity, although the mining and milling of uranium and some
other ores brings these radioactive materials into closer contact
with people, and in the case of radon and its daughter products,
speeds up their release to the atmosphere. (See also Radiation and Life.)
Wastes from the nuclear fuel
Radioactive wastes occur at all
stages of the nuclear fuel cycle - the process of producing
electricity from nuclear materials. The fuel cycle comprises the
mining and milling of the uranium ore, its processing and
fabrication into nuclear fuel, its use in the reactor, the
treatment of the used fuel taken from the reactor after use, and
finally, disposal of the wastes.
The fuel cycle is often considered
as two parts - the "front end" which stretches from mining through
to the use of uranium in the reactor - and the "back end" which
covers the removal of used fuel from the reactor and its subsequent
treatment and disposal. This is where radioactive wastes are a
Residual materials from the "front
end" of the fuel cycle
The annual fuel requirement for a
l000 MWe light water reactor is about 27 tonnes of enriched uranium
oxide. This requires the mining and milling of tens of thousands of
tonnes of ore to provide about 200 tonnes of uranium oxide
concentrate (U3O8) from the mine.
At uranium mines, dust is controlled
to minimise inhalation of radioactive minerals, while
concentrations of radon gas (seeping out of the rocks) are kept to
a minimum by good ventilation and dispersion in large volumes of
air. At the mill, dust is collected and fed back into the process,
while radon gas is diluted and dispersed to the atmosphere in large
volumes of air.
At the mine, residual ground rock
from the milling operation contain most of the radioactive
materials from the ore, such as radium. This material is discharged
into tailings dams which retain the remaining solids and prevent
any seepage of the liquid. The tailings contain about 70% of the
radioactivity in the original ore.
Eventually these tailings may be put
back into the mine or they may be covered with rock and clay, then
revegetated. In this case considerable care is taken to ensure
their long-term stability and to avoid any environmental impact
(which would be more from acid leaching or dust than from
radioactivity as such).
The tailings are usually around ten
times more radioactive than typical granites, such as used on city
buildings. If someone were to live continuously on top of the
Ranger mine tailings they would receive about double their normal
radiation dose from the actual tailings (ie they would triple their
With in situ leach (ISL) mining,
dissolved materials other than uranium are simply returned
underground from where they came, as the water is recirculated.
(U3O8) produced from the mining and milling
of uranium ore is only mildly radioactive - most of the
radioactivity in the original ore remains at the mine site in the
Turning uranium oxide concentrate
into a useable fuel has no effect on levels of radioactivity and
does not produce significant waste.
First, the uranium oxide is
converted into a gas, uranium hexafluoride (UF6), as
feedstock for the enrichment process.
Then, during enrichment, every tonne
of uranium hexafluoride becomes separated into about 130 kg of
enriched UF6 (about 3.5% U-235) and 870 kg of
'depleted' UF6 (mostly U-238). The enriched
UF6 is finally converted into uranium dioxide
(UO2) powder and pressed into fuel pellets which are
encased in zirconium alloy tubes to form fuel rods.
Depleted uranium has few uses,
though with a high density (specific gravity of 18.7) it has found
uses in the keels of yachts, aircraft control surface
counterweights, anti-tank ammunition and radiation shielding. It is
also a potential energy source for particular (fast neutron)
Wastes from the "back end" of the fuel cycle
It is when uranium is used in the
reactor that significant quantities of highly radioactive wastes
are created. When the uranium-235 atom is split it forms fission
products, which are very radioactive and make up the main portion
of nuclear wastes retained in the fuel rods.
About 27 tonnes of used fuel is
taken each year from the core of a l000 MWe nuclear reactor. This
fuel can be regarded entirely as waste (as, for 40% of the world's
output, in USA and Canada), or it can be reprocessed (as in Europe
and Japan). Whichever option is chosen, the used fuel is first
stored for several years under water in cooling ponds at the
reactor site. The concrete ponds and the water covering the fuel
assemblies provide radiation protection, while removing the heat
generated during radioactive decay.
Storage pond for spent fuel at UK reprocessing
costs of dealing with this high-level waste are built into
electricity tariffs. For instance, in the USA, consumers pay 0.1
cents per kilowatt-hour, which utilities pay into a special fund.
So far more than US$ 32 billion has been collected thus.
also a relatively small amount of radioactivity induced in the
reactor components by neutron irradiation. When the reactor is
retired and dismantled these materials become wastes.
If the used
fuel is later reprocessed, it is dissolved and separated chemically
into uranium, plutonium and high-level waste solutions. About 97%
of the used fuel can be recycled leaving only 3% as high-level
waste. The recyclable portion is mostly uranium depleted to less
than 1% U-235, with some plutonium, which is most valuable.
Arising from a year's operation of a
typical l000 MWe nuclear reactor, about 230 kilograms of plutonium
(1% of the spent fuel) is separated in for recycle. This can be
used in fresh mixed oxide (MOX) fuel (but not weapons, due its
The separated high-level wastes -
about 3% of the typical reactor's used fuel - amounts to 700 kg per
year and it needs to be isolated from the environment for a very
Immobilising separated high-level waste
Solidification processes have been developed in several
countries over the past fifty years. Liquid high-level wastes are
evaporated to solids, mixed with glass-forming materials, melted
and poured into robust stainless steel canisters which are then
sealed by welding.
Borosilicate glass from the
first waste vitrification plant in UK in the 1960s. This block
contains material chemically identical to high-level waste from
reprocessing. A piece this size would contain the total high-level
waste arising from nuclear electricity generation for one person
throughout a normal lifetime.
The vitrified waste from the
operation of a 1000 MWe reactor for one year would fill about
twelve canisters, each 1.3m high and 0.4m diameter and holding 400
kg of glass.
Loading silos with canisters containing vitrified high-level waste in UK, each disc on the floor covers a silo holding ten canisters
sophisticated method of immobilising high-level radioactive wastes
has been developed. Called 'SYNROC' (synthetic rock), the
radioactive wastes are incorporated in the crystal lattices of the
naturally-stable minerals in a synthetic rock. In other words,
copying what happens in nature. This process is still
being developed for specialist application.
Final disposal of high-level waste is delayed for 40-50 years to allow its radioactivity to decay, after which less than one thousandth of its initial radioactivity remains, and it is much easier to handle.
Hence canisters of vitrified waste, or used fuel assemblies, are
stored under water in special ponds, or in dry concrete structures
or casks, for at least this length of time.
The ultimate disposal of vitrified
wastes, or of used fuel assemblies without reprocessing, requires
their isolation from the environment for a long time. The most
favoured method is burial in stable geological formations some 500
metres deep. Several countries are investigating sites that would
be technically and publicly acceptable, and in Sweden and Finland
construction is proceeding in 1.9 billion year-old granites.
One purpose-built deep geological
repository for long-lived nuclear waste (though only from defence
applications) is already operating in New Mexico, in a salt
After being buried for about 1000
years most of the radioactivity will have decayed. The amount of
radioactivity then remaining would be similar to that of the
corresponding amount of naturally-occurring uranium ore from which
it originated, though it would be more concentrated.
Layers of protection after disposal
To ensure that no significant
environmental releases occur over a long period after disposal, a
'multiple barrier' disposal concept is used. The radioactive
elements in high-level (and some intermediate-level) wastes are
immobilised and securely isolated from the biosphere. The principal
- Immobilise waste in an insoluble matrix, eg borosilicate glass
(or leave them as uranium oxide fuel pellets - a ceramic).
- Seal inside a corrosion-resistant container, eg stainless
- Surround containers with bentonite clay to inhibit any
groundwater movement if the repository is likely to be wet.
- Locate deep underground in a stable rock structure.
For any of
the radioactivity to reach human populations or the environment,
all of these barriers would need to be breached, and this would
need to happen before the radioactivity decayed to innocuous
particular example in nature provides strong reassurance concerning
final disposal of high-level wastes underground. Two billion years
ago at Oklo in Gabon, West Africa, chain reactions started
spontaneously in concentrated deposits of uranium ore. These
natural nuclear reactors continued operating for hundreds of
thousands of years forming plutonium and all the highly radioactive
waste products created today from exactly the same processes in a
nuclear power reactor. Despite the existence at that time of large
quantities of water in the area, these materials stayed where they
were formed and eventually decayed into non-radioactive elements.
The evidence remains there.