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A Russian fissile material storage facility
Radioactive waste is waste material containing radioactive chemical
elements which does not have a practical purpose. It is often the product of a nuclear process, such as nuclear fission. Waste can also be generated from the processing of fuel for
nuclear reactors or nuclear weapons.
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. Eventually all waste decays into non-radioactive
elements.
The faster a radioisotope is decaying, the more radioactive it will be.
The factor in deciding how dangerous a pure radioactive substance will be is the energy of the radiation. Some decays yield more
energy than others. This is further complicated by the fact that few radioisotopes decay immediately to a stable state, but
rather to a radioactive decay product leading to decay chains.
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 for the more dangerous wastes, the preferred
technology to date has been deep and secure burial. Transmutation,
long-term retrievable storage, and removal to space have also been suggested.
Types of radioactive waste
Removal of very low-level waste
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. It does not
require shielding during handling and transport and is suitable for shallow land burial. To reduce its volume, it is often
compacted or incinerated before disposal.
Intermediate level Waste (ILW) contains higher amounts of radioactivity and some requires shielding. It
typically comprises 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 in a shallow repository, while long lived waste (from fuel reprocessing) will be
disposed of deep underground.
High level Waste (HLW) arises from the use of uranium fuel in a
nuclear reactor and nuclear weapons processing. It contains the
fission products and transuranic elements generated in the reactor core. It is highly radioactive and hot. It can be considered
the "ash" from "burning" uranium. HLW accounts for over 95% of the total radioactivity produced in the process of nuclear
electricity generation.
Transuranic Waste arises mainly from weapons production, and consists of clothing, tools, rags, residues,
debris and other such items contaminated with small amounts of radioactive elements -- mostly plutonium. These elements have an
atomic number greater than uranium -- thus transuranic (beyond uranium). Because of the long half-lives of these elements, this
waste is not disposed of as either low level or intermediate level waste. It does not have the very high radioactivity of high
level waste, nor its high heat generation. The United States currently permanently disposes of transuranic waste at the Waste Isolation Pilot Plant.
Wastes from nuclear reactor fuel processing
Uranium oxide concentrate from mining is not significantly radioactive - barely more so than the granite used in buildings. It
is refined to form 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 3.5%. 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, principally the U-238 isotope, which 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 and
to dilute highly enriched uranium from weapons stockpiles which is now being redirected to become reactor fuel.
Disposing of high-level wastes
High-level radioactive waste is stored temporarily in spent fuel pools and in dry cask
storage facilities.
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 utilised. Away-from-reactor storage capacity was 78,000 tonnes, with 44%
utilised. Annual arisings are about 12,000 tonnes. Final disposal is therefore not urgent.
France is furthest ahead with preparation for HLW disposal. In 1989 and 1992 it
commissioned commercial plants to vitrify HLW left over from reprocessing oxide fuel, although there are adequate facilities
elsewhere, notably in the UK 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 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 curently being developed for US
military wastes).
The process of selecting appropriate deep final repositories is now under way in several countries with the first expected to
be commissioned some time after 2010. 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 its location next to the border to the former GDR. Actually this place is used to
store radioactive waste non-permanently. The US has opted for a final repository at Yucca Mountain in Nevada. There is also a proposal for an international HLW repository in optimum geology -
Australia or Russia are possible locations - however, when the proposal for a global repository for Australia has been raised
domestic political objections have been loud and sustained, making such a dump in Australia unlikely.
In 2003 the UK government
appointed a committee on radioactive waste management, the UK does, afterall, have 500,000 tonnes of such waste. Deciding to look at this form of disposal in a new light, they looked at 14 methods of disposal
- all possible, though each has its drawback. They are as follows:
- Send it into space aiming for it to exit the solar system or hit the sun.
This method has the potential for rocket failure, and hence the release of radioactive waste into the atmosphere. It is also
prohibitively expensive.
- Forcefully insert it on the edge of tectonic plates so as to allow
it to enter the Earth's mantle.
- Three options involve Antarctica:
- Allow it to sink two miles through the ice to the bedrock.
- Allow it to sink through ice, but keep it on chains so as to not lose it.
- Place it on the surface of ice, and superficially cover it with ice.
The problem with this method is the Antarctic Treaty,
maintaining it as the last pristine continent. Furthermore, future climate change could potentially cause the Antarctic icecap to
melt and expose the waste.
- Drop the waste to the bottom of seas and oceans
packaged in concrete, as previously done by the UK.
- Attach it to torpedos so that it to becomes deeply embedded in the seabed.
The above two options are technically supreme. See ocean
floor disposal for detailed discussion.
- Liquify the waste and pump it into underground reservoirs, as previously done by Russia and Sweden.
- Store on the surface of Earth.
- Store it underground, safer than the above option.
The above three options are limited by the geologic conditions of the country.
Also, there is the potential danger of nuclear theft.
- Construct nuclear plants to recondition waste.
- Dilute the waste and pump it into the
sea, as done previously by the early nuclear industry.
References
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