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Radioactive waste

From Academic Kids

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A Russian fissile material storage facility

Radioactive waste is waste material containing radioactive chemical elements that 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. Another 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.

Contents

Types of radioactive waste

Removal of very low-level waste
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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 very radioactive - only a thousand or so times as radioactive as 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% (enriched uranium). 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, with a U-235 content of ca. 0.3%. It is stored, either as UF6 or as U3O8. Some is used in applications where its extremely high density makes it valuable, such as the keels of yachts, and anti-tank shells. It is also used (with recycled plutonium) for making mixed oxide fuel and to dilute highly enriched uranium from weapons stockpiles which is now being redirected to become reactor fuel. This dilution, also called downblending, means that any nation or group that acquired the finished fuel would have to repeat the (very expensive and complex) enrichment process before assembling a weapon.

It's important to distinguish the processing of uranium to make fuel, from the processing of used fuel. Used fuel contains the highly radioactive products of fission (see High Level Waste above). Many of these are neutron absorbers. These eventually build up to a level where they absorb so many neutrons that the chain reaction stops, even with the control rods completely removed. At that point the fuel has to be changed, even though there is still plenty of U235 (or plutonium) present. Currently, in the USA, this used fuel is stored. In other countries (UK, France, and Japan in particular) the fuel is reprocessed to remove the fission products, and the fuel can be re-used. The reprocessing process involves handling highly radioactive materials, and the fission products removed from the fuel are a concentrated form of High Level Waste.

Proliferation concerns

When dealing with uranium and plutonium, the possibility that they may be used to build nuclear weapons, perhaps by terrorists (nuclear proliferation), is often a consideration. Active nuclear reactors and nuclear weapons stockpiles are very carefully safeguarded and controlled. However, high-level waste from nuclear reactors contains plutonium. Ordinarily, this plutonium is reactor-grade plutonium, containing a mixture of Pu-239 (highly suitable for building nuclear weapons) and Pu-240 (an undesirable contaminant and highly radioactive); the two isotopes are difficult to separate. Moreover, high-level waste is full of highly radioactive fission products. However, most fission products are relatively short-lived. This is a concern since if the waste is stored, perhaps in deep geological storage, over many years the fission products decay, decreasing the radioactivity of the waste. Thus as time passes, these deep storage areas have the potential to become "plutonium mines", from which nuclear weapons can be made with relatively little difficulty.

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 currently 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, after all, 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:

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, melting its way via its own decay heat. Theft would require major work in basic engineering to even consider.
    • 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 torpedoes so that it to becomes deeply embedded in the seabed.

The above two options are technically supreme. See ocean floor disposal for detailed discussion.

  • Liquefy 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.

There have been proposals for reactors that consume nuclear waste and transmute it to other, less-harmful nuclear waste. In particular, the Integral Fast Reactor was a proposed nuclear reactor with a nuclear fuel cycle that produced no transuranic waste; in fact, it could consume transuranic waste. It proceeded as far as large-scale tests but was then cancelled by the US Government.

Accidents involving radioactive waste

While nuclear waste is not as sensitive to disruption as an active nuclear reactor, it is often treated as waste and forgotten. A number of incidents have occurred when nuclear waste was improperly disposed of.

Perhaps the worst is the Goinia accident, in which a rod of caesium salts, previously used in a hospital, was left behind when the hospital was abandoned. Scavengers collected the rod and its casing, and the mysterious glowing rod was passed around. Several people were killed, and many more suffered radiation poisoning, before the rod was recognized as radioactive.

See also

References

  • The Nuclear Energy Option (http://www.phyast.pitt.edu/~blc/book/BOOK.html), book online by Bernard L. Cohen with chapter on nuclear waste.
  • Radwaste.org (http://radwaste.org) Very comprehensive list of links.
  • Radwaste (http://radwaste.blogspot.com) niche blog Radwaste


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