Nuclear Fuel Reprocessing

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Reprocessing spent nuclear fuel is examined as an answer to the nuclear industry issues of waste, storage and proliferation especially with nuclear power expected to expand. Nuclear processes are explained to establish some familiarity with how nuclear energy is generated. The generation of mixed oxide from spent nuclear fuel is demonstrated as a safe efficient use of existing plutonium to answer proliferation concerns and waste storage issues.? The reprocessing strategy could provide services to other countries and jobs for U.S. citizens.? The U.S. could benefit from a new political view of reprocessing its resource of spent nuclear fuel and reduce its high level waste storage issues and safely consume existing plutonium stockpiles for a growing electrical power grid.

Nuclear waste

  • 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.
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Radioactivity arises naturally from the decay of particular forms of some elements, called isotopes. Some isotopes are radioactive, most are not, though in this publication we concentrate on the former.

Three general principles are employed in the management of radioactive wastes:

  • concentrate-and-contain
  • dilute-and-disperse
  • delay-and-decay.
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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.

Types of 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.

Intermediate-level Waste 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) will be disposed of deep underground.

High-level Waste may be the used fuel itself, or the principal waste 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 deep underground.

 

Waste disposal

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 long periods. The most favoured method is burial in dry, stable geological formations some 500 metres deep. Several countries are investigating sites that would be technically and publicly acceptable. The USA is pushing ahead with a repository site in Nevada for all the nation's used fuel.
One purpose-built deep geological repository for long-lived nuclear waste (though only from defence applications) is already operating in New Mexico.
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 naturally-occurring uranium ore from which it originated, though it would be more concentrated.

Layers of protection 

To ensure that no significant environmental releases occur over a long perio after disposal, a 'multiple barrier' disposal concept is used to immobilise the radioactive elements in high-level (and some intermediate-level) wastes and to isolate them from the biosphere. The principal barriers are:

  • Immobilise waste in an insoluble matrix, eg borosilicate glass, Synroc (or leave them as uranium oxide fuel pellets - a ceramic).
  • Seal inside a corrosion-resistant container, eg stainless steel.
  • 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 before the radioactivity decayed.

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