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Thursday, November 6, 2008

Accelerator-driven Nuclear Energy

More information at this site:

- Powerful accelerators can produce neutrons by spallation.
- This process may be linked to conventional nuclear reactor technology in Accelerator-Driven Systems (ADS) to transmute long-lived radioisotopes in used nuclear fuel into shorter-lived fission products.
- There is also increasing interest in the application of ADS to running subcritical nuclear reactors, powered by thorium.

The essence of a conventional nuclear reactor is the controlled fission chain reaction of U-235 and Pu-239. This produces heat which is used to make steam which drives a turbine. The chain reaction depends on having a surplus of neutrons to keep it going (a U-235 fission requires one neutron input and produces on average 2.43 neutrons).

For many years there has been interest in utilising thorium (Th-232) as a nuclear fuel since it is three to five times as abundant in the Earth's crust as uranium. Also, all of the mined thorium is potentially useable in a reactor, compared with the 0.7% of natural uranium, so some 40 times the amount of energy per unit mass might theoreticlly be available. A thorium reactor would work by having Th-232 capture a neutron to become Th-233 which decays to uranium-233, which fissions. The problem is that insufficient neutrons are generated to keep the reaction going.

More recently there has been interest in transmuting the long-lived transuranic radionuclides (actinides - neptunium, americium and curium particularly) formed by neutron capture in a conventional reactor and reporting with the high-level waste. If these could be made into shorter-lived radionuclides such as fission products, the management and eventual disposal of high-level radioactive waste would be easier and less expensive. As it is, most radionuclides (notably fission products) decay rapidly, so that their collective radioactivity is reduced to less than 0.1% of the original level 50 years after being removed from the reactor. However, a significant proportion of the separated high-level wastes is long-lived actinides.

Accelerator-driven systems (ADS) address both these issues. They are seen as safer that a normal fission reactor because they are subcritical and stop when the input current is switched off. This is because they burn material which does not have a high enough fission-to-capture ratio for neutrons to enable criticality and maintain a fission chain reaction. It may be thorium fuel, or actinides which need 'incineration'. An ADS can only run when neutrons are supplied to it.

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