An appreciable advantage of thorium-based fuel is the potential to reach very high burn-ups so that the number of fuel assemblies needed to achieve a given energy output is reduced. This would produce savings in fuel manufacture and reduce the amount of waste that was produced although burn-up cannot be increased indefinitely, of course.

The interim storage of thorium spent fuel shows characteristics a little less constraining than those of uranium-based spent fuel because of the relative chemical inertness of thorium. In consequence, maximum acceptable temperatures for dry storage of spent UO2 fuel are lower than for thorium fuel because at higher temperatures, UO2 fuel may oxidize to U3O8 with a volume expansion that may rupture the fuel cladding. Matrix oxidation is not an issue with thorium-based fuels. Further, oxidation of minor solid-solution components such as uranium and plutonium can be easily accommodated within the thorium fuel matrix. Consequently, fuel oxidation is unlikely to be a concern during dry storage of thorium-based fuels and the maximum storage temperature may be limited by other factors such as cladding degradation.11 Similar points concerning the reaction of thorium fuel with water may be made with respect to wet interim storage.

Direct disposal of thorium-based fuels is attractive from the standpoint of long­term behaviour in a geological repository, because thorium oxide is chemically stable and almost insoluble in ground water. The most important chemical difference between thorium and uranium oxides is that thorium is present in its maximum oxidation state whereas uranium is not. Under oxidizing conditions, uranium can be converted into the water-soluble uranyl cation UO2 2+ and its various derivatives. Not only does this produce a mobile radionuclide, it also degrades the fuel, releasing the actinides and fission products, which are contained within it. Conversely, radionuclide release from thorium oxide fuel is expected to be limited by the low solubility of ThO2 and its low cation diffusion coefficient.

No credible aqueous or geochemical process has yet been identified that would greatly accelerate ThO2 fuel-matrix dissolution under disposal conditions.11

Disposal of fission product waste after reprocessing of thorium-based fuel would require treatment similar to that of waste from reprocessed UO2 fuels. Although thorium-based fuel cycles may produce much less plutonium and associated minor actinides than uranium-based fuels, they will instead generate other radionuclides such as Pa-231, Th-229 and U-230, which will have a long­term radiological impact.

Nevertheless, the global radiotoxic inventory (GRI) of waste to be disposed when using a thorium cycle appears to be significantly less than for the standard uranium-plutonium cycle, for the same energy output. This is a real asset for thorium-based fuels which has been confirmed in several studies, such as in a recent one, performed under an EC contract.7 The main findings of these studies are as follows:

• Where only the major actinides are recycled and reused with a thorium matrix (i. e. assuming that all other actinides such as Np, Am, Cm or Pa go to waste), the GRI of the as-disposed thorium cycle waste is reduced by a factor of 10 compared to the uranium-plutonium cycle. As the disposed waste decays, the two GRI values come closer together so that, after 10 000 years, the ‘thorium GRI’ is greater than the ‘uranium GRI’. This is not seen as a major problem, however, because beyond a few tens of thousands of years, both GRI values are relatively low. They are, for example, lower than that of the amount of natural uranium needed to feed a once-through reactor programme of equivalent energy output.

• In the case of recycling of all minor actinides (assuming 0.1% losses to the waste), the ‘thorium GRI’ is less than the ‘uranium GRI’ by a factor of between about 5 to 20 for all times up to 10 000 years. After 20 000 to 30 000 years or so, the ‘thorium GRI’ becomes the greater but, as in the previous case, the absolute values are relatively low being, again, lower than that of the equivalent amount natural uranium used in a once-through cycle.