Multi-recycling of actinides in thorium based fuels

Burnup calculations have been performed to assess reactor physics and waste toxicity characteristics of a PWR fuelled with either low enriched UO₂ (LEU) fuel, medium enriched UO₂ (MEU) fuel mixed in ThO₂, or highly enriched UO₂ (HEU) fuel mixed in ThO₂. For all fuels the reactivity coefficients have been calculated as a function of burnup, and the radiotoxicity curves of the spent fuels and loss fractions as a function of storage time. Without recycling, the fuel temperature coefficient of the MEU fuel is strongest negative due to the large quantities of both ²³⁸U and ²³²Th in the fuel. For the HEU fuel, the moderator temperature coefficient is very small negative or even positive, which means that special measures have to be taken. Up to 20,000 years of storage, the radiotoxicity of HEU spent fuel is lowest and that of LEU spent fuel largest. After this period the reverse is true. After two times recycling of Pa+U in MEU fuel, the radiotoxicity of the reprocessing losses is a factor of 2 less than the radiotoxicity of the LEU spent fuel. If in addition Pu is recycled in MEU fuel, the reduction factor equals a factor of 5 to 10. The highest reduction (a factor of 10 to 20) can be obtained when Pa and U are recycled in HEU fuel. Only in this case, the reduction factor is significantly larger than that obtained with recycling of Pu in standard (U,Pu) MOX fuel. In general, multi-recycling worsens the reactivity coefficients with several tens of percents