In fast neutron spectrum reactors, the recycling of plutonium and MAs in critical systems can be carried out in either a homogeneous or a heterogeneous mode.810 In the homogeneous recycling mode, all the TRU elements are recycled together (i. e. ‘grouped’ TRU) and, after mixing with other fissile material, form the standard fuel for the reactor. In heterogeneous mode, on the other hand, MAs are separated from plutonium and placed as targets in specific sub-assemblies and managed independently from the standard fuel, which contains plutonium. When comparing the two modes in terms of core and fuel cycle performance, key issues are a potential increase in the (already positive) sodium coolant void reactivity coefficient, which is associated with homogeneous TRU recycling in, e. g., a sodium-cooled fast reactor (which could limit the MA content of the fuel), and unwanted increases in decay heat and spontaneous neutron emission, which are of interest for the fuel cycle, especially when considering fuel fabrication (see below).

Recent studies (see e. g. Ref 11) have concluded that there are no theoretical ‘upper limits’ for the MA content of fast reactor fuel and that any potential issues for reactor safety can be managed through appropriate design measures. A recent JAEA study 1 2 calculated the different MA compositions that will arise in fast reactor fuel when, during a transition from a LWR fuel cycle to a FR fuel cycle, it is used for continuous homogeneous recycling of grouped TRU material: see

Fig. 17.3. It is observed that the MA content varies from 1 to 4 wt% corresponding to natural in-core equilibrium values. It is possible that the actual content could be higher if there is the need to deal with MA legacies from previous operation of LWRs. This is one advantage of partitioning the Pu and MAs as it allows the option to utilize and manage the two materials in any favorable ratio, even if they were to be subsequently recombined for use in homogeneous recycling. Such flexibility could be used to alleviate any potential safety issues arising from positive sodium void reactivity coefficients or from radiation doses during fabrication.