Characteristics of TRU Transmutation in an LMFBR

It is generally acknowledged that LMFBRs would be superior TRU transmutation devices, mainly due to their hard neutron spectrum. In the present study, the characteristics of TRU transmutation in a 1000 MWe-class LMFBR with mixed-oxide(MOX) fuel were quantitatively investigated to substantiate the feasibility. We considered here two typical methods transmuting TRU in the LMFBR core. One is the method dispersing TRU homogeneously throughout the entire core (homogeneous TRU-loading method). It was found that the homogeneous method had no serious penalties of TRU ioading to the reactor core performance, and the TRU transmutation rate reached approximately 11% per cycle (15months) with the loading of the weight ratio of 5% TRU in MOX fuel. The amount of the TRU transmutation in the LMFBR is almost six times as much as that of the TRU production from a 1000 MWe-class LWR. As another desirable property, the TRUloaded core was found to reduce the burnup reactivity loss by 40% compared with the reference core without TRU. It can be concluded that the TRU transmutation in an LMFBR has no serious problems from the viewpoint of the core performance, provided that the homogeneous method could be employed. The other possible method is the use of small number of subassemblies (target S/As) which concentrate TRU in fuel (heterogeneous TRU-loading method). The weight ratio of 50% TRU fuel was assumed to be loaded in the target S/As. The calculated results showed that the swing of power peaking near the target S/As became quite severe and critical for the thermal design criteria, although the TRU transmutation rate and other reactor core performances of the heterogeneous method were roughly comparable to those of the homogeneous method. It seems from the present study that the heterogeneous method has a serious problem to be overcome and much effort would be needed to make the method feasible.