Characteristics of a Heterogeneous Tru-loaiiing Core

LMFBRs are considered to have the potentiality of transmuting the TRU nuclides effectively because of their hard neutron spectrum(’TRU’ means eventually minor actinide(MA) in the present study). Since MA Ioadkg considerably affects not only core characteristics but fuel material properties, it is necessary to investigate MA loading method taking into account the influence upon core characteristics and fuel material properties. A heterogeneous M&loading method, where a few number of subassemblies with concentrated MA fuel (target fuel subassemblies) are loaded in the core, can have an advantage in fabric@&g and managing the MA-loaded fuel since the number of the MA-loaded fuel subassemblies is smaller compared with the method loading MA homogeneously. A study has been carried out on the feasibility of the heterogeneous MA-loading in an oxide-fueled 1000MWe LMFBR core. Based on the experimental data on fuel properties of MA fuel published up to now, it was found that MA loading significantly reduces the linear power limit almost proportionally to the MA loading ratio because of degradation of the thermal conductivity and the melting point. Furthermore, core analyses showed that heterogeneous MA-loading leads to a significant power deformation in the core if the design of target fuel subassemblies is the same as that of the normal fuel subassemblies with no MA fuel loaded. These cause a serious thermal problem. An effort was made so as to make the heterogeneous method feasible. The fuel pin design and the loading pattern of the target fuel subassemblies were studied. It was found that reduction of the fuel pin diameter and the Pu enrichment is essential to reduce the power of MA-loaded fuel. It is concluded that the heterogeneous MAloading method is feasible by optimizing fuel design, loading pattern and coolant flow of MA-loaded fuel subassemblies.