Reduced-moderation LWR configurations for TRU burning in thorium fuel

Multiple recycle of transuranic (TRU) isotopes in thermal reactors results in a degradation of the Pu fissile quality with build-up of higher actinides (e.g., Am, Cm, Cf), some of which are thermal absorbers. These phenomena lead to increasing amounts of plutonium feed being required to sustain criticality and accordingly larger TRU content in the multi-recycled fuel inventory, ultimately resulting in a positive moderator temperature coefficient (MTC) and void reactivity coefficient (VC). Due to the favorable impact fostered by use of thorium on these coefficients, the feasibility of thorium-TRU multiple-recycle in reduced-moderation PWRs (RMPWRs) and BWRs (RBWRs) has been investigated using assembly and full-core models. Spatial separation of TRU from bred uranium (U3) is found to greatly improve neutronic performance. A large reduction in moderation is necessary to allow full actinide recycle. In a retrofit PWR design, a sufficient reduction of moderation may not be achievable without affecting the plant safety case, or penalizing operation. The harder neutron spectrum resulting from the reduced moderation reduces the control rod worth, which may make it difficult to achieve an adequate shutdown margin, while there is a neutronic incentive to use increased mechanical shim to maintain a negative MTC. It may therefore be desirable to increase the number of rod cluster control assemblies. Superior burn-up is achievable in a reduced-moderation BWR as a larger reduction in moderation is feasible, although the incineration rate is reduced relative to a PWR due to a higher conversion ratio