On the Use of Reduced-Moderation LWRs for Transuranic Isotope Burning in Thorium Fuel—II: Core Analysis

Multiple recycle of transuranic (TRU) isotopes in thermal reactors results in a degradation of the plutonium (Pu) fissile quality with buildup of higher actinides (e.g., Am, Cm, Cf), some of which are thermal absorbers. These phenomena lead to increasing amounts of Pu feed being required to sustain criticality and accordingly larger TRU content in the multirecycled fuel inventory, ultimately resulting in a positive moderator temperature coefficient (MTC) and void reactivity coefficient (VC). Because of the favorable impact fostered by use of thorium (Th) on these coefficients, the feasibility of Th-TRU multiple recycle in reduced-moderation (RM) pressurized water reactors (PWRs) and RM boiling water reactors (called RMPWRs and RBWRs, respectively) has been investigated. In this paper, Part II of two companion papers, the results of the single-assembly analyses of Part I are developed to investigate full-core feasibility. A large reduction in moderation is necessary to allow full actinide recycle. This increases the core pressure drop, which poses some thermal-hydraulic challenges, which are more pronounced if the design implementation is through retrofitting an existing PWR. For a given reactor cooling pump, the core flow rate is reduced. Despite this, it is possible to achieve feasible inlet and outlet temperatures and minimum departure from nucleate boiling ratio, for the reduction in moderation considered here. Reflood after loss-of-coolant accident is expected to be slower, which may lead to unacceptable peak clad temperatures and/or clad oxidation. Equilibrium cycles are presented for the RMPWR and RBWR, with a negative MTC and VC. However, the RMPWR may have positive reactivity when fully voided, and the hard spectrum makes it difficult to achieve an adequate shutdown margin, such that for the considered fuel designs, additional rod banks would be required.