Optimum core design studies for long-life small modular boiling water reactors

Faisal Yousef Odeh, Purdue University

Abstract

A BWR-based SMR called the Novel Modular Reactor (NMR-50) is being developed at Purdue University. NMR-50 takes the advantages of the two-phase flow driving head, which allows a much smaller and simpler reactor pressure vessel compared to the integral PWRs. The NMR-50 has a long-life core design and a net electrical power output of 50 MWe. In this work, optimum core design options pertaining to the application of a long-life small modular BWR are studied. The primary core design objective is to meet a 10-year cycle length with a minimum fuel cost while satisfying safety related criteria. The lattice code CASMO-4, the whole core analysis code PARCS and the thermal-hydraulics code RELAP5 were used to perform calculations from pin cell level up to whole core depletion calculations. The first core design option for the NMR-50 that was analyzed and optimized was based on the existing industry standards and constraints in order to promote near-term deployment. Through a systematic step-wise optimization approach including a simulated annealing based optimization method, an optimum core design that meets the core design objective was derived and analyzed. The optimized NMR-50 foundational core design is able to achieve a 10.2 year cycle length with an average fuel enrichment of 4.61 wt% of 235U in a 10x10 lattice fuel assembly. The minimum critical power ratio (MCPR) and the maximum fuel linear power density (MFLPD) during the cycle are 1.99 and 18.25 kW/m, respectively, providing large margins to thermal design constraints. The NMR-50 control system design is able to provide a sufficient cold shutdown margin of 1.7 %. With its small reactor core size, large negative void coefficient, and low operating thermal neutron flux, an enhanced xenon stability characteristic is possible. Peak fast neutron fluence of 8.8x1021 n/cm 2 was below the industry standard limit, which from extensive plant data records, should not be a major concern to channel distortions from a radiation damage point of view. An evolutionary fuel assembly design concept based on partial segregation of fertile material was developed as a mean of extending fuel cycle length. The additional degree of freedom in the heterogeneous fuel assembly design was thoroughly analyzed through the utilization of the fissile material and the conversion efficiency of the fertile material. Two different designs for a heterogeneous fertile material loading (HFL) within an assembly, one based on the ThO2 and UO2 once-through fuel cycle and the other based solely on existing UO2 once-through fuel cycle were assessed. The HFL-thorium/ uranium based and the HFL-uranium based assembly design options resulted in a 20% and an 18% increase in cycle length when compared to the foundational NMR-50 design option. The reactivity feedback coefficients were almost maintained at the same level at the beginning and the end of cycle for all design options. The MFLPD of the HFL-Th/DU and the HFL-DU were 35 kW/m and 31 kW/m, respectively, indicating a significant margin to the imposed thermal design limit of 45 kW/m. The MCPR for the HFL-Th/DU and the HFL-DU were 1.95 and 1.98, respectively, for the most limiting conditions, indicating sufficient margin to the critical power ratio limit of 1.3. The core-average plutonium production rate was reduced by 20% and its content was degraded in the HFL design. Importantly, the levelized fuel cycle unit cost was reduced by approximately 20% in the HFL assembly options when compared to its reference NMR-50 design counterpart.

Degree

M.S.

Advisors

Yang, Purdue University.

Subject Area

Design|Nuclear engineering

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