Fuel Behavior Implications of Reactor Design Choices in Pressurized Water SMRs

Abstract

Small pressurized water reactors can feature boron-free operation, natural circulation mode, reduced-height assemblies, and/or long refueling cycles. This paper attempts to explore core design optimization for each of these design evolutions. In consequence, five core design layouts are developed incorporating boron-free operation with continuous control rod insertion, natural circulation with low burnup/low power density design, natural circulation with high burnup/low power density design, forced circulation with standard core power density design, and forced circulation with high power density design. These cores’ performance is compared to a standard four-loop pressurized water reactor. The design process aims to improve the fuel cycle cost under safety constraints through core design optimization using the CASMO4E/SIMULATE3 reactor physics codes and the FRAPCON4.1 fuel performance assessment tool. Core modeling assumes standard 17×17 PWR fuel assemblies loaded with low enriched uranium up to 5 wt% or low enriched uranium plus (i.e. below 10 wt% enrichment) pellets with gadolinium oxide as the burnable poison. Satisfactory core and fuel performances are obtained for all the designed cores under steady state and considered overpower transients. For low power density operation, long cycle lengths are achieved reaching 2.5-year and 5-year cycles, and peak rod-average burnup is pushed to 83 MWd/kgU. Other cycle lengths are maintained at 18 months. Boron-free operation exhibits the ability to achieve longer cycle lengths at the cost of higher peaking factors leading to high local power and fuel temperatures, which prevents sizable power uprates and is deemed uneconomical. Fuel assembly height reduction allows coolant velocity retrofit, which enables higher core power density without violating the structural integrity of the fuel assembly. As a result, a core power density of 123 kW/L is reached where total cladding hoop strain becomes the limiting parameter.