Reactor physics considerations for implementing silicon carbide cladding into a PWR environment

Abstract

Silicon carbide (SiC) offers several advantages over zirconium (Zr)-based alloys as a potential cladding material for Pressurized Water Reactors: very slow corrosion rate, ability to withstand much higher temperature with little reaction with steam, and more favorable neutron absorption. To evaluate the feasibility of longer fuel cycles and higher power density in SiC clad fuel, a core design study was completed with uranium dioxide fuel and SiC cladding in a standard, Westinghouse 4-loop PWR. NRC-limited values for hot channel and hot spot values were taken into account as well as acceptable values for the reactivity feedback and control mechanisms and shutdown margin. The Studsvik Core Management System, which consisted of CASMO-4E, CMS-Link, and SIMULATE-3, provided an accurate tool to design the new core loading patterns that would satisfy current nuclear industry standards. Libraries of Westinghouse robust fuel assemblies (RFAs) were modeled in CASMO-4E with varying enrichments, burnable poison layouts, and power conditions. Using these assemblies, full core, three-dimensional analyses were performed in SIMULATE-3 for operating conditions similar to the Seabrook Nuclear Power Station. In this study, SiC-clad fuel rods held 10% less heavy metal to allow for central holes in the U0 2 pellets, limiting peak fuel temperature during anticipated operational transients but raising the average enrichment per fuel batch. The cladding dimensions remained similar to the current Zircaloy 4 cladding. Three approaches were followed in creating the PWR core designs: 1) constant core power density (or total reactor power) and cycle length, but fewer fresh assemblies loaded, 2) constant cycle length, but increased core power density to the maximum feasible level, staying within the capability of the reactor etc., and 3) constant power density, but extended fuel cycle length from 18 to 24 months. Sixteen core designs were completed with three different types of burnable poison (IFBA, WABA, and gadolinium) that achieved the desired operating cycle lengths and target values for reactor physics parameters limited by the NRC. Batch average discharge burnups ranged from ~41 to ~80 MWd/kgU, reinforcing SiC's advantage and potential appeal to power utilities. Additionally, a power uprate of 10% was found to be feasible, but beyond this value would require a redesign of the control rod material and/or layout to allow for an acceptable shutdown margin by end of cycle (EOC). Nevertheless, all other reactivity coefficients and safety margins were met, confirming the feasibility of operating to higher burnups beyond the current limits of Zr cladding.