Comparison of cobra-tf and vipre-01 against low flow code assessment problems

NuScale Power LLC (NuScale) is developing a Small Modular Reactor that relies on natural circulation to provide primary flow. NuScale, therefore, requires a subchannel analysis code that is applicable to its unique operating conditions, such as low coolant flow, in the reactor core. The primary purpose of this study is to compare the existing subchannel codes VIPRE-01 and COBRA-TF (CTF) to examine their relative strengths and weaknesses. A secondary purpose is to identify areas for code improvement at NuScale plant operating conditions. VIPRE-01 and CTF have a common lineage as both codes evolved from earlier COBRA code versions. VIPRE-01 is extensively used in the United States for safety analysis. CTF has the capability for two- fluid and void drift modeling. A representative set of experimental test cases (including well-known test models like GE 3x3', and etc.) were selected to focus on phenomena of specific interest to the NuScale reactor. Parameters such as channel flows, temperatures, pressures, void fractions, and ability to converge at low flow and pressure conditions were compared for several different power, flow, and exit quality conditions. Both the VIPRE-01 and CTF comparisons showed reasonable results for some cases while other cases indicated the need for additional investigation or model improvement. The comparative study has been useful in identifying the two codes relative strengths and weaknesses as well as providing direction for additional development. All benchmark activities have been conducted in cooperation with Zachry Nuclear Engineering, Inc. and Oak Ridge National Laboratory/CASL. References [1], R. Lahey et al, "Two-Phase Flow and Heat Transfer in Multirod Geometries: Subchannel and Pressure Drop Measurements in a Nine-Rod Bundle For Diabatic and Adiabatic Conditions," GEAP- 13049, General Electric Company, San Jose, CA, USA (1971).