Temperature Distributions in LMR Fuel Pin Bundles as Modeled by COBRA‐IV‐I

Most pin type reactor designs for space power or terrestrial applications group the fuel pins into a number of relatively large fuel pin bundles or subassemblies. Fuel bundles for terrestrial liquid metal fast breeders reactors typically use 217 – 271 pins per sub‐assembly, while some SP100 designs use up to 331 pins in a central subassembly that was surrounded by partial assemblies. Because thermal creep is exponentially related to temperature, small changes in fuel pin cladding temperature can make large differences in the lifetime in a high temperature liquid metal reactor (LMR). This paper uses the COBRA‐IV‐I computer code to determine the temperature distribution within LMR fuel bundles. COBRA‐IV‐I uses the sub‐channel analysis approach to determine the enthalpy (or temperature) and flow distribution in rod bundles for both steady‐state and transient conditions. The COBRA code runs in only a few seconds and has been benchmarked and tested extensively over a wide range of flow conditions. In this report the flow and temperature distributions for two types of lithium cooled space reactor core designs were calculated. One design uses a very tight fuel pin packing that has a pitch to diameter ratio of 1.05 (small wire wrap with a diameter of 392 μm) as proposed in SP100. The other design uses a larger pitch to diameter ratio of 1.09 with a larger more conventional sized wire wrap diameter of 1 mm. The results of the COBRA pin bundle calculations show that the larger pitch‐to‐diameter fuel bundle designs are more tolerant to local flow blockages, and in addition they are less sensitive to mal‐flow distributions that occur near the edges of the subassembly.