MARS-GCR / CAPP Coupled Calculation of Reactivity Feedback Transients for the OECD/NEA PBMR-400 Benchmark

The OECD/NEA has lunched a benchmark problem program on a neutronics/ thermal-hydraulic (T-H) coupled calculation for the PBMR-400 to test the existing analysis methods for high temperature gascooled reactors (HTGRs) and to develop more accurate and efficient tools to analyze the neutronics and thermalhydraulic behaviour for the design and safety evaluations of the PBMR-400. There are three cases for the steady state phase (Phase I) and six cases for the transient state phase (Phase II) in the benchmark. In the previous works, by using the coupled code system with CAPP and MARS-GCR for a neutronics and T-H calculations respectively, we performed three steady state calculations which are the neutronics standalone, T-H standalone and neutronics / T-H coupled calculations that will be used as the initial condition of the cases in the transient state. Recently, however, the problem specification such as a graphite thermal conductivity has been modified by the OECD/NEA for a more realistic transient behaviour and so we have performed new calculations for two exercises in the steady state phase (T-H standalone and coupled steady) and the new coupled steady state result has been used for the initial condition of the transient problems. Among various transient cases, we have performed three selected transient cases, which are the reactivity insertions by a TCRW (total control rod withdrawal) and a TCRE (total control rod ejection), and a cold helium inflow transient