Benchmark analyses for EBR-II shutdown heat removal tests SHRT-17 and SHRT-45R - (2) subchannel analysis of instrumented fuel subassembly

Two kinds of Loss of Flow experiments conducted in EBR-II are the subject of an IAEA benchmark exercise. The present work discusses the subchannel analysis in an instrumented subassembly, analyzed with the COBRA-IV-I code. Boundary conditions are provided by a 1D thermal-hydraulic system code (NETFLOW++) for a protected loss-of-flow and an unprotected-loss-of-flow tests conducted at the EBR-II reactor. The instrumented subassembly with 61 pins is installed in the 5th row of the reactor core in order to measure the temperature profile across the mid-plane and the top region of the fuel subassembly. The pin lattice in the subassembly consists of 126 subchannels. Calculated temperature profiles at three levels are compared with the measured results. Biased temperature profiles with respect to the center line of the subassembly measured in the experiments are appropriately simulated by the code. The presence of the spacer wire by itself introduces a biased temperature profile. The biased temperature profile is also shown by a CFD calculation using a 7-pin partial model of the instrumented subassembly. In order to obtain close agreement with measured temperatures, the power profile within the subassembly must be taken into account. Temperature evolutions during the transients are simulated by the subchannel code.