Development of a Flow Network Calculation Code (FNCC) for High Temperature Gas-Cooled Reactors (HTGRs)

Abstract A core of the prismatic high temperature gas-cooled reactor (HTGR) consists of fuel blocks, control rod guide blocks and replaceable reflector blocks with surrounding permanent reflector blocks. The core flowrate available for effective cooling of fuels, fuel blocks and control rod guide blocks has to be evaluated to confirm that maximum fuel temperature satisfies fuel design limits. In the evaluation, unintended flows such as gap flows between columns, cross flows between column layers and gap flows between permanent reflectors should be analyzed for the prismatic-type HTGR design. The flow distribution has to be evaluated preciously with minimizing the unintended flows. In the HTGR design, flow distribution analysis has to take into account the interaction of the gaps between the graphite structures forming unintended flow paths and flow conditions such as coolant temperature and pressure by thermal and radiation deformations of the graphite structure. In the present study, a user-friendly flow network calculation code (FNCC) has been developed on the basis of experiences of High Temperature engineering Test Reactor (HTTR) design for HTGR design with enhanced compatibility with other HTGR design codes and with considering graphite block deformation in iteration process without manual control. The code is able to solve the flow distribution based on conservation equations on the mass, momentum and energy by converging iteration calculations along with update of gap information in each calculation step. The validation result using FNCC provides good agreement with a result of the one-column flow distribution experiment.