Analysis of a BWR Turbine Trip Experiment by Entire Plant Simulation with Spatial Kinetics

Abstract

An analysis of a boiling water reactor turbine trip was performed with the THYDE-NEU code. In spatial kinetics, reactivity was not used since the three-dimensional transient diffusion equation was solved with the implicit direct integration method. The plant was treated as a closed coolant system, and hence, it was necessary to cope with thermal-hydraulic behaviors at pressures as low as the atmospheric pressure. At low pressures, nonlinearity of the thermal-hydraulic equation is enhanced, and hence, a thermal nonequilibrium model is required. To satisfy the measured initial pressure distribution within the reactor, it was necessary to have the moisture separator model and to account for a reversible pressure drop at a junction with a flow area change. Among the parameters in THYDE-NEU is γ in the thermal nonequilibrium model in addition to C₁ and C₂ regarding the manner in which to express the coolant density used in the table look-up of cross sections. For a pair of C₁ and C₂, it is possible to find parametrically a value of γ, namely, γ_C, so that THYDE-NEU can reproduce the experimental fact that the core-averaged local power range monitor output R_APRM reached 0.95 at 0.63 s to generate a scram signal. One of the calculations with γ_C was compared with the experiment. It was shown that the spatial kinetics results are sensitive to the temporal behavior of the bypass valve opening. Among the assumptions in use, those to be scrutinized before further performing sensitivity calculations were indicated.