Simulation of a Simultaneous Closure of All Main Steam Isolation Valves Without Scram for a Boiling Water Reactor/4 Using RETRAN-01

Abstract

An analysis of a boiling water reactor/4 plant response to a simultaneous closing of all main steam isolation valves (MSIVs) followed by a failure of the reactor protection system scram function was performed using the RETRAN-01 computer code. The purpose of the analysis was to determine what power level the plant must operate at such that the vessel pressure does not exceed 1500 psig during the postulated transient. No credit was taken for an anticipated transient without scram recirculation pump trip. Analyses were performed both at 100% flow conditions and at reduced core flow conditions. The results of the analyses show that the 1500-psig vessel pressure criterion is met for operating conditions of ≤85% power.

Sensitivity studies were performed to identify important input parameters and modeling techniques. Steamline inertial effects, direct moderator heating fraction, feedback reactivity data, fuel rod gap conductance, MSIV closure rates, and upper downcomer modeling were investigated. The peak vessel pressure was found to be most sensitive to the value assumed for gap conductance and to the amount of liquid assumed to be in thermodynamic equilibrium with the vapor phase in the upper downcomer (region outside of the separators between the feedwater sparger and steam dryers). It is recommended that nonequilibrium effects in the upper downcomer region be investigated with models such as the RETRAN nonequilibrium pressurizer model.