Improvement in the RELAP4 ECC bypass model

RELAP4 is a thermal-hydraulic code which predicts the transient response of a nuclear power plant subjected to postulated accidents such as a loss of coolant (LOCA). When loss of coolant occurs, the Emergency Core Cooling System (ECCS) will respond automatically and the cold water from accumulators and high and low head pumps is injected into the reactor core. The ECC bypass model currently used in RELAP4 is a conservative approach which removes the total amount of injected water (up to the end of bypass) from the reactor coolant system and replaces it with saturated steam. A new model, which is consistent with the current model yet gives a better prediction of ECC water distribution, is presented in this paper. This model, which calculates the distribution of ECC water in the reactor coolant system, only removes the amount remaining