Performance Assessment of the Two-Phase Pump Degradation Model in the RELAP5-3D Transient Safety Analysis Code

摘要

Two-phase pump performance in a nuclear reactor plant directly affects the ability of the system to provide adequate core cooling during the blowdown phase of a Loss-Of-Coolant Accident (LOCA). Under such conditions the system depressurizes causing the coolant to flash into a two-phase steam/water mixture. This leads to significant degradation of reactor coolant pump performance. Therefore, in order to obtain an accurate representation of the resulting core flow and fuel/cladding temperatures for these scenarios it is important that safety analysis codes, such as RELAP5-3D, accurately model reactor coolant pump two-phase behavior. However, this degradation behavior has been shown to be highly pump specific and difficult to characterize. The RELAP5-3D transient safety analysis code currently calculates two-phase pump performance using an empirical model developed by the Aerojet Nuclear Corporation (ANC). This model relates two-phase pump performance to single-phase pump performance using two-phase degradation multipliers, which can only be a function of void fraction. The most widely-adopted set of degradation multipliers were developed using experimental data collected from the radial pump used in the Semiscale testing program. However, other work has shown that radial pumps degrade more severely under two-phase conditions and therefore the predictions associated with this data do not accurately represent the degradation behavior of a typical reactor coolant pump.