Accurate representation of primary coolant system depressurization for high flux isotope reactor transients

摘要

Because the High Flux Isotope Reactor primary coolant system is typically all liquid, the pressure falls very rapidly if a leak forms in the primary piping. This depressurization is the predominate phenomenon in the consideration of the loss-of-coolant-accident analysis that has been completed recently for the High Flux Isotope Reactor Safety Analysis Report. Small differences in the rate of depressurization can significantly affect the safety margin. A RELAP5 thermal-hydraulic input model has been developed, but the capabilities of the RELAP5 code do not automatically take into account the effect on the system pressure of a stretching or shrinkage in the pressure boundary. Because this change in the pressure boundary is so important in an all-liquid system, a scheme has been developed to account for the effect implementation of the structural elasticity model involved using the control variable capability of RELAP5. During simulated transients involving rapid pressure changes, mass is added to or taken away from the primary system depending on whether the system is decreasing or increasing in pressure. By doing so, a more realistic system response to transients involving significant pressure changes is obtained. The model has been used to perform various HFIR transient simulations including loss-of-coolant accidents (LOCAs), loss of offsite power, and loss-of-secondary-cooling transients. Results for a small break LOCA are presented with and without the elasticity model in place.