RELAP5 MOD3.2 modification and application to the transient analysis of a fluoride-salt-cooled high-temperature reactor

Highlights: • The thermo-physical and transport properties of liquid and vapor fluoride salts are implemented into RELAP5 MOD3.2 code. • The modification extends the application of the RELAP5 code to transient analysis of the FHRs. • Two basic transient conditions are investigated on the MK1 PB-FHR by the modified RELAP5 code. - Abstract: The Fluoride-salt-cooled High temperature Reactors (FHRs) are an advanced concept using a novel combination of high-temperature coated-particle fuel, low-pressure fluoride-salt coolant and air-Brayton power conversion system. RELAP5, developed for the best-estimate transient simulation of light water reactors, is still recommended to perform safety analysis of FHRs during postulated accidents. Thermo-physical properties and transport properties of liquid and vapor fluoride salts are implemented into the RELAP5 MOD3.2 source code, through which the application of the RELAP5 MOD3.2 can be extended from the water-cooled reactors to the FHRs. Specific heat transfer correlations are also incorporated into the code for this type of reactors. A RELAP5 model is developed for the MK1 Pebble Bed Fluoride-Salt-Cooled High-Temperature Reactor (MK1 PB-FHR). Using the modified code, two basic postulated transients including the unprotected loss of forced circulation (ULOFC) and unprotected transient overpower (UTOP), are investigated to explore the safety characteristics of the MK1 PB-FHR. Results show that the core outlet temperature may exceed its limit in the ULOFC caused by two main pumps trip. It can be also concluded that the passive residual heat removal system can mitigate the consequences of the ULOFC. Results of the sensitivity analysis of the UTOP indicate that the reactor power and core temperatures show a linear increment trend versus the positive reactivity insertion.