Publications Details
Safety Relief Valve cyclic failure analysis for use in discrete dynamic event trees
Safety Relief Valves (SRVs) are an important component of the safety case for a Light Water Reactor (LWR). The number and types of SRVs in LWRs vary from plant to plant, but they generally operate to perform the same safety function. During accidents in which the coolant pressurizes beyond a predetermined set-point, the SRV will open, releasing coolant from the primary system and into the containment. Once enough coolant has been released to lower the coolant pressure, the SRV will reset. This cycle will continue until the SRV fails, in either a "Failed to Open (FTO)" or "Failed To Close (FTC)" mode. These failures can be caused either through cyclic loading or as a result of thermalinduced stresses from the coolant passing through the valve. SRV failures can be important, because an SRV that has FTC will cause a small "Loss of Cooling Accident", which depressurizes the primary system. Alternatively, SRVs that have FTO will allow system pressure to rise until it reaches the next SRV set-point. If the pressure is not reduced through the successful operation of other safety systems, either creep rupture elsewhere in the system, such as in the steam line, or high-pressure core damage may occur. While some SRV failure data is recorded in NUREG/CR-6928, the spread of the epistemic uncertainty distributions for FTO and FTC are wide. These large uncertainties may cause an analyst to be overconfident in the results of a severe accident simulation that uses only point-estimates calculations of FTO and FTC.