Water Relief Through Safety or Relief Valves is not Just a Leak

Abstract

Two Nuclear Safety Advisory Letters (NSALs), advise operators of PWRs that they may assume, in accident analyses, that certain safety and relief valves can open, relieve water, and then reseat properly. The NSALs claim water that exits the reactor coolant system (RCS) can be made up by water that is delivered into the RCS by the emergency core coolant system (ECCS). For example, during an inadvertent operation of the ECCS (IOECCS) event, The NSALs claim, “since the cause of the water relief is the ECCS flow, the magnitude of the leak will be less than or equivalent to that of the ECCS (i.e., operation of the ECCS maintains RCS inventory during the postulated event and establishes the magnitude of the subject leak).” A comparison of ECCS flow to water relief, at relevant RCS pressures, indicates that water relief, during an IOECCS, cannot be dismissed as a leak. Critical flow calculations indicate that ECCS flow cannot replace RCS inventory that is relieved, as water, through the pressurizer relief and safety valves when RCS pressures are near nominal operating levels. At much lower pressures, the ECCS could offset the water relief. However, by then the IOECCS will have been either resolved, or will have developed into a loss of coolant accident (LOCA). The NSALs were published in 1993 and 2007. NRC did not question the NSALs’ advice until 2015, when it was found in a licensee’s application for a power uprating. The licensee might have submitted a false statement, in support of its application.