Pressure locking and thermal binding gate valves

The nuclear industry has been aware of the potential for valve inoperability caused by pressure locking and thermal binding for many years. Pressure locking or thermal binding is a common-cause failure mechanism that can prevent gate valve opening on demand, and could render redundant trains of safety systems, or multiple safety systems, inoperable. In spite of numerous generic communications issued by the U.S. Nuclear Regulatory Commission (USNRC) and industry, gate valves in nuclear plant safety-related systems continue to experience pressure locking and thermal binding. USNRC investigations of gate valve pressure locking and thermal binding events and surveys of licensee evaluations and corrective measures concerning the locking mechanisms indicate that the valve locking is a result of inadequate design consideration under specific system and plant operating conditions. Most valve binding events occurred during infrequent plant evolutions, system transients, or unusual system alignments. Hence, the inadequate design would not necessarily be discovered during plant startup testing or regular surveillance testing. The USNRC staff described the results of its analysis in NUREG-1275, Volume 9. A subsequent survey has found that (a) most licensees had made multiple attempts to address the issue with limited implementation of industry guidance to prevent or correct the problem, (b) three incorrect assumptions were used that prevent licensees from identifying valves susceptible to pressure locking or thermal binding, (c) low-pressure emergency core cooling system valves (low-pressure coolant injection, low-pressure core spray, residual heat removal, and pressurized water reactor containment sump sunction) appear most susceptible and risk significant, and (d) a programmatic approach involving system, operations, and component personnel is needed in order to identify the potential for valve locking