Pressure Locking and Thermal Binding (PL/TB) are two different but related physical phenomena, which under certain conditions may prevent the opening of some types of valves.
According to the US Nuclear Regulatory Commission (NRC) NPP operators should evaluate all safety.related power operated gate valves in all operational configurations for susceptibility to PL/TB. Corrective actions are required for susceptible valves that shall assure performance of safety function within plant licensing bases.
NEK (NEK) first addressed this issue through the analysis of the Motor Operated Valves (MOV) that were included in the NEK MOV program. This approach resulted with 31 valves in Krško NPP being found as susceptible to TB, 21 of which to both TB and PL. Subsequently, a more thorough analysis was performed, which took into account realistic operational parameters and detailed deterministic and probabilistic assessment evaluation of accident scenarios. This produced a list of 8 valves susceptible to PL, and another 8 susceptible to TB.
Valves were screened according to their safety significance, safety function, type, operational history, and operational conditions. Where applicable, Probabilistic Safety Analysis (PSA) has been used as a tool to screen the risk from PL/TB to the safe plant operation. To reduce the risk from PL/TB occurrence, either physical modification of the valve (alteration of actuator gear ratio to provide more force), relevant part of the entire fluid system (flooding of sump suction lines), or simulation and testing to confirm actual ability to overcome increased forces (initiating a controlled TB condition, and testing to confirm actuator’s ability to perform), was used.
Methods used and results produced for TB are the subject of this paper.