Safety significance of inadvertent operation of motor operated valves in nuclear power plants

This report addresses concerns about the consequences of valve mispositioning which were brought to the forefront following an event at Davis Besse in 1985 (NRC, 1985a). The concern related to the ability to reposition ''position changeable'' motor operated valves (MOVs) in the event of their inadvertent operation from the control room and was documented in Nuclear Regulatory Commission (NRC) Bulletin 85-03 (NRC, 1985b) and Generic Letter (GL) 89-10 (NRC, 1989). The mispositioned MOVs may not be able to be returned to their required position due to high differential pressure (dP) or high flow conditions across the valves. The inability to reposition such valves may have significant safety consequences as in the Davis Besse event. However, full consideration of such mispositioning in safety analyses and in MOV test programs can be labor intensive and expensive. Industry raised concerns that consideration of position changeable valves under GL 89-10 would not decrease the probability of core damage to an extent which would justify licensee costs. As a response, Brookhaven National Laboratory (BNL) has conducted separate scoping studies for both Boiling Water Reactors (BWRS) and Pressurized Water Reactors (PWRs) using Probabilistic Risk Assessment (PRA) techniques to determine if such valve mispositioning by itself is significant to safety. The approach utilized internal events PRA models to survey the order of magnitude of the risk significance of valve mispositioning by considering the failure of selected position changeable MOVS. The change in core damage frequency (CDF) was determined for each valve considered and the results were presented as a risk increase ratio for each of four assumed MOV failure rates. The risk increase ratios resulting from this failure rate sensitivity study can be used as a basis for a judgement determination of the risk significance of the MOV mispositioning issue for BWRs and PWRS