The contribution of control systems in LWR safety

The objective of this paper is to estimate the effects of non-safety-grade control systems in light water reactor (LWRs) on the overall likelihood of accidents leading to severe core damage or core melt, as determined from operating experience. One hundred ninety operational events which involve failure of non-safety-grade control systems that have ovvurred at commercial PWR plants during 1969-1981 and which are considered to be precursors to potential severe core damage have been identified; eighty such evens have been identified for /bWR plants. These events are treated as initiating or concurrent events to be fit into appropriate event trees, which are then quantified using component failure rates and system unavailabilities taken from available PRA studies to estimate the frequency of severe core damage arising from non-safety-grade control system failures. An example is worked out in detail. Considerable uncertainty is introduced into the results by the choice of system and operator failure rates and the results reported herein are considered preliminary. No allowance is made in these estimates for plant changes made because of operational experience. (orig.)