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SSINS No.: 6870 Accession No.: 8002280654 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C. 20555 March 31, 1980 Information Notice No. No.: 80-12 INSTRUMENT FAILURE CAUSES OPENING OF PORV AND BLOCK VALVE Description of Circumstances: On August 13, 1979, during full, steady-state power operation at the Connecticut Yankee Haddam Neck Plant, the pressurizer power operated relief valve (PORV) and its isolation (block) valve opened as a result of failure of a light source in the sigma bistable of the pressurizer pressure controller. The failure of the light source was equivalent in control system response to an overpressure condition in the pressurizer. By system design both PORV and block valves are closed during normal operation and therefore both valves are required to open in response to an overpressure condition. The opening of the PORV and block valve allowed the pressurizer pressure to drop from its normal value of 2000 psig to 1950 psig before the PORV and block valves could be closed. The operator immediately overrode the open signal to the valves and closed the valves to stop depressurization. The pressurizer pressure then immediately returned to normal. The light source was subsequently replaced in the bistable and the pressure control system was returned to normal. The licensee reported that failures of light sources in the bistable of this pressure controller and in other similar plant bistables had been experienced previously. Also, subsequently on February 4, 1980, a spurious signal from the pressure controller caused both PORV and block valve to open. In both events, the operator responded immediately and effectively to limit the consequences of the event to a relatively mild pressure transient on the reactor coolant system. The licensee also indicated that it had changed out the type of bistable with a solid state design which does not depend on a light source and which is expected to be more reliable. This information is provided as notification of a possibly significant matter. It is expected that recipients will review the information for possible applicability to their facilities. No specific action or response is requested at this time. If you have questions regarding this matter, please contact the Director of the appropriate NRC Regional Office.
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