United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No.80-12 – Instrument Failure Causes Opening of PORV and Block Valve

                                                           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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