Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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.
