Information Notice No. 80-40:Excessive Nitrogen Supply Pressure Actuates Safety-Relief Valve Operation to Cause Reactor Depressurization
SSINS No.: 6835
Accession No.:
8008220269
IN 80-40
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
November 7, 1980
Information Notice No. No.: 80-40: EXCESSIVE NITROGEN SUPPLY PRESSURE
ACTUATES SAFETY-RELIEF VALVE OPERATION TO
CAUSE REACTOR DEPRESSURIZATION
Description of Circumstances:
On October 7 and 31, 1980, the reactor coolant system was spuriously
depressurized at Boston Edison Company's Pilgrim Nuclear Power Station Unit
No. 1. The Pilgrim Station Unit 1 uses a GE BWR. The depressurizations
resulted when excessive pressure in the nitrogen supply system caused the
"A" Target Rock (TR) safety-relief valve to open and remain open until the
excessive supply pressure could be isolated, pneumatic operator pressure
vented and the main steam system depressurized sufficiently to allow the
main disk to reseat. These two events involved a failure in the pressure
regulation of the supply nitrogen and not a failure of the TR safety-relief
valve to function as designed.
The safety-relief valves at Pilgrim are designed to be supplied, with other
drywell instrumentation, from either the compressed air system or
containment atmosphere control system (CACS). The CACS uses nitrogen for
containment inerting. The supply for the safety-relief valves is provided
from the CACS through an ambient air vaporizer and then through one of two
parallel pressure regulators or a parallel bypass line. Shortly before the
time of each event a new supply of liquid nitrogen had been added to the
storage tanks. At the time of the events the two pressure regulators were in
service with the bypass closed. Nitrogen pressure supplying the valves
increased to 160 - 165 psi. This may have been caused by liquid nitrogen
reaching the pressure regulators or by a failure in a pressure regulator.
The result was excessive pressure which was sufficient to leak through the
solenoid actuator and initiate the pneumatic operator of the safety-relief
valve.
The design normal operating pressure of the compressed air or nitrogen
systems supplying the safety-relief valves is 90-110 psi. At a pressure of
145 psi the solenoid valve may begin to leak since excessive pressure acts
to unseat the disk. The supply pressure must then decrease to 135 psi or
less for the solenoid disk to reseat. In addition, the design of the
safety-relief valve is such that as the main steam pressure increases, less
instrument pressure is necessary to initiate the pneumatic operator.
Approximately 3 to 5 psi at the pneumatic operator is sufficient to initiate
the safety-relief valve opening. Such a pressure begins to build with
leakage through the solenoid actuator and was reached in the "A" valve at
160 psi supply pressure. According to information from GE and TRC,
approximately 180 psi pressure is necessary for all of the safety-relief
valves to open as result of supply overpressure. It would appear that under
such conditions of overpressure that safety-relief valve openings would be
sequential rather than simultaneous.
The particular solenoid actuator valves used with these two-stage safety-
relief valves are manufactured by TRC. Their design is such that excessive
control pressure tends to unseat the solenoid valve disk. The three-stage TR
safety-relief valves use either AVCO (Automatic Valve Company) or ASCO
(Automatic Switch Company) solenoid valves according to information from GE.
The AVCo solenoid
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IN 80-40
November 7, 1980
Page 2 of 2
valve has been tested to 300 psi and found not to leak and the ASCo solenoid
valve tends to seat with increasing supply pressure. Therefore, the NRC
believes that the tendency for solenoid leakage and hence safety-relief
valve opening is confined to the two-stage safety-relief valve
installations.
This information is provided as a notification of a possibly significant
matter which is still under review by the NRC staff. It is anticipated that
the results of continuing NRC review will culminate in issuance of an IE
Bulletin which will recommend or require specific licensee action. In the
interim, we expect that recipients will review the information for possible
applicability to their facilities, particularly those with installations of
the TR two-stage safety-relief valves. If you have questions regarding this
matter, please contact the Director of the appropriate NRC Regional Office.
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