United States Nuclear Regulatory Commission - Protecting People and the Environment

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 
.

                                                           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. 

Page Last Reviewed/Updated Tuesday, November 12, 2013