United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 82-17: Overpressurization of Reactor Coolant System

                                                            SSINS No.: 6835 
                                                            IN 82-17 

                               UNITED STATES 
                       NUCLEAR REGULATORY COMMISSION 
                    OFFICE OF INSPECTION AND ENFORCEMENT 
                          WASHINGTON, D. C. 20555 

                               June 11, 1982 

Information Notice No. 82-17:   OVERPRESSURIZATION OF REACTOR COOLANT 
                                   SYSTEM 

Addressees: 

All nuclear power reactor facilities holding an operating license (OL) or 
construction permit (CP). 

Purpose: 

This information notice is provided as a notification of two events that may
have safety significance. It is expected that recipients will review the 
information for applicability to their facilities. No specific action or 
response is required. 

Description of Circumstances: 

On November 30, 1981, Florida Power and Light .Company r .@ported that the 
Turkey Point Unit 4 reactor coolant system (RCS) was overpressurized on 
November,28 and 29 during startup following a refueling outage. The reactor 
was shut down and the RCS was in a water solid condition with a pressure and
temperature of approximately 310 psig and 1100F, respectively. Two separate 
transients that resulted in overpressure conditions of 1100 and 750 psig at 
1100F occurred for which the overpressure mitigating system-(OMS) failed to 
operate. These events exceeded the pressure limit of 480 psig at 1100F 
specified in Technical Specifications which prescribe the allowable pressure
and temperature limits to prevent reactor vessel brittle fracture. 

The OMS is specifically designed to prevent this type of overpressurization.
The OMS did not operate as designed because: 

1.   After the first event a pressure transmitter isolation valve was found 
     closed and was opened. This transmitter provides input into the OMS 
     circuit to automatically open a power operated relief valve (PORV) if 
     the reactor coolant system exceeds the allowable pressure for RCS 
     temperature; 

2.   The summator failed in the electrical circuitry which prescribes the 
     pressure at which the OMS is to initiate PORV actuation. The failed 
     summator was identified and corrected after the second event. The OMS 
     surveillance procedure in use before the event did not include testing 
     the summator, in that the test signal bypassed the summator; and  

3.   The redundant OMS circuit was out of service for calibration. 


8204210383  
.

                                                           IN 82-17 
                                                           June 11, 1982  
                                                           Page 2 of 3 

Before each event the reactor coolant system inventory was being maintained 
by charging from the chemical and volume control system and letdown through 
the residual heat removal (RHR) system. Each event was initiated with a 
pressure spike caused by the start of a reactor coolant pump which resulted 
in isolation of letdown by automatic closure of the RHR system isolation 
valve. During both occurrences, the operator took immediate action to stop 
the charging pump which was providing the source of rapid pressurization. 
Within two minutes the operator decreased pressure to the desired level by 
manually opening the PORV and securing the pressurizer heaters in addition 
to securing the charging flow. Timely operator action to completely prevent 
the overpressurization was precluded by the rapidity of the transient. 

Following the events, OMS surveillance procedures were revised to include 
testing of the summator. Other procedural changes include additional 
equipment checks to ensure OMS operability. Westinghouse performed a 
fracture mechanics analysis based on the method of Appendix G, Section III 
of the ASME Boiler and Pressure Vessel Code. The analysis showed that these 
transients had neither impaired the integrity of the reactor vessel, nor 
significantly affected the fatigue life of the vessel. A Florida Power and 
Light Company consultant reviewed the analysis and concurred with the 
Westinghouse conclusion. 

In a separate event, on May 23, 1982 Virginia Electric Power Company (VEPCO)
reported that the overpressure protection system (OPS) at North Anna was 
inoperable from May 19-22. The OPS had not been called upon to operate 
during this time. The reactor was in cold shutdown and for two days the 
reactor coolant system was in the water solid condition. Initially, one OPS 
system was declared inoperable when the pressure in the "A" nitrogen supply,
reservoir dropped below the minimum pressure required to maintain the PORV 
operable. Two days later, a low pressure alarm occurred on the "B" nitrogen 
supply reservoir. An isolation valve between the reservoir and the "B" OPS 
system had been closed for an indeterminant period (possibly as long as 
eight days), isolating the nitrogen supply to the "B" PORV. Therefore both 
PORVs were inoperable. Initial investigation discovered that system 
procedure did not include OPS valve lineups, and the incorrectly positioned 
valve was not shown on plant drawings in use at that time. VEPCO is 
presently taking action to correct these problems. 

Each of the above events involved failure of two redundant systems designed 
to provide overpressure protection. The concern is that without prompt 
operator action such failures increase the potential for brittle fracture of
the reactor pressure vessel from overstress during pressure transients. 
.

                                                           IN 82-17 
                                                           June 11, 1982  
                                                           Page 3 of 3 


If you have any questions regarding this matter, please contact the Regional
Administrator of the appropriate NRC Regional Office, or this office. 



                                   Edward L. Jordan, Director 
                                   Division of Engineering and  
                                     Quality Assurance 
                                   Office of Inspection and Enforcement 

Technical Contact:  R. A. Holland 
                    301-492-4791 

                    W. R. Mills 
                    301-492-4791 

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