Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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
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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.
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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
Attachment:
List of Recently Issued IE Information Notices
