Information Notice No. 82-45: PWR Low Temperature Overpressure Protection
SSINS NO. 6835
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, DC 20555
November 19, 1982
Information Notice No. 82-45: PWR LOW TEMPERATURE OVERPRESSURE PROTECTION
All pressurized water reactors (PWRs) holding an operating license (OL) or
construction permit (CP).
This notice is to provide PWR licensees and construction permit holders with
pertinent information related to operation of the low temperature
overpressure protection system. This information updates that presented in
Information Notice No. 82-17 (June 10, 1982) and reflects the initial
findings of NRC staff review of operating experience, Licensee Event
Reports, and technical specifications related to low temperature
overpressure protection. NRC staff review of this issue is continuing.
Recipients of this information notice should review the information herein
for applicability to their facilities. No specific action or response is
required at this time.
Description of Circumstances:
In August of 1976, the issue of low temperature overpressure protection was
raised and licensees initiated procedures and proposed systems to mitigate
postulated overpressure events. The main concern was with the low
temperature modes of cooldown and heatup, during which overpressurization
could cause brittle fracture of the reactor vessel. In most cases, licenses
proposed a manually enabled low pressure setpoint on the existing
pressurizer power-operated relief valves (PORVs) supplemented by procedures
and technical specifications.
The low temperature overpressure events at Turkey Point Unit 4, on November
28 and 29, 1981 have been designated by the Commission as abnormal
occurrences. These events were described in Information Notice No. 82-17.
The events were caused by failure of the backup train of the low temperature
overpressure protection system (LtopS) because of inadequate surveillance
and valve lineup procedures. Following the Turkey Point events,
investigation of the contributing factors led to a review of the Turkey
Point Unit 4 LtopS surveillance procedures which showed that the
surveillance requirement did not include a test of the complete instrument
Staff review of LERs indicates that no overpressure events similar to those
at Turkey Point have occurred at operating PWRs since 1978. However, events
have occurred in which both trains of LtopS have been inoperable
November 19, 1982
Page 2 of 3
apparently from common cause factors. The following causes have each
resulted in both LtopS trains being inoperable at the same time.
1. Operation with both PORVs isolated (block valves closed) because of
known PORV leakage.
On June 12 and again on June 18, 1981 at the Salem 2 plant, the PORV
block valves were closed because of leaking LtopS PORVs, thus rendering
both, trains inoperable. Also, on December 12, 1978 at the Ft. Calhoun
plant, during plant heatup, a technician troubleshooting the failure of
one train of LtopS pulled fuses which caused both PORVs to open. To
stop the discharge, both PORV block valves were closed, disabling the
LtopS. The PORVs were returned to service within 15 minutes.
2. Operator error during maintenance.
On May 21, 1981 at the Surry 2 plant, one train of LtopS was inoperable
because of a wiring error while the isolation valve for the pressure
transmitter for the second train of LtopS was closed. Also, on May 6,
1980 at the Ginna plant, during post-installation test of the reactor
vessel head vent, DC power switches for both trains of LtopS were found
in the off position.
3. Isolation and venting of instrument air to the PORV actuators during,
integrated leak rate testing. (ILRT)
On June 18, 1980 at the Zion 2 plant, the accumulators for both PORVs
were vented and the instrument air source was isolated, rendering both
trains of LtopS inoperable. To prevent recurrence, a procedure change
was made to block both PORVS open during the ILRT. Also, on May 27,
1980 at the Surry 2 plant the LtopS was inoperable due to ILRT.
4. Low nitrogen pressure to both PORV actuators.
On numerous occasions at North Anna 1 and 2, leakage in the backup
nitrogen supply to the PORVS degraded the nitrogen supply pressure and
rendered the LtopS inoperable.
The events involving low nitrogen pressure were caused by excessive leakage,
from the pneumatic system coupled with a limited supply of bottled nitrogen.
Although, these events occurred in a LtopS where the backup air supply is
bottled nitrogen, the events could have direct applicability to those
systems which employ air accumulators to provide opening force for the PORVS
in, case of loss of air. Because these air-operated systems are normally
continuously supplied from the plant air compressors, even when in shutdown,
the lack of effectiveness of the pneumatic system and the air accumulators
may not be discovered unless the plant experiences a loss-of-air event or
unless the normal air supply to the accumulators is deliberately interrupted
to perform an operability check. In these cases, periodic inspection or
surveillance may be needed to detect excessive leakage and to ensure
operability of the backup pneumatic supply.
November 19, 1982
Page 3 of 3
In addition to instances in which both LtopS trains were found to be
inoperable, some LtopS may have been in a degraded condition as a result of
failure to update the LtopS setpoints to correspond to changes in the
Appendix G temperature pressure limits. This condition was found at both
Kewaunee and Turkey Point.
If you have any questions regarding these matters, please contact the
administrator of the appropriate Regional Office or this office.
Edward L. Jordan, Director
Division of Engineering and
Office Inspection and Enforcement
Technical Contact: George F. Lanik, IE
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