05000323/LER-2001-004
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| 3232001004R00 - NRC Website | |
I. Plant Conditions
Unit 2 has operated in various plant modes with the described condition.
II. Description of Problem
A. Background
Technical Specification (TS) 3.4.10, "Pressurizer Safety Valves," requires that three pressurizer safety valves (PSVs) shall be operable with a lift setting of greater than 2460 psig and less than 2510 psig corresponding to ambient conditions of the valve at nominal operating temperature and pressure.
Surveillance Test Procedure (STP) M-77, "Safety and Relief Valve Testing," verifies the PSVs lift setting in accordance with the requirements of the ASME Boiler and Pressure Vessel Code, Section Xl. The initial lift setting is evaluated for TS compliance. STP M-77 requires that the valves lift within the required tolerance in order to declare them operable.
STP M-77 test methodology obtains the as-found lift setting by placing the PSVs in an environmentally controlled enclosure and heating the ambient air to the temperature conditions typical at Diablo Canyon Power Plant (DCPP). The loop seal is also heated to simulate the piping temperature conditions at DCPP. Testing is accomplished by the addition of steam at a defined ramp rate. Steam is added until physical evidence of stem movement is visible on the remote data acquisition display screen. The data is then reviewed to ascertain "first discernible stem movement" and the pressure at which it took place.
B. Event Description
During the Unit 2 ninth refueling outage in 1999, all three PSVs were placed in service and declared operable without any additional adjustment of the lift settings until the PSVs were checked following removal in May 2001.
On June 26, 2001, two of the three Unit 2 PSVs were identified with lift setting outside TS 3.4.10 requirements. One PSV was found with a lift setting less than 2460 psig and one PSV was found with a lift setting of greater than 2510 psig. The valves were found to lift 3.4 percent low and 2.8 percent high, respectively during offsite testing.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION LER NUMBER (6) FACILITY NAME (1) DOCKET NUMBER (2) PAGE (3) Diablo Canyon Unit 2 0 PSV lift setting repeatability has been recognized as an industry-wide problem. PG&E has participated in extensive investigative test programs, both jointly with the Nuclear Steam Supply System vendor Westinghouse Owners Group, and independently. The results of the industry investigations are documented in WCAP-12910, "Pressurizer Safety Valve Set Pressure.
C. Inoperable Structures, Components, or Systems that Contributed to the
Event
None.
D. Other Systems or Secondary Functions Affected
None.
E. Method of Discovery
This condition was discovered by PG&E while performing a routine scheduled surveillance test in accordance with STP M-77.
F. Operator Actions
None.
G. Safety System Responses
None.
Ill. � Cause of the Problem
A. Immediate Cause
Two of three PSVs did not lift within the TS 3.4.10 tolerance.
B. Root Cause
The cause of the lift setting change has been determined to be the random lift setting spread.
C. Contributory Cause None.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) OF 3J2 1 2001 6 4 0 0 4 0 3 0 0 0 5 0 Diablo Canyon Unit 2 0 IV. A Analysis of the Event The limiting event for evaluating the lift setting is the loss of load analysis that requires the maximum reactor coolant system (RCS) pressure of 2750 psia not be exceeded. The RETRAN computer code model was run to determine if RCS pressure would exceed 110 percent of ASME design or 2750 psia. The RETRAN computer code model for DCPP was used to determine the potential effect on peak pressure due to the high PSV lift setting. The following cases were evaluated and found to fully bound the range of as-found PSV lift settings for the past operation of Unit 2 Cycle 10. For simplicity the three PSVs are labeled 1, 2, and 3 in the RETRAN model. All pressure values are in psia.
CASE PSV1 PSV2 PSV3 Peak RCS Pressure 1 2453 2525 2580 2702 psia 2 2500 2500 2580 2714 psia 3 2500 2525 2580 2730 psia 4 2525 2525 2580 2748 psia Assuming the most conservative Case 4, with both of the other PSVs at their maximum 1 percent drift combined with the high lift of 2580 psia, the analysis would not have resulted in exceeding the 2750 psia limit. In actuality, one PSV had drifted low sometime during the cycle such that one of the less limiting cases is more representative of the peak pressure results for a loss of load event. All of these cases are conservative. There is no analytical credit taken for the redundant pressurizer spray valves or the three power operated relief valves' actuation. The analysis also assumes 102 percent reactor power, a maximum RCS average temperature of 587.5 degrees Fahrenheit, and a maximum beginning of life moderator temperature coefficient and Doppler feedback coefficients.
The conservatism of these RETRAN results provide more than reasonable engineering assurance that the drift in PSV lift settings would not have resulted in a challenge to the PSV operability with respect to RCS over pressure protection.
Therefore, the event:
- Is of very low risk significance;
- did not adversely affect the health and safety of the public; and
- was not a Safety System Functional Failure.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) Diablo Canyon Unit 2 0 V. � Corrective Actions
A. Immediate Corrective Actions
The valves were disassembled, inspected, reset within tolerance, and returned to warehouse stock.
B. Corrective Actions to Prevent Recurrence
No corrective action to prevent recurrence was required because this inherent characteristic of the valve is within the analysis basis of DCPP.
VI. � Additional Information
A. Failed Components
None.
B. Previous Similar Events
Voluntary LER 1-88-018, submitted in PG&E Letter DCL-89-172, dated June 27, 1989, regarding PSVs found outside TS limits during refueling outages. No root cause or corrective actions could be established for the generic industry problem of lift setting drift of the PSVs. Therefore, the corrective actions taken for LER 1-88-018 did not prevent this event.
November 7, 1995, regarding PSVs found outside TS limits during the Unit 1 sixth refueling outage. The root cause of this event was determined to be random lift setting spread. No corrective action to prevent recurrence was required because this inherent characteristic of the valve was within the analysis basis of DCPP. However, a prudent action to replace the PSV upper spring washer was recommended. The implementation of this prudent action has been deferred until NRC concerns regarding valve performance can be acceptably resolved.
seventh refueling outage. The root cause of this event was determined to be random lift setting spread. No corrective action to prevent recurrence was required because this inherent characteristic of the valve was within the analysis basis of DCPP. However, a prudent action to replace the PSV upper spring washer was recommended. The implementation of this OF 2001 6 4 3 2 3 0 0 0 5 0 Diablo Canyon Unit 2 6 0 0 0 0 LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) prudent action has been deferred until NRC concerns regarding valve performance can be acceptably resolved.
Adjusting Nut pper Spring .Washer Nozzle ring Typical PSV Cross Section