05000323/LER-2005-001

LER-2005-001, TS 3.4.10 Not Met During Pressurizer Safety Valve Surveillance Testing Due to Random Lift Spread

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
3232005001R01 - NRC Website

I. Plant Conditions

Unit 2 has operated in various plant modes with the described condition.

A. Background

Technical Specification (TS) 3.4.10, "Pressurizer Safety Valves" (PSVs), requires that three PSVs [AWN/ shall be operable with a lift setting greater than or equal to 2460 psig and less than or equal to 2510 psig corresponding to ambient conditions of the valve at nominal operating temperature and pressure. This upper and lower pressure limit is based on a nominal pressure of 2485 psig, with an upper and lower tolerance limit of plus or minus one percent. TS 3.4.10 and associated bases did not discuss the as-found testing analysis limits for the PSVs.

Surveillance Test Procedure (STP) M-77, "Safety and Relief Valve Testing," verifies the PSV's lift setting in accordance with the requirements of the ASME Boiler and Pressure Vessel Code, Section Xl.

The initial (as-found) 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.

El.�Event Description Following the Unit 1 eleventh refueling outage in May 2002, the subject PSVs lift settings were verified to be within the range required by TS 3.4.10. The PSVs were then returned to warehouse stock. During the Unit 2 eleventh refueling outage in February 2003, these three PSVs were placed in service in Unit 2 without any additional adjustment of the lift settings. The valves were replaced during the Unit 2 twelfth refueling outage in November 2004 and tested offsite in January 2005.

LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Diablo Canyon Unit 2 0 5 0 0 0 3 2 3 2005 0 0 1 0 1 3 OF On January 27, 2005, during scheduled offsite testing in accordance with STP M-77, "Safety and Rellief Valve Testing," Pacific Gas and Electric Company (PG&E) identified two of three PSVs outside the TS 3.4.10, "Pressurizer Safety Valves, ... lift setting of >2460 and initial lift settings were 4.4 and 3.6 percent low. The second valve lifts of these same two PSVs were found to be 3.1 percent low and 0.3 percent low, respectively.

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, Systems, or Components that Contributed to the

Event

None.

D. Other Systems or Secondary Functions Affected None.

E. Method of Discovery This condition was identified during routine scheduled testing of the three PSVs performed in accordance with STP M-77 at the offsite testing facility.

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 lift setting tolerance.

LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Diablo Canyon Unit 2 0 5 0 0 0 3 2 3 2005 0 0 1 0 1 4 OF 13.CRoot Cause The cause of the as-found lift setting has been determined to be random lift spread.

C. Contributory Cause None.

IV. Assessment of Safety Consequences

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 model was run to determine if, with the as-found PSV lift setpoint, the RCS pressure would exceed 110 percent of ASME design acceptance criteria, or 2750 psia. The analysis confirmed that the as-found set points would have maintained adequate RCS pressure relief capacity, such that the plant remained bounded by the limiting loss of load analysis provided in Final Safety Analysis Report Update, Section 15.2.7, "Loss of External Electrical Load and/or Turbine Trip." Also, the as-found lift setting was reviewed for potential interaction with the pressurizer power-operated relief valves and the potential for inadvertent low pressure lifting, and were found acceptable.

Therefore, this event was of very low risk significance, was not a Safety System Functional Failure, and did not adversely affect the health and safety of the public.

V. Corrective Actions

A.CImmediate Actions The valves were disassembled, inspected, reset within tolerance, and returned to warehouse stock. No degraded PSV condition was identified.

El.CCorrective Actions 1. PG&E has revised the loss-of-load analysis to determine the upper and lower as-found allowable PSV lift setting. This reanalysis formed the technical basis for a change in the acceptance criteria utilized for the evaluation of the as-found lift setting of the PSVs.

LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Diablo Canyon Unit 2 0 5 0 0 0 3 2 3 2005 0 0 1 0 1 5 OF 2. PG&E has updated the TS 3.4.10 bases to reflect the above reanalysis conditions and establish the as-found surveillance test criteria.

3. PG&E has revised STP M-77 to reflect the applicable testing conditions and as-found lift setting criteria in the revised TS 3.4.10 bases.

VI.�Additional Information A.�Failed Components None.

E3.�Previous Similar Events Licensee Event Report (LER) 1-94-009, Revision 2, submitted in PG&E Letter DCL-95-248, dated 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.

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.

August 27, 2001, regarding one PSV found 3.4 percent low and one PSV found 2.8 percent high during offsite testing. 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.

August 9, 2002, regarding one PSV found 1.9 percent low and one PSV found 2.6 percent high during offsite testing. 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.