05000400/LER-2013-002
| Shearon Harris Nuclear Power Plant, Unit 1 | |
| Event date: | 10-23-2013 |
|---|---|
| Report date: | 12-19-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4002013002R00 - NRC Website | |
Background
Energy Industry Identification System (El IS) codes are identified in the text as poq .
On October 23, 2013, while operating at 100% power in mode 1, safety valves [RV] in the main steam system [SB] were being tested in accordance with technical specification surveillance requirement 4.7.1.1. The Harris design has a total of 15 main steam safety valves (MSSV), and five of the 15 are tested each operating cycle on a rotating basis in accordance with the Harris inservice testing program.
The MSSVs were manufactured by Crosby, model DS-C-57374.
No other systems, structures or components were inoperable at the time that contributed to the event.
This event is reportable in accordance with 10 CFR 50.73(a)(2)(i)(B) because two MSSV setpoints were discovered outside of the tolerance prescribed by table 3.7-2 of technical specification 3/4.7.1.1.
Event Description
Two of the five tested MSSV setpoints were found outside the +/- 1% tolerance of table 3.7-2 of technical specification 3/4.7.1.1. Valve 1MS-44 setpoint was found approximately 0.2% lower than the acceptable limit, and valve 1MS-53 was found approximately 0.9% higher than the acceptable limit.
Upon discovery of the out of tolerance condition, the valves were adjusted to restore the setpoint to within tolerance, which occurred later in the same day. Three other valves were tested and setpoints were found to be within tolerance.
Because the condition was discovered during routine surveillance testing, and two valves were discovered outside of tolerance, the condition is assumed to have occurred prior to discovery as discussed in NUREG-1022, Revision 3. 1MS-44 and 1MS-53 had been previously tested in April 2009, and both valves were found within tolerance at that time. The specific date and time at which the valves became out of tolerance is not known and cannot be determined.
Testing of the valves does not cause the main valves to lift to relieve pressure, so there is no transient caused by testing of the valves. No other systems, structures or components were inoperable at the time that contributed to the event. There were no automatically or manually initiated safety system responses to the out of tolerance conditions identified.
Shearon Harris Nuclear Power Plant, Unit 1 05000400 Causal Factors The cause was determined to be setpoint drift incompatible with analysis specified criteria. As described in Electric Power Research Institute report TR-105872S and Nuclear Regulatory Commission Information Notice 2006-24, setpoint drift is a common phenomenon and the timing or degree of the drift cannot be predicted or determined with a high level of confidence. The common causal factors are an aggregate of aging, binding, bonding, and corrosion. Each of these alone are failure causes in their classic definition. The factors above preclude accurate determination or prediction of when and how much setpoint drift can occur on the MSSVs. Therefore, the MSSV setpoint out of tolerance conditions at Harris are principally driven by the close tolerance between technical specification requirements and the ability of the valve to perform within the required pressure band.
Corrective Actions
Upon discovery of the out of tolerance conditions, the two setpoints were adjusted to within technical specification tolerances which restored compliance with the technical specifications.
Because the setpoint drift does not have a highly reliable resolution path, avoiding the condition of MSSV's not meeting acceptance criteria can be accomplished by revision of the safety analysis and changing the Technical Specification to accommodate the observed drift. The corrective action to preclude recurrence will be implementation of a revised safety analysis that accommodates increased setpoint drift and supports revised technical specification setpoints.
Safety Analysis
The MSSVs are used to satisfy American Society of Mechanical Engineers (ASME) Code requirements for overpressure protection and are designed to prevent the system pressure from exceeding 110% of the design operating pressure. The valves are also credited in mitigating the effects of postulated accidents (e.g., loss of external electrical load and loss of normal feedwater).
The consequences of exceeding the ASME pressure limit could include damage to system components, leakage, or a requirement to perform additional stress analyses prior to resumption of reactor operation.
The applicable limits are 110% of the main steam design pressure for American Nuclear Society (ANS) condition I and II events and 120% of the main steam system design pressure for ANS condition III and IV events. The limits are documented in the Harris Final Safety Analysis Report (FSAR), Chapter 15, for the respective events. As documented in the Safety Analysis Report for Cycle 18, the most limiting transients are turbine trip for ANS condition II events, and main feedline break for ANS condition III and IV events.
The safety analyses are predominantly concerned with main steam system overpressure. As a consequence, the out of tolerance reading for 1MS-44 (slightly below 1% tolerance) does not produce a result which is outside of the safety analysis.
The event with the smallest amount of overpressure margin is turbine trip at 18.9 pounds per square inch. The lowest margin is for a case with Temperatureaverage at 588.8° Fahrenheit. The event is analyzed crediting all the MSSVs at the nominal setting with the technical specification tolerance of +1.0% added. All five stages of the MSSVs open for the limiting event. Therefore, all the safety valves would have to exhibit measured setpoints higher than the technical specification tolerance of +1% to invalidate the analysis of record. Therefore, the safety analysis remains valid, and the impact on safety is very minor.
Additional Information
No previous Harris licensee event reports describing main steam safety valve setpoints outside technical specification tolerances within the last ten years were identified. Although isolated cases of MSSV setpoints were found outside of technical specification tolerances in the last three years, the isolated discrepancies did not meet reportability criteria. Multiple cases of MSSV setpoints outside technical specification tolerance did occur in 2001 and 2006, but a root cause analysis was not performed for those conditions because the failures were within the tolerance of the ASME code.
Therefore, corrective actions for the 2001 and 2006 events were not expected to prevent these conditions.
As stated previously, Energy Industry Identification System (EllS) codes are identified in the text as POq
- This report contains no regulatory commitments.