05000410/LER-2011-001

LER-2011-001, As-Found Safety Relief Valve Lift Setpoints Exceed Technical Specification Allowable Values

Nine Mile Point Unit 2
Event date:	04-01-2011
Report date:	08-12-2011
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
4102011001R01 - NRC Website

A. PRE-EVENT PLANT CONDITIONS:

At the time of discovery, Nine Mile Point Unit 2 (NMP2) was operating at 100 percent power.

Operation of NMP2 was unaffected by the event since the entire compliment of eighteen (18) Main Steam Safety Relief Valves (SRVs) had been replaced with pre-tested, certified spare SRVs during the prior 2010 refueling outage.

B. EVENT:

During the 2010 NMP2 refueling outage, all 18 of the SRVs were removed and replaced with pre-tested, certified SRVs. The removed SRVs were sent to an offsite test facility for as-found testing, refurbishment, and re-certification. In accordance with the American Society of Mechanical Engineers (ASME) Operation and Maintenance Code, for replacement of a full complement of SRVs, the SRVs removed from service must be tested within 12 months of removal from the system. On April 1, 2011, Nine Mile Point Nuclear Station, LLC (NMPNS) determined that, based on the results of the completed as-found testing, four (4) SRVs mechanically actuated at pressures that exceeded the allowable Technical Specification (TS) limit, which is the TS-specified setpoint plus or minus 3 percent. The following is a tabulation of the as-found test results for the 18 SRVs:

SRV ID No. SRV Serial No.

TS Setpoint (psig) TS Setpoint Acceptance Band (psig) As-Found Setpoint (psig) (1) Percent Difference (1) 2M55*PSV120 160953 1185 1149 - 1221 1229 3.7 2M55*PSV121 160967 1195 1159 - 1231 1226 2.6 2M55*P5V122 160952 1185 1149 - 1221 1217 2.7 2M55*P5V123 160914 1175 1140 - 1210 1193 1.5 2M55*P5V124 160906 1175 1140 - 1210 1212 3.1 2M55*P5V125 160950 1185 1149 - 1221 1220 3.0 2M55*P5V126 160939 1195 1159 - 1231 1229 2.8 2M55*P5V127 160905 1205 1169 - 1241 1234 2.4 2M55*P5V128 160903 1165 1130 - 1200 1196 2.7 2M55*P5V129 160904 1205 1169 - 1241 1234 2.4 2M55*PSV130 160976 1195 1159 - 1231 1226 2.6 2M55*PSV131 160974 1175 1140 - 1210 1192 1.4 2M55*P5V132 160969 1185 1149 - 1221 1197 1.0 2M55*P5V133 160959 1165 1130 - 1200 1214 4.2 2M55*P5V134 160955 1205 1169 - 1241 1211 0.5 2M55*P5V135 160936 1195 1159 - 1231 1222 2.3 2M55*P5V136 160962 1175 1140 - 1210 1204 2.5 2M55*P5V137 160970 1205 1169 - 1241 1245 3.3 (1) The shaded values indicate the as-found test results that exceeded the TS-required 3 percent setpoint tolerance.

NMP2 TS 3.4.4 requires the safety function of sixteen (16) SRVs to be operable in reactor operating modes 1, 2, and 3. With one or more required SRVs inoperable, the unit is required to be placed in Mode 3 (hot shutdown) within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> and in Mode 4 (cold shutdown) within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. Since the as-found testing determined that 4 of the 18 SRVs were inoperable for an indefinite period of time during the operating cycle that preceded the 2010 refueling outage, it is probable that NMP2 operated longer than the TS allowed Completion Time.

THE EVENT:

None

D. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES:

April 2010 During the 2010 refueling outage, all 18 SRVs are removed and replaced with pre-tested SRVs that had completed set pressure certification lifts within plus 1 percent to minus 0.5 percent of the specified set pressure.

5/2/2010 NMP2 startup from the 2010 refueling outage commences with replacement SRVs installed.

4/1/2011 NMPNS documents in Condition Report 2011-003045 that the as-found lift pressure for 4 SRVs removed during the 2010 refueling outage exceeded the TS-specified setpoint plus or minus 3 percent.

E. OTHER SYSTEMS OR SECONDARY FUNCTIONS AFFECTED:

None

F. METHOD OF DISCOVERY:

The out-of-tolerance SRV lift setpoints were discovered during the performance of as-found testing conducted at NWS Technologies in Spartanburg, South Carolina.

G. MAJOR OPERATOR ACTION:

None. No operational conditions requiring operator action occurred as a result of this event.

H. SAFETY SYSTEM RESPONSES:

None. No operational conditions requiring the response of safety systems occurred as a result of this event.

II. CAUSE OF EVENT:

The immediate cause for this reportable condition is out-of-tolerance lift pressures that exceeded Completion Time. The 4 SRVs that failed the as-found test were disassembled and inspected at NWS Technologies in Spartanburg, SC, using the guidance provided in the Dikkers instruction manual and Electric Power Research Institute (EPRI) TR-105872, "Safety and Relief Valve Testing and Maintenance Guide." There was no evidence of degradation, corrosion, binding, rubbing, foreign material intrusion, or parts out of adjustment noted during the inspections, and no test method irregularities at the NWS test facility were identified that would account for the test failures.

The cause for the 4 as-found test failures is attributed to inaccurate as-left lift pressure settings that resulted from the use of nitrogen as the test medium for SRV testing performed prior to the 2010 refueling outage. Onsite nitrogen testing of the NMP2 SRVs was conducted from 1997 to 2008.

Prior to commencing nitrogen testing, NMPNS performed analyses to establish a nitrogen-steam correlation that, when combined with a 95% confidence limit, would provide conservative nitrogen pressure limits for establishing an equivalent SRV steam set pressure within the TS as-left set pressure tolerance limit of plus or minus 1 percent. The analyses considered the concerns expressed in General Electric Service Information Letter (SIL) No. 577, "Nitrogen Setting of the Dikkers SRV," and incorporated the implementation considerations described in the SIL. The testing of the 18 SRVs removed during the 2010 refueling outage, performed at NWS Technologies using saturated steam as the test medium, indicates that all of the lift pressures were greater than the nominal TS setpoint values; therefore, it appears that the nitrogen-steam correlation was not sufficiently conservative. Onsite nitrogen testing of the NMP2 SRVs is no longer being performed.

III. ANALYSIS OF THE EVENT:

This event is reportable in accordance with 10 CFR 50.73(a)(2)(i)(B) as an operation or condition prohibited by the NMP2 TS. NMP2 TS 3.4.4 requires the safety function of 16 SRVs to be operable in reactor operating modes 1, 2, and 3. With one or more required SRVs inoperable, the unit is required to be placed in Mode 3 (hot shutdown) within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> and in Mode 4 (cold shutdown) within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. The as-found testing determined the lift pressures for 4 of the 18 SRVs to be outside of the TS requirements. Consistent with the guidance provided in NUREG-1022, Revision 2, Section 3.2.2 (Example (3), Multiple Test Failures), the condition is considered to have existed during the plant operating cycle preceding the 2010 refueling outage (Cycle 12) and is reportable under 10 CFR 50.73(a)(2)(i)(B).

The ASME Boiler and Pressure Vessel Code requires that the reactor pressure vessel be protected from overpressure during upset conditions by self-actuated safety valves. As part of the nuclear pressure relief system, the size and number of SRVs are selected such that peak pressure in the nuclear system will not exceed the ASME Code limits for the reactor coolant pressure boundary.

The NMP2 SRVs are Dikkers Model G471-6 valves. There are a total of 18 installed SRVs divided into 5 groups, with each group having a different lift pressure setpoint, as follows:

Number of SRVs Setpoint (psiq) 2 1165 +/- 35.0 4 1175 +/- 35.0 4 1185 +/- 36.0 4 1195 +/- 36.0 4 1205 +/- 36.0 The SRVs are located on the main steam lines between the reactor vessel and the first isolation valve within the drywell. Each SRV discharges steam through a discharge line to a point below the minimum water level in the suppression pool.

The overpressure protection system must accommodate the most severe pressure transient. For NMP2, the most severe transient is the closure of all main steam isolation valves (MSIVs) followed by a reactor scram on high neutron flux (assumes failure of the direct scram associated with MSIV position). The analysis results demonstrate that the design SRV capacity is capable of maintaining reactor pressure below the ASME Code limit of 1375 psig (110 percent of the 1250 psig vessel design pressure). For the purpose of the overpressure protection analysis, 16 of the SRVs with the highest setpoints are assumed to operate in the safety mode (i.e., operation of the two SRVs with setpoints of 1165 psig is not credited in the analysis), with assumed setpoints that are about 3 percent above the nominal setpoints. Since the SRV with the largest deviation from the TS- required lift pressure has a TS-required setpoint of 1165 psig, the as-found setpoint deficiency for this SRV has no impact on the overpressure protection analysis. The setpoints for the other 3 SRVs with out-of-tolerance lift pressures exceeded the TS required as-found values (including 3 percent tolerance) by only small amounts, which would have a minimal impact on the overpressure protection analysis results. In addition, parametric analyses presented in Appendix 15C of the NMP2 Updated Safety Analysis Report (USAR) demonstrate that if only 14 SRVs are assumed to operate rather than 16 SRVs, the peak calculated vessel pressure increases by less than 10 psig.

Based on the above discussion, the margin between the calculated peak vessel pressure evaluated for Cycle 12 (1301 psig) and the ASME Code limit of 1375 psig, and the fact that all 18 SRVs actually lifted during the as-found testing, the peak vessel pressure would not have exceeded 1375 psig had an overpressure transient occurred that required SRV operation. Furthermore, the peak reactor steam dome pressure would also have remained below the TS safety limit of 1325 psig.

Overpressure analyses for the limiting NMP2 Anticipated Transients Without Scram (ATWS) event (MSIV closure) have also previously been performed to demonstrate that the reactor pressure does not exceed the ASME Service Level C design limit of 1500 psig. For this analysis, two SRVs were assumed to be unavailable. The analysis results, presented in NMP2 USAR Appendix 15C, showed a peak calculated vessel bottom head pressure of 1279 psig. Based on the margin between this calculated value and the 1500 psig limit, the small amount by which the 4 SRVs exceeded the TS setpoint limits, and the fact that all 18 SRVs actually lifted during the as-found testing, the peak vessel pressure would not have exceeded 1500 psig had an ATWS event occurred that required SRV operation.

One of the 4 SRVs with an out-of-tolerance lift pressure (ID No. 2MSS*PSV137; Serial No. 160970) is also an Automatic Depressurization System (ADS) valve. The lift pressure deficiency had no impact on the ADS function of this SRV.

Based on the above, it is concluded that the safety significance of this event is low and the event did not pose a threat to the health and safety of the public or plant personnel.

IV. CORRECTIVE ACTIONS:

A. ACTION TAKEN TO RETURN AFFECTED SYSTEMS TO PRE-EVENT NORMAL STATUS:

During the 2010 NMP2 refueling outage, all 18 of the SRVs were removed and replaced with pre-tested SRVs that had completed set pressure certification lifts within plus 1 percent to minus 0.5 percent of the specified set pressure, thereby meeting the NMP2 TS 3.4.4 requirements.

The replacement SRVs were all tested using saturated steam as the test medium. The removed SRVs will be refurbished, tested, and certified prior to future use at NMP2.

B. ACTION TAKEN OR PLANNED TO PREVENT RECURRENCE:

• The 4 SRVs that failed the as-found set pressure test were disassembled and inspected to identify any conditions that would cause the out-of-tolerance test results.

• Onsite testing of the SRVs using nitrogen as the test medium has been discontinued.

Testing of the SRVs is now being conducted at an offsite test facility using saturated steam as the test medium.

A. FAILED COMPONENTS:

The identified condition for the 4 SRVs is considered to be a Maintenance Rule functional failure since the 4 SRVs did not lift within the setpoint tolerance requirements of the ASME Operation and Maintenance (OM) Code - 2004 Edition.

B. PREVIOUS LERs ON SIMILAR EVENTS:

None C. THE ENERGY INDUSTRY IDENTIFICATION SYSTEM (EllS) COMPONENT FUNCTION

COMPONENT

Reactor Pressure Vessel Main Steam Lines Safety Relief Valve

D. SPECIAL COMMENTS:

None IEEE 803 FUNCTION IEEE 805 SYSTEM