05000369/LER-2004-002

LER-2004-002, Main Steam Isolation Valve Inoperable.

Mcguire Nuclear Station
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident
3692004002R01 - NRC Website

BACKGROUND

Main Steam Isolation Valves [ISV](MSIV):

The Unit 1 Main Steam System [SB](SM) contains four Main Steam Isolation Valves manufactured by Atwood and Morrill Co (Model MSI-001). Each valve is located downstream of its respective Steam Generator [SG](SG) and remains open during normal power operation. The MSIVs are designed to automatically close upon receipt of a Main Steam Line Isolation Signal (Engineered Safety Feature). The safety functions performed by these valves include the following:

• Close to isolate all four SGs to ensure that no more than one SG blows down in the event of a steam line break. This minimizes the positive reactivity effects of the break by ensuring that the Reactor Coolant System [AB](NC) does not experience excessive cooldown as a result of the increased steam flow.

O Close to isolate all four SGs to ensure that no more than one SG blows down in the event of a steam line break inside containment. This minimizes the containment temperature and pressure increase.

McGuire Technical Specification (TS) 3.7.2 - Main Steam Isolation Valves:

The above MSIV safety functions are ensured by compliance with the requirements of TS 3.7.2. The TS 3.7.2 LCO specifies that four MSIVs shall be operable in Mode 1. This LCO also states that they shall be operable in MODES 2 and 3, except when the MSIVs are closed and de-activated. TS Surveillance Requirement (TSSR) 3.7.2.1 and the TS 3.7.2 BASES indicate that an MSIV is operable when it is capable of closing in less than or equal to 8 seconds upon receipt of an isolation signal. Compliance with this operability criteria can be verified by the performance of hot stroke testing. As per TS 3.7.2, Condition A, if one MSIV is inoperable in MODE 1, the affected MSIV shall be restored to operable status within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />. If the required action and associated completion time of Condition A are not met, then TS 3.7.2, Condition B, states that the respective Unit must be in MODE 2 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />. As per TS 3.7.2, Condition C, if one or more MSIVs are inoperable in MODE 2 or 3, the affected MSIV shall be closed within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> and verified closed once per 7 days. If the required action and associated completion time of Condition C are not met, then TS 3.7.2, Condition D, states that the respective Unit must be in MODE 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and in MODE 4 within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.

EVENT DESCRIPTION

Note: All events are shown in the approximate sequence in which they occurred.� All times are approximate.

September/October 2002:

• Scheduled maintenance was performed on 1SM-3 during the Unit 1 EOC15 refueling outage. Although not known at the time, 1SM-3 was re-assembled incorrectly during maintenance such that it was incapable of closing to isolate a postulated steam line break between the "C" SG and 1SM-3 when required.

• Unit 1 was restarted, entering Mode 3 (Hot Standby) at 0638 on October 7, 2002. Since, upon entry into MODE 3, 1SM-3 was unknowingly inoperable due to incorrect re-assembly, this resulted in a TS prohibited condition.

• 1SM-3 was stroke tested at 2226 on October 7, 2002. Upon review of the test data, it was identified that the measured stroke length of 1SM-3 did not meet the overall travel acceptance criteria. Subsequent evaluation concluded that 1SM-3 was operable.

March 2004:

• At 1015 on March 6, 2004, Unit 1 entered MODE 4 (Hot Shutdown)in preparation for the Unit 1 EOC16 refueling outage.

April 2004:

• At 1655 on April 3, 2004, during startup from the Unit 1 EOC16 refueling outage, Unit 1 entered MODE 3.

• At 1340 on April 4, 2004, during the Unit 1 EOC16 refueling outage and with Unit 1 in MODE 3, 1SM-3 was hot stroke tested. 1SM-1 was hot stroke tested at 1507 on the same day. No abnormalities were identified in either test.

• On April 6, 2004 at 1216, Unit 1 entered MODE 4.

• On April 9 and 10, 2004 1SM-1 was cold stroked with no abnormalities identified.

• Unit 1 entered MODE 3 at 1156 on April 10, 2004.

• On April 27, 2004, scoring was discovered on the valve stem for 1SM-1.

Subsequent investigation concluded that the scoring was most likely introduced during the stroking of 1SM-1 on April 9 or 10, 2004. An operability assessment determined that the scoring did not render 1SM-1 inoperable.

October 2004:

• On October 18, 2004, Unit 1 entered MODE 3 for planned repair of a leak on an instrument line associated with the "B" SG. During hot stroke testing of 1SM-1 at 1028, the valve would not fully close. 1SM-1 was subsequently declared inoperable. Unit 1 entered MODE 4 at 2153 in preparation for repairing 1SM-1.

November 2004:

• On November 3, 2004 at 1650, upon completion of repairs on 1SM-1, Unit 1 entered MODE 3.

• On November 4, 2004, 1SM-1 was hot stroke tested with acceptable results. 1SM-3 was hot stroke tested at 1308. During this test of 1SM-3, station personnel observed the absence of a distinct sound associated with the valve's pilot poppet hitting its seat. As a result, applicable test data was reviewed. This review indicated the pilot poppet stroke length for 1SM-3 was shorter than expected.

Subsequent evaluation of the condition identified that, in the absence of a steam line break between the "C" SG and 1SM-3, this valve remained capable of closing within the required time to isolate forward steam flow upon receipt of an isolation signal. However, in the presence of a steam line break between the "C" SG and 1SM-3, the subject condition would have prevented 1SM-3 from remaining closed and isolating reverse MSIV concurrent with a steam line break, more than one SG would have blown down. This condition rendered 1SM-3 inoperable with respect to TS 3.7.2.

• In preparation for repairing 1SM-3, Unit 1 entered MODE 4 at 0934 on November 5, 2004.

• Following repair of 1SM-3, Unit 1 entered MODE 3 at 0331 on November 10, 2004. 1SM-3 was hot stroke tested at 1247 with acceptable results.

• On April 28, 2005, subsequent to discussions with the NRC, McGuire concluded that the scoring observed on the 1SM-1 valve stem in April of 2004 was indicative of conditions which probably prevented the valve from fully closing.

For reasons listed in the cause analysis, 1SM-3 was re-assembled incorrectly which rendered it incapable of closing to isolate a steam line break from the other Unit 1 Steam Generators when required since Unit 1EOC15 refueling was in a TS 3.7.2 MODE of applicability during the following periods:

• From when Unit 1 entered MODE 3 at 0638 on October 7, 2002 until when Unit 1 entered MODE 4 at 1015 on March 6, 2004.

• From when Unit 1 entered MODE 3 at 1655 on April 3, 2004 until when Unit 1 entered MODE 4 at 2153 on October 18, 2004.

• From when Unit 1 entered MODE 3 at 1650 on November 3, 2004 until when Unit 1 entered MODE 4 at 0934 on November 5, 2004.

During the above periods, 1SM-3 was inoperable longer than permitted by plant Technical Specifications. These instances represented an operation prohibited by Technical Specifications and are reportable as per the requirements of 10 CFR 50.73 (a)(2)(i)(B).

In addition to the above reportable conditions, 1SM-3 was simultaneously inoperable with 1SM-1 during the following period:

• From when Unit 1 entered MODE 4 on April 10, 2004 until October 19, 2004, when Unit 1 entered a MODE in which both TS 3.6.3 and TS 3.7.2 were not applicable.

During the above period when 1SM-3 and 1SM-1 were simultaneously inoperable, the MSIV safety function of preventing blowdown from more than one S/G following a steam line break could not have been performed. This represented a condition which could have prevented fulfillment of a safety function. This condition is reportable as per 10CFR50.73(a)(2)(v)(D).

The causal factors for 1SM-1 and corrective actions have been documented on Licensee Event Report 369/2005-02 revision 1.

CAUSAL FACTORS

The failure of 1SM-3 is being attributed to improper re-assembly of the valve during maintenance performed in the Unit 1 EOC15 refueling outage in 2002. This improper re-assembly was caused by deficiencies in the instructions provided in the procedure used to maintain, re-assemble, and test the MSIVs. The failure to recognize that the abnormal stroke length identified during the October 7, 2002 stroke test rendered 1SM-3 inoperable is being attributed to inadequate stroke length acceptance criteria in the procedure used to maintain, re-assemble, and test the MSIVs. The cause analysis determined that the failure mode of 1SM-3 was not transportable to the remaining three Unit 1 MSIVs or the four Unit 2 MSIVs.

CORRECTIVE ACTIONS

1) The Cause analysis determined that the failure mode of 1SM-3 was not MSIVs.

2) The procedure containing the instructions for maintaining, re- assembling, and testing the MSIVs has since been revised to ensure adequate assembly instructions and acceptance criteria are provided to prevent re-occurrence of a failure similar to that experienced on 1SM-3.

3) 1SM-3 was repaired to correct the deficiencies identified in November of 2004.

SAFETY ANALYSIS

A Westinghouse analysis specific to McGuire indicated that reactor vessel integrity would not be challenged by a loss of secondary side pressure, eliminating any Pressurized Thermal Shock (PTS) concerns for this event.

A risk assessment of this event determined that the increase in the estimated core damage frequency (CDF) or large early release frequency (LERF) was insignificant. Therefore, the event described in this LER was not significant with respect to the health and safety of the public.

ADDITIONAL INFORMATION

A review of the McGuire corrective action database identified three instances within the past three years involving the failure of 2SM-1 (2D SG MSIV), 1SM-3 (1C SG MSIV), and 1SM-7 (1A SG MSIV) to perform as designed during stroke testing. However, none of those failures were attributed to improper assembly and procedural inadequacies. Therefore, the event described in this LER is not considered a recurring event.

Applicable Energy Industry Identification (EIIS) system and component codes are enclosed within brackets. McGuire unique system and component identifiers are contained within parentheses.