05000410/LER-2012-001

LER-2012-001, Forced Shutdown Due to an Increase in Drywell Leakage in Excess of Technical Specifications Limit

Docket Number
Event date:	12-09-2011
Report date:	03-16-2012
Reporting criterion:	10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown
4102012001R01 - NRC Website

A. PRE-EVENT PLANT CONDITIONS:

Prior to this event, Nine Mile Point Unit 2 (NMP2) was operating at 100 percent rated thermal power with no inoperable systems affecting this event.

B. EVENT:

On December 9, 2011, at 0908, NMP2 was operating at 100 percent of rated thermal power when alert alarms were received for containment monitoring particulate channels. These alarms were accompanied by a rise in unidentified drywell leakage and drywell pressure. At 1046, a manual shutdown of Unit 2 was initiated due to exceeding the Technical Specifications (TS) Limiting Condition for Operation (LCO) for unidentified leakage in the drywell. TS 3.4.5 requires action to be taken if unidentified leakage increases > 2 gpm within a 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> period while in Mode 1.

There was no impact on Nine Mile Point Unit 1 (NMP1) from this event.

EVENT:

There were no inoperable components or systems that contributed to this event.

D. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES (note: all times are on December 9, 2011, unless otherwise noted):

0903 - Identified rising unidentified drywell leakage rate and a corresponding rise in drywell pressure.

0905 - Identified rising trend on particulate monitor 2CMS*CAB10A-2.

0908 - 2CMS*CAB10A-2 in alert.

0911 - 2CMS*CAB10B-2 in alert.

0915 - Reactor Coolant System (RCS) unidentified leakage is 2.35 gpm, entered TS 3.4.5 Condition B, which requires the unidentified leakage increase to be reduced to within limit within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />, or the source of the unidentified leakage increase to be identified within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />.

0918 - The shift manager directed unit shutdown.

1046 - Power reduction commenced.

1315 - Entered TS 3.4.5 Condition C, which requires the plant to be in Mode 3 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 within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />, due to not meeting TS 3.4.5 Condition B.

2237 - All Control Rods are fully inserted.

2317 - Mode Switch placed in Shutdown. Plant in Mode 3, Hot Shutdown.

12/10/2011 - 1813 - Reactor Operation is Mode 4, Cold Shutdown.

E. OTHER SYSTEMS OR SECONDARY FUNCTIONS AFFECTED:

None

F. METHOD OF DISCOVERY:

On December 9, 2011, at 0908, Operations noted that the containment particulate radiation monitors went into alarm and that RCS unidentified leakage was increasing.

G. MAJOR OPERATOR ACTION:

At 0915, TS 3.4.5 Condition B was entered for RCS unidentified leakage rate increase > 2 gpm within a 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> period. At 1046, a manual shutdown of NMP2 was initiated.

H. SAFETY SYSTEM RESPONSES:

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

II. CAUSE OF THE EVENT:

The direct cause of this event is failed packing on Reactor Coolant Pump "A" discharge blocking valve 2RCS*MOV18A. This caused the RCS unidentified leakage rate to exceed the TS 3.4.5 limit. The valve stem packing leak occurred due to a score in the packing material created by a burr on the valve stem. The burr was created during packing replacement in August 2011 when the plant was shutdown due to high unidentified drywell leakage from the packing of this same valve. In August 2011, while removing the failed packing set, destructive removal of the packing set's carbon bushing was performed. The carbon bushing is designed to be removed using a packing puller and the pre-drilled and tapped holes in the top of the carbon bushing. Due to valve stem mis-alignment, the bushing was pinched in the stuffing box and could not be removed in this manner.

Hardened tools, including a machinist's punch and hammer, were used to break apart and remove the bushing.

The physical arrangement of the stuffing box and packing set's carbon bushing allowed the punch to be at such an angle to allow a direct strike to the stem. It is believed at this point the burr was created on the valve stem.

The root cause of this event was that Nine Mile Point leadership did not exhibit a questioning attitude toward potential error likely situations. Leadership did not appropriately identify situations where precursors, such as lack of adequate procedure details and environmental conditions, were challenges to workmanship.

This event was entered into the NMPNS corrective action program (Condition Report CR-2011-010906).

III. ANALYSIS OF THE EVENT:

This event is reportable in accordance with 10 CFR 50.73 (a)(2)(i)(A), "The completion of any nuclear plant shutdown required by the plant's Technical Specifications.

There were no systems inoperable and no system failures related to this event. There were no actual safety consequences from this event. The leakage was from the blocking valve packing and was not indicative of RCS component wear. The leakage was contained within the drywell. The maximum leakage rate noted during this event was 3.7 gpm, which is within the TS limit of 5 gpm. Even if the packing had catastrophically failed, the leakage would still have been contained within the drywell and the plant would have been capable of reaching a safe shutdown condition. There were no system failures that prevented the safe shutdown of the plant. It is therefore concluded that even if a design basis accident had occurred concurrent with this event, all safety systems would have operated to safely mitigate the event. Based on the above considerations, 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.

This event does not affect the NRC Regulatory Oversight Process (ROP) Index for Unplanned Scrams because the shutdown did not involve a scram. This event increases the ROP Index for Unplanned Power Changes per 7000 Critical Hours from 1.6 to 2.5.

IV. CORRECTIVE ACTIONS:

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

1. Removed burr and polished all high spots on the stem of 2RCS*MOV18A to improve the condition of the sealing surface.

2. Installed modified packing on 2RCS*MOV18A and torqued.

3. Installed modified packing on 2RCS*MOV18B and torqued, as a precautionary measure.

4. The packing for similar RCS pump suction blocking valves 2RCS*MOV10A and 2RCS*MOV10B were re-torqued.

B. ACTION TAKEN OR PLANNED TO PREVENT RECURRENCE:

1. Revise the appropriate procedures to include required tools and precautions for proper removal of valve packing.

2. Develop and implement a case study of this event to highlight the elements of an engaged thinking workforce necessary to overcome conditions that are not conducive to completing high quality maintenance. The expected outcome will be improved behaviors in terms of supervisory intervention to mitigate error-likely situations.

3. Revise training for Mechanical Maintenance personnel to include human performance and safety dynamic learning activity course material during initial and continuing training that incorporates the lessons learned from this event.

A. FAILED COMPONENTS:

None

B. PREVIOUS LERs ON SIMILAR EVENTS:

There are three similar LERs:

1. NMP2 LER 2011-002. On August 6, 2011, NMPNS identified drywell floor drain leakage exceeding the maximum limits of TS 3.4.5 for unidentified drywell leakage. The cause of the unidentified leakage was determined to be failed packing in a reactor coolant system discharge blocking valve, 2RCS*MOV18A.

The corrective actions included repacking the valve to stop the leakage and re-torquing the packing for the remaining similar valves in the RCS to protect against leakage. The primary cause of the packing failure was determined to be vibration and flow turbulence. This caused the packing to relax and fail on 2RCS*MOV18A.

2. NMP2 LER 2001-007. On December 15, 2001, NMPNS identified drywell floor drain leakage approaching the maximum limits of TS 3.4.5 for unidentified drywell leakage. The cause of the unidentified leakage was determined to be failed packing in a reactor coolant system discharge blocking valve, 2RCS*MOV18A. The corrective actions included installing modified packing to stop the leakage and re-torquing the packing for the remaining similar valves in the RCS to protect against leakage. The primary cause of the packing failure was determined to be packing ring extrusion into the leak-off port.

3. NMP1 LER 2006-001. On June 11, 2006, NMPNS commenced a planned downpower to perform a drywell entry to determine the cause of increased drywell leakage. The source of the increased leakage was determined to be the reactor coolant system drain valve packing. The cause of the packing leak was installation of incorrect packing in March 1997. The packing that was installed did not have the same diameter as the inside diameter of the stuffing box. During the shutdown, NMPNS replaced the packing in the leaking RCS pump drain valve.

C. THE ENERGY INDUSTRY IDENTIFICATION SYSTEM (EIIS) COMPONENT FUNCTION

LER:

COMPONENT IEEE 803 IEEE 805

COMPONENT IDENTIFIER SYSTEM IDENTIFICATION

Reactor Coolant Blocking Valves V AD Reactor Protection System NA JC

D. SPECIAL COMMENTS:

None