05000410/LER-2006-002

LER-2006-002, Nine Mile Point Unit 2

Nine Mile Point Unit 2
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition
4102006002R00 - NRC Website

I. Description of Event

On February 8, 2006, the Nine Mile Point Unit 2 (NMP2) Reactor Core Isolation Cooling (RCIC) system was declared inoperable but available during planned maintenance activities during which a RCIC turbine steam exhaust line vacuum breaker isolation valve, 2ICS*MOV148, was de-energized shut. The NRC resident inspector questioned if considering RCIC available was appropriate with the vacuum breakers isolated. With the vacuum breakers isolated, vacuum relief is not available post-RCIC initiation to prevent water from the suppression pool from being rapidly drawn back into the RCIC turbine exhaust line. This alignment results in the potential for steam-condensation induced water hammer. Additionally, before NMPNS determined that the RCIC configuration represented an unanalyzed condition, similar but separate events occurred on February 10, 2006 and May 11, 2006 where RCIC vacuum breakers were isolated and RCIC remained available for automatic initiation. These additional conditions were present consistent with procedural requirements during performance of Technical Specification required surveillance N2-OSP-ICS-Q003, (RCIC vacuum breaker testing). This procedure was performed each quarter since NMP2 initial plant start-up. On May 26, 2006, NMPNS determined that an unanalyzed condition existed and was reportable per 10 CFR 73(a)(2)(ii)(B).

During normal RCIC system operation, steam would exit the RCIC turbine and be exhausted through horizontal and vertical 12" piping, a check valve (2ICS*V29), a Group 12 primary containment isolation valve (2ICS*MOV122), a containment penetration (203' elevation) and horizontal and vertical piping in primary containment to a sparger at elevation 189' in the suppression pool, approximately 11' below normal suppression pool level. With the RCIC vacuum breaker function unavailable, when steam flow to the RCIC turbine is stopped via manual or automatic action, steam in the exhaust piping would condense (due to piping cooling and steam/water interface in the steam exhaust piping in the suppression pool) and a vacuum would be drawn in the exhaust piping. The differential pressure could rapidly accelerate a water slug from the suppression pool into the exhaust piping. The subcooled water would facilitate the condensation process and increase the acceleration of the water slug. The momentum of the water slug would create large stresses on RCIC system exhaust line valves, piping and pipe supports, of which portions also serve as the primary containment pressure boundary.

In the early and mid-1970's the industry learned of the importance of vacuum breakers on HPCI and RCIC exhaust lines in preventing water hammer events through actual water hammer operating experience. EPRI reported (TR-106438) several RCIC and HPCI events from 1971 to 1977 where system damage occurred due to water hammer events caused by steam condensing in the exhaust piping (before nuclear stations added vacuum breaker lines). As a result of these events, facilities modified their systems by installing vacuum breakers, as recommended by GE Service Information Letter No. 30.

More recent industry operating experience from 1998 (Vermont Yankee) and 2004 (Riverbend) provided information that, if critically reviewed, could have led NMP2 to a better understanding of the potential water hammer impacts with RCIC exhaust line vacuum breakers isolated, resulting in actions and staff understanding that may have prevented placing NMP2 in the unanalyzed condition described in this report.

II. Cause of Event

The root cause of this condition has been attributed to the failure of original design change process to provide cautions in procedures and adequate training regarding the RCIC vacuum breaker function when the vacuum breakers were installed prior to NMP2 initial start-up.

Contributing causes to this event were determined to be:

• Industry Operating Experience related to water hammer in RCIC and HPCI systems did not receive an adequate review and response. This manifested itself in weaknesses in training and procedures which led to operations and engineering knowledge gaps, resulting in failure to recognize the impact of actions/decisions with regard to maintaining RCIC available without the vacuum breaker function.

II. Cause of Event (Continued)

• Inadequate procedures and training created a station knowledge gap regarding the safety function of the RCIC vacuum breakers and did not provide adequate barriers to prevent placing the station in a condition where RCIC remained available for automatic and manual start with the turbine steam line exhaust piping vacuum breakers isolated.

• Non-conservative decision making led to less than adequate immediate compensatory actions in response to NRC resident questions raised about RCIC vacuum breakers being isolated with RCIC line-up for automatic injection on February 8, 2006. This led to repeat occurrence of the unanalyzed condition while performing scheduled Technical Specification surveillance testing on the RCIC vacuum breakers.

Ill. Analysis of Event This event is reportable in accordance with 10 CFR 50.73(a)(2)(ii)(B), "Any event or condition of the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety.

No actual water hammer occurred, therefore, there were no actual safety consequences associated with this condition.

There were no structures, systems or components that were inoperable during the time this condition existed that contributed to this event.

An engineering analysis determined that under non-loss of coolant accident (LOCA) conditions the forces on the piping would not likely breach the piping, but could result in damage to pipe hangers when containment pressure is normal. This analysis is consistent with operating experience in the 1970's (before vacuum breakers were installed as standard components in RCIC systems) that demonstrated water hammer events did not damage piping, but, in some cases, damaged piping components, valves and piping supports when HPCl/RCIC was shutdown with no vacuum breaker function and where no additional pressure was present in the containment.

Engineering judgment and preliminary analysis indicated that, under worst case conditions, the configuration may result in forces significantly above allowables for RCIC piping and piping components. Under design basis LOCA conditions, with the containment pressurized, operation of RCIC without the vacuum breaker function represents an unanalyzed condition.

In this condition, there is an increased likelihood that piping can be breached. In this postulated scenario, the worst case could result in draining the suppression pool below the downcomers, into the lower level of the reactor building. In this case, emergency core cooling systems net positive suction head would be insufficient, reactor vessel and containment flooding post LOCA would not be possible, and there would be direct communication of the wetwell atmosphere to the reactor building.

Due to the small amount of time in this configuration (approximately 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> over the past year), the cumulative risk was below the threshold for the significance determination process risk significance (i.e., Green results).

Based on the above, the event did not pose a significant 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

• When the condition was identified on February 8, 2006, the RCIC steam admission valve was closed and automatic initiation was defeated to prevent automatic start and considered unavailable until the vacuum breaker function was restored.

B. Action Taken or Planned to Prevent Recurrence NOTE: There are no NRC regulatory commitments in this Licensee Event Report.

• Revisions to operating procedures, surveillance procedures, and maintenance work order files have been implemented so that these documents provide guidance to defeat the automatic initiation of RCIC if the RCIC vacuum breaker function is unavailable and to provide cautions, as applicable, about water hammer concerns.

• Operations and appropriate Engineering personnel will be briefed on water cannon/hammer mechanism and the RCIC vacuum breaker function. The brief will include discussion of the need to prevent RCIC initiation if the vacuum breaker function is lost and the potential consequences on RCIC and primary containment if a water cannon were to occur. The briefing to plant operators will describe the importance of adequate and timely response to questions in maintaining a conservative approach to issues that could impact nuclear safety.

• Operator and engineering training programs will be improved with regard to potential RCIC water hammer events and water hammer mechanisms by revising lesson plans for initial and continuing training for the RCIC system to describe water cannon initiation mechanisms. This training material will clearly describe the vacuum breaker function and the potential consequences of operating RCIC while the vacuum breaker function is unavailable.

• A list of standard questions will be developed to assist industry operating experience (OE) reviewers in maintaining a broad view when reviewing OE. The list will include questions regarding enhancing procedures to assure appropriate cautions are given to operating and surveillance procedures and questions regarding enhancing operations and engineering training.

• External OE that is screened as applicable or potentially applicable will be entered into the corrective action program or, when implemented, into a new database, as individual records and assigned a single individual for review and response.

A. Failed Components:

None

B. Previous similar events:

None C. Identification of components referred to in this Licensee Event Report:

Components� IEEE 805 System ID� IEEE 803A Function Reactor Core Isolation Cooling System (BWR)� BN� — High Pressure Coolant Injection System (BWR)� BJ� — Containment Leakage Control System� BD� — Vacuum Breaker Valves� CKV Isolation Valves� ISV RCIC Turbine� TRB