05000369/LER-2003-003
| Mcguire Nuclear Station, Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 3692003003R00 - NRC Website | |
BACKGROUND
Applicable Energy Industry Identification System (EIIS) and component codes are enclosed within brackets. McGuire unique system and component identifiers are contained within parentheses.
McGuire Nuclear Station (MNS) Unit 1 is a Westinghouse Pressurized Water Reactor with an ice condenser containment design.
10 CFR 50.44 "Standards for combustible gas control system in light-water- cooled power reactors" requires units with ice condenser containments to install suitable hydrogen control systems to reduce the potential for breach of primary containments due to a hydrogen oxygen reaction in post accident environments. The system must be capable of handling an amount of hydrogen equivalent to that generated from a metal water reaction involving 75% of the fuel cladding surrounding the active fuel region (excluding the plenum volume.) This requirement was placed on ice condenser units because of their small containment volume and low design pressure (compared with pressurized water reactor dry containments). The MNS Hydrogen Mitigation System (HMS) [BB] provides this required capability.
The function of the HMS is to employ a method of controlled ignition, using thermal ignitors, to reduce the hydrogen concentration in an ice condenser containment following a degraded core accident. The HMS was installed to address beyond design basis accidents as a post-TMI requirement according to 10 CFR 50.44. Per emergency procedures, the HMS is utilized in conjunction with the Hydrogen Recombiners and the Containment Air Return and Hydrogen Skimmer System to maintain hydrogen concentrations in containment below explosive limits. At McGuire, a total of 70 ignitors (35 per train) are distributed throughout the various regions of containment in which hydrogen could be released or to which it could flow in significant quantities. Each containment region has two ignitors, one per train, controlled and powered redundantly so that ignition would occur in each region even if one train failed to energize. McGuire utilizes glow plugs, typical of those used in diesel engines, as the hydrogen ignitors. The ignitors are non-safety related.
Technical Specification (TS) 3.6.9 governs the HMS and is applicable in Modes 1 and 2. TS 3.6.9 requires that two HMS trains be operable in Modes 1 and 2. With one HMS train inoperable per Condition A, Required Action A.1 requires that the HMS train be restored to operable status within 7 days, or alternatively, per Required Action A.2, SR 3.6.9.1 may be performed on the operable train once per 7 days. There is no condition that allows more than one train of HMS to be inoperable.
EVENT DESCRIPTION:
On May 1, 2003, Unit 1 was in Mode 1 at 100 percent power. Train B of the HMS system was declared inoperable at 1230 hours0.0142 days <br />0.342 hours <br />0.00203 weeks <br />4.68015e-4 months <br /> to complete a modification to replace cables feeding the Train B ignitors in the ice condenser region. The day crew began the cable replacement effort and turned the job over to the night shift. While the night shift was performing this task, a cable feeding the Train A ignitors in the ice condenser region was inadvertently cut. Operations was immediately notified. Since Technical Specification (TS) 3.6.9, "Hydrogen Mitigation System' does not contain a condition statement for two inoperable trains, TS 3.0.3 was entered on May 2, 2003 at 0115 hours0.00133 days <br />0.0319 hours <br />1.901455e-4 weeks <br />4.37575e-5 months <br /> and a unit shutdown was initiated. The Train A cable was repaired and Unit 1 exited TS 3.0.3 at 0741 on May 2, 2003 prior to completion of the unit shutdown. During the time that both HMS trains were inoperable, 16 required ignitors (6 on Train A and 10 on Train B) were rendered inoperable.
The event is being reported pursuant to the following requirements:
■ 10CFR50.73(a)(2)(i)(B), Any operation or condition prohibited by the plant's Technical Specifications
- 10CFR50.73(a)(2)(v)(D), Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident
CAUSAL FACTORS
The cause of this event has been attributed to human error. Personnel involved with inadvertently cutting the wrong (Train A) cable failed to appropriately apply human error prevention elements during task turnover and demonstrated a lack of rigor during task execution.
CORRECTIVE ACTIONS
Immediate:
1. Technical Specification 3.0.3 was entered and unit shutdown commenced.
2. The Train A cable was repaired and TS 3.0.3 was exited prior to entry into Mode 3.
Subsequent:
1. The individuals involved with inadvertently cutting the wrong (Train A) cable were counseled.
2. Prior to this event, training regarding verification techniques including correct components was initiated for Maintenance personnel.
The individuals on the Maintenance crew involved in this event had not yet received the training when the event occurred. This training is now complete.
Planned:
1. Maintenance turnover guidance, including necessary protocol and roles and responsibilities, will be enhanced as appropriate.
SAFETY ANALYSIS
The HMS provides no function required to safely shutdown the reactor.
The system provides an additional means of preventing the accumulation of high concentrations of hydrogen which may be generated after a Loss of Coolant Accident (LOCA). The system's function is not required in the mitigation of any design basis event.
A probabilistic risk assessment evaluation shows that the worst case assumption results in a negligible increase in the large early release probability for the time period where both trains were inoperable in the ice condenser upper plenum. The increase in LERF is expected to come from those sequences where early containment failure would normally be prevented by operation of the ignitors. The frequency of these sequences is approximately 2.6E-05/year. With the assumed containment failure probability and a 6.5 hour5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> period with no upper plenum ignitors, the increase in large early release probability is estimated to be less than 5E-09. This increase is very conservative in its estimation and represents an insignificant increase in the probability of a large early release at McGuire.
Based upon the above, the risk attributable to the short period of time that both trains were inoperable in the ice condenser upper plenum is not significant with respect to the health and safety of the public.
Additional Information
This event is considered a safety system functional failure under the Reactor Oversight Process. A search of the McGuire Electronic Licensing Library did not identify any safety system functional failures during the past three years that involved the HMS or that had the same underlying concern or reason as this event. Therefore, no previous similar events were identified.
i �