05000336/LER-2001-006
| Event date: | 06-12-2001 |
|---|---|
| Report date: | 08-13-2001 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 3362001006R00 - NRC Website | |
1. Event Description On June 12, 2001, a Condition Report was generated to document a concern expressed by the NRC Resident Inspector relating to a work activity conducted on May 11, 2001 at Millstone Unit No. 2 (MP2). On May 11, 2001, with the unit operating in Mode 1 at 100% reactor power, welding was performed on the MP2 Turbine Driven Auxiliary Feedwater [BA] Pump [P] (TDAFWP) enclosure door [DR]. The work involved a weld buildup on the door closure mechanism to improve the overall sealing capability of the door. The previous degraded condition of the closure mechanism had been evaluated and was determined to meet the minimum acceptance standard for HELB mitigation. The work was intended to restore the door to a fully qualified condition.
This door is designated as a High Energy Line Break (HELB) barrier due to the four inch steam supply line feeding the TDAFWP. The nature of the work required the door to be open for approximately 45 minutes. During this time, although operators remained cognizant of the work activity, no compensatory measures were taken to protect the adjacent Motor Driven Auxiliary Feedwater Pumps (MDAFWP) from the consequences of a HELB originating from within the enclosure and no Technical Specification ACTION statements were acknowledged during the work evolution. Additionally, the Final Safety Analysis Report (FSAR) credited safe shutdown strategy for this event calls for a normal plant shutdown using the main feedwater [SJ] and condensate [SG] systems. This equipment would also be impacted by the adverse environmental conditions created by the break. Because the main feedwater and condensate systems are non-safety related, their continued availability under extreme environmental conditions is unanalyzed.
This event was determined to be reportable under the provisions of 10 CFR 50.73(a)(2)(ii)(B) as an unanalyzed condition that significantly degraded plant safety. This condition has not been classified as a safety system functional failure.
2. Cause The cause of this event is under investigation and will be reported in a supplement to this report. Corrective actions to date include the revision of existing plant procedures governing work on hazard boundary doors to require entry into applicable Technical Specification ACTION Statements.
3. Assessment of Safety Consequences There were no actual consequences experienced as a result of this condition and the safety significance of this condition is judged to be low. The probability of the initiating event is on the order of 9.8 E-10/hr and the actual exposure time was less than one hour. The corresponding Incremental Core Damage Probability for this condition is estimated at 1.2E-10. The potential outcome and significance of a TDAFWP steam supply line rupture concurrent with the TDAFWP enclosure door being open are described below.
The increased steam flow from the four inch TDAFWP steam supply line break is not sufficient to cause a plant trip.
Consequently, the MP2 licensing basis recovery strategy credits normal plant shutdown using the main feedwater and condensate systems. The rupture itself would be isolated from a remote location in accordance with existing plant procedures. The event duration is expected to be less than one hour.
With the enclosure door impaired, environmental conditions (i.e., temperature, humidity) outside of the enclosure following a TDAFWP steam supply line break would rapidly escalate. Similar environmental effects would be experienced by the main feedwater and condensate system components. These components are non-safety related and not subject to qualification testing. As such, their continued availability under these conditions is unanalyzed. Additionally, the predicted environmental conditions following the TDAFWP steam supply line rupture exceed those used for qualifying the MDAFWP's. If the main feed and condensate systems and the MDAFWP's become unavailable as a result of the event, existing plant emergency procedures would require a reactor trip followed by a rapid cooldown and depressurization of the RCS to support reaching conditions necessary for initiation of decay heat removal using the residual heat removal system.
4. Corrective Action An investigation is in progress and corrective actions will be addressed in accordance with the Millstone Corrective Action Program. Corrective actions to date include the revision of existing plant procedures governing work on hazard boundary doors to require entry into applicable Technical Specification ACTION Statements.
5. Previous Occurrences No similar events/conditions were identified during the 24 months preceding this condition.
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].