05000423/LER-2004-002
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| Event date: | |
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| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4232004002R01 - NRC Website | |
1. Event Description On May 26, 2004, while operating in Mode 1 at 100 percent power, an investigation determined that a potential existed for the operability of the 'B' motor driven auxiliary feedwater (MDAFW) [BA] pump to have been impacted by a degraded service water [BI] piping connection that was identified and corrected during the unit's most recent refueling outage on April 15, 2004. The degraded piping condition was inadequate braze bonding and a 360 degree crack of the brazing in a socket joint of the 3 x 3 x 3/4 inch tee in the 3 inch diameter 'A' service water Engineered Safety Features (ESF) supply line (3-SWP-003-67-3) to the residual heat removal and containment recirculation pump ventilation units (HVQ), and the safety injection pump lube oil cooler (CCI).
No unavailability of safety related equipment or systems, piping failure or flow diversion, had resulted from the affected joint. However, upon disassembly and inspection of the joint, the braze bond was found to be deficient. I Conservatively assuming that there had been no bond strength (no braze bond material), analysis of the affected socket joint shows a potential for pipe separation under seismic conditions. A pipe separation at this location would create a service water spray hazard to the motor of the 'B' MDAFW pump. The spray hazard was I determined to represent a potential to render the pump unavailable. The 'A' MDAFW pump and turbine driven auxiliary feedwater pump would have been unaffected by the break.
This historical condition potentially affected the credited design function of the auxiliary feedwater system coincident with a seismic event. Technical specifications 3.7.1.2, Auxiliary Feedwater System, requires restoration of an inoperable auxiliary feedwater pump within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. Installation of the affected joint with the deficient braze bonding was original construction. The inoperability of the 'B' MDAFW pump would constitute a condition that is prohibited by the plant's technical specifications and is being reported pursuant to reporting criteria of 10 CFR 50.73(a)(2)(i)(B). This is revision 1 to the LER 2004-002-00 that was issued on July 26, 2004. I 2. Cause This condition is historical in nature and the apparent cause for the condition was a poor work practice during original installation.
The affected socket joint and piping in the 3 inch tee remained intact until the socket was cut away and disassembled during inspection. Disassembly of the joint showed that the silver braze had cracked and was not fully bonded to surfaces within the piping socket. The joint was quartered axially to examine the condition of each of the tee's brazes. In addition, the corresponding piping tee on the opposite train ('B' train) ESF supply was also removed and axially quartered. These inspected brazes were concluded satisfactory, with the exception of the one affected piping socket that is described by this condition.
3. Assessment of Safety Consequences The condition resulted in a potential for a loss of service water from the 3 inch diameter 'A' service water supply line to the residual heat removal and containment recirculation pump ventilation units (HVQ) and the safety injection pump lube oil cooler (CCI) following a postulated seismic event. The 'B' service water header was not adversely affected by this condition. An evaluation of the potential for service water flow diversion concluded that all other safety related loads on the 'A' service water header would have received adequate flow. For the postulated scenario, the loss of cooling to the affected 'A' train ESF equipment could be mitigated as there is sufficient time to restore ventilation to the affected residual heat removal pump room by installation of portable fans, opening doors, and other similar temporary measures that would maintain required cooling following a seismic event. The containment recirculation pump ventilation unit and safety injection pump lube oil cooler are not required equipment following a seismic event. The postulated piping separation from a seismic event could cause flooding in the cubicle above the 'B' MDAFW pump that could overflow upon the MDAFW pump motor and adversely impact the availability of the pump. The AFW system provides a supply of high pressure feedwater to the secondary side of the steam generators for reactor coolant system (RCS) heat removal. There are two motor driven AFW pumps, and one turbine driven AFW pump. The design of the AFW system protects against single failures as well as providing power source diversity. This condition would not have impacted the redundant 'A' train MDAFW pump or turbine driven AFW pump. An evaluation of the flooding associated with this scenario concluded that no other equipment required for safe shutdown following a seismic event would be adversely affected.
The seismic fragility of the subject service water piping connection could not be analyzed. However, a sensitivity analysis was performed to assess the potential core damage and large early release risk impact of this condition.
The scenario analyzed in the sensitivity analysis is based upon the following:
a. a seismic event at the Safety Shutdown Earthquake (SSE) level (0.17g), b. the 3 inch diameter 'A' train service water supply line to the ESF building coolers fails, c. the 'A' service water train continues to supply all its loads following the supply line failure except those supplied from the affected ESF header (i.e., the 'A' train residual heat removal and containment recirculation pump ventilation units (HVQ) and the safety injection pump cooler (CCI) are assumed failed), d. offsite power is lost due to the seismic event and cannot be restored within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, e. the station blackout (SBO) diesel fails due to the seismic event and cannot be recovered, f. the 'B' train MDAFW pump fails from the service water flooding/spray, and g. the operators are successful in isolating the service water flood via a manual isolation valve before affecting any other risk significant equipment in the ESF building.
Based on the seismic hazard analysis for Millstone Unit 3, the frequency of exceeding the SSE is approximately 1E-4 per year. Based on the SSE exceedence frequency and the above assumptions, the changes in core damage and large early release frequencies from this condition were calculated to be 4.3E-07 per year and 3.4E-09 per year, respectively. This level of risk is characterized as very small per Regulatory Guide 1.174, "An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis.
4. Corrective Action The corrective action to prevent recurrence of this specific condition has been completed during the refueling outage. The 'A' and 'B' ESF supply header tees were cut out and replaced with new 3 inch butt-welded piping and branch weldolets.
A multi-year program is continuing to replace major portions of the brazed joint service water system piping to preclude similar brazed joint failures. The corrective actions associated with this condition are being addressed in accordance with the Millstone Corrective Action Program.
5. Previous Occurrences No previous similar events/conditions were identified.
Energy Industry Identification System (EDS) codes are identified in the text as [XX].