05000263/LER-2009-002
| Docket Number | |
| Event date: | 04-02-2009 |
|---|---|
| Report date: | 05-29-2009 |
| 2632009002R00 - NRC Website | |
Event Description
As part of an on-going station HELB improvement plan a review of piping structures for potential HELB effects were performed based on industry benchmarking, lessons learned and corrective actions from previous plant events. As part of this plan, a series of plant walk downs for pipe whip and jet impingement were scheduled for the plant. On April 2 and 14, 2009 with the plant shutdown and in Mode 5 for a refueling outage, plant personnel performed the scheduled HELB walk downs of inaccessible areas. During the walk down, two potential HELB pipe whip issues were found.
1. Potential Reactor Core Isolation Cooling (RCIC) [BN] HELB in the Steam Chase (April 2, 2009): A postulated break at MO-2107 would result in a jet force from the 4 inch RCIC pipe which would force the cantilever end of the broken RCIC pipe up and into the 3" Essential Service Water (ESW) [BI] Div 2. This force was determined to be enough to cause the ESW Div 2 pipe to fail. The result is no safe shutdown path would be available assuming a concurrent loss of off-site power and worse case single active failure would occur at the same time.
2. Potential Main Steam [SB] HELB in the Condenser Room [SG] (April 14, 2009): A postulated Main Steam HELB would produce a jet reaction force which could potentially whip into the steam bypass line and force it into the 3" Emergency Service Water (FSW) [BI] Div 2 line. The result is no safe shutdown path would be available assuming a concurrent loss of off-site power and worse case single active failure would occur at the same time.
Event Analysis
The station determined there was no current operability concern due to the condition being discovered while the plant was shutdown for a refueling outage and therefore the event was not reportable under 10 CFR 50.72. However, due to past operability concerns, the event is reportable under 50.73(a)(2)(ii) (B) "Any event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degrades plant safety," and a Licensee Event Report is required for this event.
The event is not considered a safety system functional failure since a single active failure is not required to be considered for the opposite division not affected by the HELB for the safety function determination.
Safety Significance
The Probabilistic Risk Assessment (PRA) group performed an evaluation of the risk. Additional risk from these deficiencies is considered to be very small based on the following:
- Specific HELB failures have very low frequency of occurrence. For the failures of interest, EPRI TR-102266 (Pipe Failure Study Update) provides estimates of 2.02 E 09/hr for the RCIC HELB and 9.63 E-10/hr for the steam line break. These failures equate to 1.8 E-5/yr and 8.4 E-6/yr respectively for a total frequency (one RCIC line and 4 steam lines) of approximately 5 E-05/yr.
o Given the RCIC or Main Steam Line break, it is not certain that the effect would be to impact the FSW supply line such that cooling would be lost to the FSW loads.
- FSW supplies emergency control room ventilation and low pressure Emergency Core Cooling System (ECCS) [BM] motor and room cooling, which are not important systems in the PRA. Loss of emergency control room ventilation [VI] has no impact on the frequency of core damage. Loss of room and motor cooling from FSW to the ECCS rooms and pumps does not impact the ability for the ECCS systems to perform their function as modeled in the PRA. Although FSW is required for Technical Specification operability, ECCS success was determined to exist upon failure of the FSW system.
o Conservatively assuming failure of the Division II ECCS systems as a result of FSW failure leaves many alternate injection systems that are fully capable of supporting the reactor vessel makeup and decay heat removal critical safety functions.
In conclusion, the risk of core damage attributable to either of the potential events discussed above is considered to be very small.
Cause
The issues were identified as part of an on-going station HELB improvement plan. A review of piping structures for potential HELB effects was implemented based on industry benchmarking, lessons learned and corrective actions from previous plant events.
The cause for the potential pipe whip issues was a failure by station personnel to identify and correct these potential issues during a HELB re-evaluation completed in 1986.
Corrective Action 1. The conditions of these events were corrected by a modification which eliminated the potential HELB issues on the 3" FSW-B line.
Failed Component Identification None
Previous Similar Events
MNGP LER 263-2000-004: An analysis of a high energy line break (HELB) on the 911ft elevation of the Turbine Building indicated flooding of the Division I 4kV switchgear room and possible loss of the Division I 4kV switchgear. With an assumed loss of offsite power, Division II Emergency Diesel Generator was considered the worse case single active failure.
Therefore, this event could potentially result in loss of the station AC power from both divisions of the 4kV distribution system. Modifications were installed to prevent water from entering the Division I 4kV switchgear room.
MNGP LER 263-2008-001: On January 31, 2008 during a review of the High Energy Line Break (HELB) calculations for the plant's Extended Power Uprate (EPU) project, it was determined that the existing HELB calculations failed to consider the actuation of the fire sprinklers in the condenser bay and the resultant flooding impact on the lower Division 1 4kV equipment. The station had previously installed a flood barrier near the 4kV Switchgear room door therefore present operability was not impacted. The station determined that prior to the installation of the barrier, there was a potential for the loss of the lower Division 1 4kV equipment. The cause of the event was a failure to consider the impact of the fire sprinklers.
Corrective actions taken or planned are: the flood barrier will remain in place and a revision of the affected HELB calculations will be performed.