05000335/LER-2014-001
| St. Lucie Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 3352014001R03 - NRC Website | |
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Description of the Event
On January 9, 2014, St. Lucie Unit 1 was in Mode 1 operation at 100 percent reactor power. The St. Lucie site was experiencing heavy rainfall. In the early afternoon the site storm drain system was challenged as the storm water basins started to back up.
At —1610 hours, operators reported that water was backing up through the -0.5 feet elevation floor drains in the reactor auxiliary building (RAB) and into the emergency core cooling system (ECCS) pump room sumps at the -10.0 feet elevation. The water was observed entering the RAB through an electrical junction box which was below the elevation for which the RAB flood protection was designed.
At —1620 hours, the control room received the B Safeguards Pump Room Sump Level High/High-High alarms.
At —1628 hours the operators entered procedure 1-AOP-24.01, "RAB Flooding," and immediately closed the ECCS sump isolation valves to preclude flooding of the ECCS pump room. However, the water level in the RAB -0.5 feet elevation continued to rise.
At —1635 hours, after pre-briefing the evolution and potential contingency actions, the operators took control and managed the RAB flooding by cycling the ECCS sump isolation valves to allow batch removal of RAB water via the ECCS sump pumps.
At —1732 hours, a higher capacity temporary pump at the yard sump was used to reduce the rate of water entry into the RAB.
At —1803 hours, an Unusual Event (UE) was declared as the significant rainfall exceeded the site's storm drain system capacity.
The UE was terminated at —0001 hours on January 10, 2014, after the significant rains subsided and the storm drain were observed draining site water accumulation.
All safety related accident mitigation and safe shutdown equipment remained operable prior to, during, and after the event.
Cause of the Event
The cause of the RAB flooding was due to legacy installation issues with two non-safety electrical conduits [EllS:CND] in the ECCS pipe tunnel that lacked the required internal flood barriers. Degradation of the conduits provided a water entry path into the RAB. The extent of condition identified four more conduits with the same legacy installation issue, for a total of six conduits without internal flood seals. Qualified flood seals have now been installed where these conduits terminate in the building in order to restore the RAB to a waterproofed condition.
The direct cause of the storm water basin back up that allowed the ECCS pipe tunnel to flood was blockage of the piping that connects east storm water basin to the overflow basin. The blockage that contributed to the storm drain system flow restriction has been removed.
St. Lucie Unit 1 05000335
Analysis of the Event
This condition is reportable in accordance with; a) 10 CFR 50.73(a)(2)(ii)(B) as an event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety; and b) 10 CFR 50.73(a)(2)(v)(A)/(B)/(D) as any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to shut down the reactor, remove residual heat, or mitigate the consequences of an accident.
Although this event did not result in any loss of function, postulated external flooding events could result in RAB water levels exceeding the analyzed RAB internal flooding event, as a result of the discovery of these missing conduit seals. This is contrary to design as it is assumed that design features seal the RAB below the +17.2 feet elevation from water sources external to the RAB.
The site is comprised of an east and a west drainage system that outflows to the east and west storm water basins, which in turn flow to the southwest overflow basin and to the southeast overflow basin through a 36-inch interconnection. The southeast overflow basin has a high level outflow valve that can be opened during excessive rainfall events. Due to flow blockage from the east storm water basin to the overflow basins, the severe rainfall caused the water level of the east storm water basin to rise above its banks and flood the plant area west of the G1 and G2 warehouses. Once the water level exceeded the elevation of the Unit 1 component cooling water (CCW) building floor (elevation +12 feet) the building no longer drained and allowed the backflow to flood the adjacent ECCS pipe tunnel and yard sump. The degraded conduit within the ECCS pipe tunnel, along with the lack of internal conduit flood barriers in the RAB side, provided a direct path for water to enter the RAB.
The water intrusion into the RAB continued until the storm drain blockage was corrected and the remaining volume of water in the ECCS pipe tunnel drained into the building. Operators took action to remove the flood water, by cycling the ECCS room sump isolation valves in order to utilize the installed sump pumps.
As previously noted, during this event there was no loss of safety related accident mitigation or safe shutdown equipment. Florida Power & Light (FPL) evaluated various external flooding scenarios to determine if any postulated conditions could have resulted in the loss of safety related accident mitigation or safe shutdown equipment.
Safety Significance
FPL concluded that although there were scenarios that may result in the loss of the chemical and volume control system pumps, manual operator actions to close the ECCS room sump isolation valves would protect the ECCS pumps from the flooding.
Corrective Actions
1. All conduits identified as lacking internal flood barriers during the extent of condition reviews were provided with qualified seals.
2. FPL is revising the original St. Lucie Unit 1 and 2 Fukushima Walkdown Reports to include additional information discovered during the extent of condition assessments resulting from this event. This action is being tracked under the Site Corrective Action Program (CAP).
Failed Component(s) Various non-safety conduits Similar Events On December 27, 2012, FPL reported degraded penetration seals within electrical manways that bypassed St.
Lucie Unit 1 RAB external flood protection features in LER 50-335/2012-010. The insights developed from the detailed St. Lucie flooding models being developed for the January 9, 2014 flooding event was applied to the 2012 LER condition, with the conclusion that St. Lucie Unit 2's internal flood analyses results were not bounded as previously documented. The 2012 LER was revised to reflect this information.
St. Lucie Unit 1 05000335