05000306/LER-2012-002

LER-2012-002, Unit 2 Emergency Diesel Generators Inoperable Due To Missing Flood Control Barrier Seal

Docket Number
Event date:	07-20-2012
Report date:	04-04-2013
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor
3062012002R01 - NRC Website

Prairie Island Nuclear Generating Plant (PINGP) Administrative Work Instruction 5AWI 8.9.0, Internal Flooding Drainage Control, Section 6.2.9 (Pipe Trench in D5/D6 Building) step a.3 states that The concrete trench covers and the sealant between the trench covers and the surrounding floor must be in place whenever there is a possibility of a High Energy Line Break (HELB) in the Unit 2 Turbine Building." Section 6.2.10 (D5/D6 Emergency Diesel Generator Building) step c.2 states that The sealed concrete trench covers must be in place whenever Unit 2 is in Mode 1, Mode 2, Mode 3, and Mode 4 when the Main Steam Isolation Valves (MSIVS) are open.

Contrary to 5AWI 8.9.0, on June 25, 2012, insulators removed sealant around a concrete pipe trench cover located in the D6 future inverter room under Work Order (WO) 437670 task 04 without implementing compensatory actions to prevent internal flooding from impacting the D5/D6 Emergency Diesel Generators (EDGs)1 in case of a HELB event. Although not recognized, this condition made D5 and D6 inoperable. Then on June 28, 2012, site maintenance removed the D6 future inverter room concrete pipe trench cover under WO 437670 task 02 without implementing compensatory actions to prevent internal flooding from impacting the D5/D6 EDGs in case of a HELB event. The concrete pipe trench cover was re-installed on June 29, 2012, under WO 437670 task 03.

On July 9, 2012 the concrete pipe trench cover was removed under WO 437670 task 06. During a walkdown, on July 20, 2012 a station engineer identified that the trench covers and sealant were missing. At 1400, D5 and D6 EDGs were declared inoperable and Limiting Condition for Operation (LCO) 3.8.1 AC Sources — Operating Condition B (One DG inoperable) and Condition E (Two EDGs inoperable) were entered. This condition is reportable per 10 CFR 50.73(a)(2)(i)(B), operations or condition prohibited by Technical Specifications. On July 20, 2012 at 1505 hrs the concrete pipe trench cover and flood control barrier seal was restored and the LCO conditions were exited associated with D5 and D6 EDGs.

EVENT ANALYSIS

An analysis has determined that in the event of a Turbine Building HELB there is a potential for damage to other piping systems with unlimited water sources as a result of displacement of the failed high energy pipe (pipe whip). Depending on the location of the HELB, the Cooling Water (CL) system can be significantly damaged. In addition, the HELB environment can cause the Fire Protection (FP) sprinkler system to actuate depending on the location and size of the break. This results in HELB induced internal flooding. The CL and FP systems would then supply a continuous flow of water into the Turbine Building until operators take manual action to stop the flow. Procedures and isolation methods are in place to isolate the source of water from the HELB.

The trench that runs between the Unit 2 Turbine Building and the D5/D6 building creates a flow path for floodwater to enter the D5/D6 building. The trench contains several pipes and conduits that displace water and increase the hydraulic resistance of the flow through the trench. This hydraulic 1 EIIS System Code DG resistance is incorporated into the flow path parameters through the use of the hydraulic diameter and flow path length. For conservatism and simplicity, additional hydraulic resistance due to piping hangers is not credited. The trench covers in the D5/D6 building are in place as flood barriers to stop the water from entering into the building. Based on engineering evaluation, both D5/D6 EDGs (and associated D5/D6 building equipment) will perform their safety function with the southernmost trench cover plug open. This condition is not reportable per 10 CFR 50.73(a)(2)(v)(A) as an event or condition that could have prevented the fulfillment of a safety function, and not reportable per 10 CFR 50.73(a)(2)(ix)(A), single cause that could have prevented fulfillment of the safety functions of trains or channels in different systems. This condition is reportable per 10 CFR 50.73(a)(2)(i)(B), operations or condition prohibited by Technical Specifications.

SAFETY SIGNIFICANCE

This issue had no nuclear, radiological, industrial, or environmental impact. Internal flooding of the Turbine Building and D5/D6 building did not occur, so the Structures, Systems and Components (SSCs) in the D5/D6 building were not affected and both EDGs could have started and run if required.

In addition, off site power was available. Therefore, this event did not affect the health and safety of the public.

CAUSE

The Root Cause of this event is attributed to a previously identified flood barrier issue, for which an Apparent Cause Evaluation (ACE) was performed. At the time this ACE was performed, there were no requirements to perform an extent of cause evaluation for ACEs that were identified as a Significant Condition Adverse to Quality (SCAQ). The ACE did not evaluate the extent of cause and an opportunity was missed to identify and correct this condition.

CORRECTIVE ACTION

The flood barriers were restored by the work order process.

The trench covers were labeled "Flood Barrier" The Corrective Action to Prevent Recurrence (CAPR) was to proceduralize the requirement that ACEs that address SCAQs require performance of an extent of cause. The corrective action process procedure, FP-PA-ARP-01, "CAP Action Request Process," was revised subsequent to the event to include the requirement to perform an extent of cause evaluation.

PREVIOUS SIMILAR EVENTS

Cooling Water System," May 3, 2010.