05000305/LER-2012-005
| Docket | |
| Event date: | 06-27-2012 |
|---|---|
| Report date: | 08-23-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| Initial Reporting | |
| ENS 48051 | 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition |
| 3052012005R00 - NRC Website | |
Event Description:
On June 27, 2012, at 0900, with the Kewaunee Power Station (KPS) in MODE 1, operating at 100% power, with no contributing structures, systems, or components inoperable, a gas void was identified in the Safety Injection (SI) [BQ] 12-inch diameter Common Suction Piping and in the 2-inch diameter SI Bypass Piping.
The void was discovered during the routine quarterly monitoring for gas voids. The SI Bypass Piping was found to be completely voided from valve SI-31 [V] to the connection to the 12-inch SI Common Suction Piping near valve SI-4B. The SI Common Suction Piping at valves SI-4A and SI-4B was found to be partially voided. An Operability Determination was performed which concluded that the SI System was Operable but Nonconforming.
To correct the Nonconforming condition KPS staff vented the gas voids. While venting of the void in the common suction line and bypass line for the safety injection pumps [P] from 1446 to 1500 June 27, 2012, both Trains of Safety Injection were declared inoperable, as documented in Kewaunee's corrective action program (CAP) condition report (CR) 480163. This condition was determined a NRC reportable event under 10 CFR 50.72(b)(3)(ii)(B) as an unanalyzed condition because current KPS analyses do not support the vent line on the SI pumps' common suction or bypass line being open. An event notification report was made to the NRC on June 27, 2012, reference Event Notification number 48051.
Post-venting ultrasonic test (UT) examination of the SI piping determined the SI Common Suction Piping to be full of water, while a void remained in the SI Bypass Piping which was determined to be less than 2 cubic inches (less than 0.001 cubic feet), well below 0.01 cubic feet (procedural definition of sufficiently full).
The Emergency Core Cooling System (ECCS) for KPS consists of safety injection and residual heat removal (RHR)[BP] pumps, accumulators [ACC], containment sump, RHR heat exchangers [HX], and the refueling water storage tank (RWST) [TK], along with the associated piping, valves, instrumentation, and other related equipment.
In Modes 1, 2, and 3, plant Technical Specification (TS) 3.5.2 "ECCS - Operating," requires two independent ECCS trains be operable, with each ECCS train consisting of an SI subsystem and an RHR subsystem.
Each train includes the piping, instruments, and controls to ensure an OPERABLE flow path capable of taking suction from the RWST upon an SI signal and manually transferring suction to the containment sump.
During an event requiring ECCS actuation, a flow path is required to provide an abundant supply of water from the RWST to the reactor coolant system (RCS) [AB] via the ECCS pumps and their respective supply headers to each of the two cold leg injection nozzles (SI pumps) and RCS vessel injection nozzles (RHR pumps). In the long term, this flow path may be switched to take its supply from the containment sump and to supply its flow to the RCS cold legs or vessel.
In Mode 4, plant TS 3.5.3 "ECCS Subsystems — Shutdown," requires one ECCS train be operable. In MODE 4, an ECCS train consists of a safety injection subsystem and an RHR subsystem. Each train includes the piping, instruments, and controls to ensure an OPERABLE flow path capable of taking suction from the RWST and transferring suction to the containment sump.
Because the effects of an open vent line on the common SI Pump suction or bypass line are not analyzed, potentially rendering both trains of SI inoperable, this condition is being reported pursuant to 10 CFR 50.73(a)(2)(ii)(B), as any event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety, 10 CFR 50.73(a)(2)(v), as any condition or event that could have prevented fulfillment of a safety function, and 10 CFR 50.73(a)(2)(vii), as any event or condition where a single cause or condition caused at least one independent train or channel to become inoperable in multiple systems or two independent trains or channels to become inoperable in a single system.
Event and Safety Consequence Analysis:
Current analyses do not support operability of the Safety Injection pumps with either of the vent valves on the common pump RWST suction line open or the vent valve on the RWST SI pump suction isolation valves bypass line open. The SI pumps are required during a design bases accident to makeup a loss of RCS inventory where the inventory loss is greater than the capacity of the charging pumps and not enough to depressurize the RCS to less than the shutoff head of the RHR pumps.
Prior to the time the vent valves were open when the nonconforming void existed in the piping, there was no increased risk because the pumps were capable of performing their intended function. While the vent valves were open, a dedicated operator was standing by with written instructions and in constant communication from the control room, ready to close the valve if a Safety Injection signal had occurred. The pumps were in auto and all other valves were aligned in their normal alignment. Therefore, there was minimal increased risk due to this event.
Based on the above, there was minimal safety consequence associated with this condition.
Cause:
The cause of this event was determined to be the planned and controlled opening of vent valves to vent a void in the SI pumps' common suction piping. The most probable cause of the void in the common SI pump suction piping and bypass piping was a pressure drop across the throttled butterfly valve RHR-101 [RHR HX Bypass Flow Control Valve] during reactor cavity drain down, which can cause dissolved gasses to come out of solution.
Corrective Actions:
As an immediate corrective action, the sections of piping, where the void was located, were vented to remove the gas void and return the SI System to a conforming and fully operable condition.
Additional planned corrective actions include:
1 Analyzing the affects of performing venting activities while at-power for this section of piping to determine a process by which acceptable accident analysis results can be obtained.
2. Develop a scheduling link to ensure the UT examination of this location is performed after completing the Reactor Cavity drain down and before entering a Mode where the Safety Injection System is required to be Operable.
Similar Events:
A review of Licensee Event Reports covering the previous three years did not identify any similar events.