05000461/LER-2010-001
| Clinton Power Station, Unit 1 | |
| Event date: | 10-07-2009 |
|---|---|
| Report date: | 01-26-2011 |
| 4612010001R01 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
General Electric — Boiling Water Reactor, 3473 Megawatts Thermal Rated Core Power Energy Industry Identification System (EllS) codes are identified in the text as [XX].
EVENT IDENTIFICATION
Unanalyzed Leakage Pathway Affecting Residual Heat Removal A Pump Room Flooding Analysis
A. CONDITION PRIOR TO EVENT
Unit: 1 Event Date: October 7, 2009� Event Time: 2122 Reactor Mode: 1 Mode Name: Power Operation� Power Level: 97 percent
B. DESCRIPTION OF EVENT
A review of plant drawings on October 7, 2009, revealed a potential issue with a floor drain [DRN] in Residual Heat Removal (RHR) [BO] A Pump [P] Room (located on the 707' — 6" elevation). Plant drawings indicated that floor drain piping was connected to floor drains in the Radwaste Pipe Tunnel via a 4-inch diameter pipe embedded in the floor that is located along the western wall of the adjacent Control Building [NA] at elevation 720' - 6".
Most of the floor drain piping in this area is embedded in the concrete base mat of the Auxiliary Building or in the concrete floor of the Radwaste Pipe Tunnel at top of concrete elevation 720'-6". There is about a 10 foot long section of drain piping exposed in the Low Pressure Core Spray (LPCS) [BM] pump room. Attempts to determine if the floor drains were interconnected were inconclusive; therefore, as a conservative measure, the floor drains were assumed to be interconnected, and an isolation plate was welded into the exposed 4-inch pipe in the LPCS pump room on October 8, 2009, in accordance with Work Order 1274884-01. As a result of the plate installation, there is no longer a potential for the floor drains to communicate.
The floor drain piping was originally installed prior to the concrete pours in the lower elevations of the plant.
This installation pre-dates startup of the plant, and as such, it is a historical legacy issue. An apparent cause evaluation was performed under Issue Report (IR) 976295 which determined that the cause was inadequate design. This evaluation was completed on November 9, 2009.
The evaluation concluded that with respect to internal flooding as required by General Design Criteria (GDC) 4, Environmental and Dynamic Effects Design Basis, safe shutdown is assured as required by the licensing basis and all functional design requirements were met.
It was questioned whether this condition was a serious degradation of a plant safety barrier or an unanalyzed condition that significantly degraded plant safety. While it could be postulated that the RHR pump room would flood up to the elevation of the drain line, the suppression pool volume would decrease by the same amount. This condition contradicted Updated Safety Analysis Report (USAR) Section 3.8.4.1.1 auxiliary building design which states that in the event of a pipe rupture, the flooding in one compartment would not result in the flooding of any other compartment, and the failure of a pump suction line would not drain the suppression pool.
USAR Section 3.6 describes the acceptability of effects associated with the postulated rupture of piping. This section states that coincident with the piping failure, the functional failure of any single active component, a seismic event the level of a safe shutdown earthquake and a loss of offsite power are assumed to occur. The USAR states that the safety function would not be impaired beyond that required to bring the plant to a safe shutdown.
A moderate energy break of the RHR A suction piping has been calculated to result in a leakage rate of 206 gallons per minute (gpm) into the RHR A pump room. Assuming that the motor-operated RHR pump suction valve, 1 E12F004A, fails to close (i.e., failure of single active component), a seismic event and a loss of offsite power occur at the same time, the plant must be brought to a safe shutdown.
GDC 34, Residual Heat Removal, is satisfied through the use of redundant components and features assuming a single failure. (Reference USAR 3.1.2.4.5).
Leakage into the RHR A Pump Room is assumed to occur. Using the 206 gpm leakage rate, the room would fill up at a rate of 0.031 ft/min or 1.86 ft/hr.
The elevation of the RHR A Pump Room floor is 707"-6" and the room is designed to be watertight up to elevation 731'-5". The elevation of the line that connects to the RHR A pump room is 720'-6", or 13'-0" off the floor. Based on the leakage rate into the room, it would take approximately 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br /> to reach the elevation of 720'-6". The room would continue to fill.
The suppression pool design parameters, such as level and volumes, are shown in USAR Table 6.2-1. The suppression pool water volume at low suppression pool water level (730"-11") is 135,220 cubic feet (cu ft) (containment) plus 10,707 cu ft (drywell). The minimum suppression pool water level is 727'-1" (minimum vent coverage) per USAR 6.2.4.3.3 and mechanical design drawing M05-1069. This volume of water is approximately 224,000 gallons. Upper pool dump, which adds 14,748 cu ft (110,000 gallons), provides an additional 2'-0" (approx.) to the suppression pool, if needed. If suppression pool level were to approach the minimum suppression pool water level, Emergency Operating Procedures would direct the Control Room operators to dump the upper pool prior to reaching 727'-1". At a rate of 206 gpm, without upper pool dump, the suppression pool could lower to the minimum vent coverage level in approximately 18 hours2.083333e-4 days <br />0.005 hours <br />2.97619e-5 weeks <br />6.849e-6 months <br />. If the upper pool dump volume is considered, this would add 9 hours1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br />, for a total of 27 hours3.125e-4 days <br />0.0075 hours <br />4.464286e-5 weeks <br />1.02735e-5 months <br />, to reach the minimum design level irrespective of any flooding issues in the RHR A pump room. Additionally, operators can add water to the suppression pool from the Cycled Condensate Storage Tank (CST) [KA] using CPS Procedure 3208.01, Cycled/Makeup Condensate. This procedure provides a means of gravity draining of the CST to the suppression pool. The CST has a usable volume of 312,000 gallons and provides additional several hours of water inventory.
Assuming that a loss of offsite power occurs at the time of the piping failure in the RHR A pump room, a reactor scram would occur accompanied with an isolation of the main steam isolation valves. The Reactor Core Isolation Cooling (RCIC) [BN] system would initiate on low water level and would inject automatically.
RCIC system operation would continue to occur until reactor pressure is reduced to 150 psig. No credit is taken for water additions to the suppression pool from the RCIC tank due to RCIC operations. However, the volume of the RCIC tank contains several hours of RCIC operation (about 125,000 gallons) prior to switching to the suppression pool.
High Pressure Core Spray [BG], Low Pressure Core Spray, and RHR C system are available to continue the cooldown to cold shutdown and RHR B can be operated in the Shutdown Cooling mode to maintain the unit in cold shutdown. Cold shutdown can be achieved in about 11 hours1.273148e-4 days <br />0.00306 hours <br />1.818783e-5 weeks <br />4.1855e-6 months <br /> with only one heat exchanger in operation (reference USAR Fig. 5.4-12). This is well within the time before the suppression pool would lower to the minimum suppression pool water level of concern.
Based upon the NRC's review of Exelon's contest letter (Reference) regarding non-cited violation 05000461/2010003-01, for the failure to meet the requirements of 10 CFR 50.72, Immediate notification requirements for operating nuclear reactors, and 10 CFR 50.73, Licensee event report system, it was determined that the as-found condition had the potential, given a postulated pipe rupture of the RHR A pump suction line, to drain the suppression pool via the radwaste pipe tunnel outside secondary containment until the suppression pool level was below the Control Building floor drain level (720'-6", or 8' 6" suppression pool level). This condition would result in the plant being in an unanalyzed condition that significantly degraded plant safety and could have prevented the fulfillment of the emergency core cooling system since this level is below any of the levels described in the Technical Specification Bases for the ECCS systems.
This LER revision replaces the voluntary LER previously submitted on March 25, 2010.
C. CAUSE OF EVENT
The cause of this event was determined to be a historical design oversight during plant construction that allowed the RHR A pump room floor drains to be connected to the radwaste pipe tunnel.
D. SAFETY CONSEQUENCES
This event is reportable under 10 CFR 50.73 (a)(2)(ii)(B) and 50.73 (a)(2)(v)(D) as a condition that would result in the plant being in an unanalyzed condition that significantly degraded plant safety and could have prevented fulfillment of the emergency core cooling systems.
While it is possible that operators could take actions to stop the leak and suppression pool level decrease as described above, these actions are not described in the USAR and cannot be credited in determining if ECCS can perform its function in the postulated event. This event had the potential to drain the suppression pool below the level that is analyzed in the Technical Specification Bases for the ECCS pumps.
E. CORRECTIVE ACTIONS
The original design issue was corrected with the installation of a welded plate in the floor drain line. An Engineering Change 377321 was initiated to capture the welded plate into plant design documents.
F. PREVIOUS OCCURRENCES
None
G. COMPONENT FAILURE DATA
None
H. REFERENCE
H. REFERENCE
Letter EA-10-185 from C. D. Pederson (NRC) to M. J. Pacilio (Exelon Generation Company, LLC), "Response to Disputed Non-Cited Violation — Clinton Power Station NRC Inspection Report 05000461/2010-003," dated November 29, 2010