05000461/LER-2015-001
| Clinton Power Station, Unit 1 | |
| Event date: | 02-06-2015 |
|---|---|
| Report date: | 04-07-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| Initial Reporting | |
| ENS 50794 | 10 CFR 50.72(b)(3)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 4612015001R00 - NRC Website | |
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PLANT AND SYSTEM IDENTIFICATION
General Electric -- Boiling Water Reactor, 3473 Megawatts Thermal Rated Core Power Energy Industry Identification System (EIIS) codes are identified in text as [XX].
EVENT IDENTIFICATION
A. Plant Operating Conditions Before the Event
Unit: 1 Event Date: 2/6/2015 Mode: 1 Mode Name: Power Operation
B. DESCRIPTION OF EVENT
Event Time: 2355 Central Time Reactor Power: 98 percent On 2/6/15 at 2300 CST, during an operator surveillance performed every shift, Division 1 Reactor Water Cleanup (RT) system flow indicator [Fl] instrument 1E31R614A read 12.9 gallons per minute (gpm), which is less than normal and greater than 10 gpm difference from its sister channel resulting in a failed channel check. The operators declared this instrument inoperable in accordance with Clinton Power Station Technical Specification (TS) 3.3.6.1, Primary Containment and Drywell Isolation Instrumentation, which required placing the channel in trip within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> per Required Action D.1.
At 2355, Division 2 RT system flow indicator instrument 1E31R614B read 0 gpm, which was a change from its previous reading with no process flow path changes. Based on this reading not being credible, operators declared this instrument inoperable, requiring entry into Required Action E.1 for two channels inoperable. Action E.1 requires restoration of isolation capability within one hour.
At 0036 on 2/7/15, a fill and vent of the Division 1 RT leak detection instrumentation was completed, restoring Division 1 to an operable status. At this time, the operators exited Action E.1.
At 0225, a fill and vent of the Division 2 RT leak detection instrumentation was completed, restoring Division 2 to an operable status. At this time, the operators exited Action D.1 and restored the RT high flow system isolation function to operable status.
During the time that both channels were inoperable, the leakage detection system was incapable of performing its containment isolation function for RT differential flow. This condition required an eight hour Event Notification phone call under 10CFR50.72(b)(3)(v)(C). Event Notification #50794 was made at 0637.
C. CAUSE OF EVENT
RT Isolation Valves [ISV] to the Main Condenser [COND] are leaking by resulting in air voids being allowed to enter into RT system piping causing the differential flow instrumentation to provide false readings. Water is leaking by either valves [V] 1G33F041 or 1G33F046.
Clinton Power Station, Unit 1 05000461
D. SAFETY CONSEQUENCES
RT Differential Flow greater than 59 GPM for longer than 45 seconds causes an automatic isolation of the RT System. The isolation function of RT is performed by differential pressure transmitters [PDT] arranged such that all the inputs and outputs are summed to determine losses from the system. The differential pressure transmitters [PDT] that failed were both on the letdown line flow path to the main condenser, which was isolated by multiple valves. Both instruments were found reading further away from the isolation signal than expected due to the presence of an air pocket in the system. This rendered the automatic leak detection / isolation function of the RT system inoperable and as such operations appropriately entered into the applicable TS Required Actions.
There were no actual consequences as a result of the loss of automatic containment isolation function of the affected RT containment isolation valves. This loss of function lasted for approximately 41 minutes.
During this time the manual isolation function was not affected.
Per CPS 4001.02, Automatic Isolation, a manual Group 4 (Reactor Water Cleanup System) isolation could have still been performed in the event that an automatic isolation was required. At no time during this event was the isolation capability of the system compromised, however the automatic isolation based on differential flow instrumentation was compromised until the fill and vent was completed for the system.
Since the automatic isolation capability was compromised, this event is being reported under 10CFR50.73(a)(2)(v)(C) as a loss of safety function condition.
E. CORRECTIVE ACTIONS
The letdown line was pressurized in accordance with CPS 3303.01, Reactor Water Cleanup (RT) to prevent the development of voids in this line from the suspected leaking valves 1G33F041 or 1G33F046.
Work Order packages have been created to repair internal leakage of valves 1G33F041 or 1G33F046.
F. PREVIOUS SIMILAR OCCURRENCES
No previous reportable events have occurred as a result of a loss of both divisions of RT differential flow.
G. COMPONENT FAILURE DATA
Component Description: 1G33F041, RWCU Blowdown Warming Bypass Valve Manufacturer: Fisher Controls Model: WCB TYPE EZ Component Description: 1G33F046, RWCU Drain Flow to Condensate Valve Manufacturer: Anchor Darling Valve Model: C20036