05000278/LER-2009-005
| Docket Number Sequential Revmonth Day Year Year Month Day Year 05000Number Na | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 2782009005R00 - NRC Website | |
Unit Conditions Prior to the Event The 'E' Wide Range Neutron Monitor (WRNM) was identified to have a degraded signal on 12/6/08. At this time, Unit 3 was operating at 100% rated thermal power. The 'C' WRNM was identified to have a degraded signal on 8/10/09. Unit 3 was operating at approximately 98% power on 8/10/09. At the time of these failures, the WRNMs were not required to be operable.
The 'A' WRNM was also identified to be degraded in January 2009. However, the cause of the 'A' WRNM degradation was not related to the 'C' and 'E' failures. There were no other structures, systems or components out of service that contributed to this event. Unit 3 entered Mode 2 on 9/13/09 at approximately 2220 hours0.0257 days <br />0.617 hours <br />0.00367 weeks <br />8.4471e-4 months <br /> in preparation for a planned refueling outage.
The WRNMs are required to be operable in Mode 2.
Description of the Event
As a result of a cause evaluation completed by Engineering personnel on 9/15/09 involving WRNM (EIIS: MON) failures, it was identified that the 'C' and 'E' WRNMs had failed as a result of an apparent common cause. The cause evaluation determined that both of these WRNMs failures were likely the result of a suspected degraded connection (EIIS: CON) between the upper cable (EIIS: CBL) and the WRNM detector element (EIIS: DET).
The 'E' WRNM was identified to have a degraded signal on 12/6/08 and the 'C' WRNM was identified to have a degraded signal on 8/10/09. When these WRNMs failed, they were not required to be operable. Technical Specification Limiting Condition for Operation (LCO) 3.3.1.1, Reactor Protection System (RPS) Instrumentation requires that at least three of the four channels of WRNMs be operable for each RPS (EllS: JC) trip system. There are 2 RPS WRNM trip systems. The 'A', 'C', 'E' and 'G' WRNMs belong to the 'A' RPS trip system, while the 'B', 'D', 'F' and 'H' WRNMs belong to the 'B' RPS trip system. The TS requires the WRNMs to be operable in Mode 2 (startup / hot standby operations) and Mode 5 (refuleing operations).
The WRNMs provide trips for a reactivity short period during these operations. Unit 3 entered Mode 2 on 9/13/09 at approximately 2220 hours0.0257 days <br />0.617 hours <br />0.00367 weeks <br />8.4471e-4 months <br /> in preparation for a planned refueling outage.
This report is being submitted pursuant to:
10CFR 50.73(a)(2)(vii) — Common Cause Inoperability — This occurrence is reportable under this criterion since more than one channel of the WRNMs associated with the 'A' RPS trip system were inoperable for an apparent common cause.
The 'C' and 'E' WRNMs were replaced during the refueling outage.
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Analysis of the Event
There were no actual safety consequences associated with this event. The 'A' RPS trip system was placed in trip prior to entering Mode 2 on 9/13/09. Therefore, the safety function of the 'C' and 'E' WRNMs was ensured to be met prior to entering the mode of applicability for the WRNMs.
The safety objective of the WRNM system (EIIS: IG) is to detect conditions in the reactor core that could potentially threaten the overall integrity of the fuel barrier. In Mode 2, the most significant source of reactivity change is due to control rod withdrawal. The WRNM provides mitigation of the neutron flux excursion. The safety analysis evaluates the consequences of control rod withdrawal events during Mode 2 operations that are mitigated only by the WRNM period-short trip function.
There are a total of eight WRNMs in the reactor core. The A and B RPS trip systems have independent WRNM inputs. The A, C, E, and G channels of WRNM provide inputs to the A RPS trip system. The B, D, F, and H channels of WRNM provide inputs to the B RPS trip system.
The safety analysis assumes that one channel in each trip system is bypassed. Therefore, six channels with three channels in each trip system are required for WRNM operability to ensure that no single instrument failure will preclude a scram from this function on a valid signal.
In Mode 2 (Startup / Hot Standby) and Mode 5 (Refueling), the WRNM system provides short period trips to RPS. The WRNM system provides inputs into the RPS circuitry to ensure a reactor scram occurs in the event that core reactivity increase (shortening period) exceeds a predetermined reference rate. The TS allowable value for WRNM short period is > 13 seconds.
The WRNM provides diverse protection from the Rod Worth Minimizer (RWM), which monitors and controls the movement of control rods at low power. The RWM prevents the withdrawal of an out-of-sequence control rod during startup that could result in an unacceptable neutron flux excursion. The RWM was not affected by this event.
In MODE 1, the Average Power Range Monitor (APRM) system and the Rod Worth Minimizer (RWM) provide protection against control rod withdrawal error events and the WRNMs are not required. The WRNMs are automatically bypassed when the mode switch is in the Run position (i.e., Mode 1 operations).
The WRNMs are provided by General Electric Company, NUMAC Chassis, Model # 304A3712G005.
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Cause of the Event
The cause of the degraded signal from the 'C' and 'E' detectors is suspected to be due to a degraded connection between the upper cable and the detector element. This conclusion was based on tests run on the WRNMs to determine the location of the failure.
There are several resistance spot welds and gas tungsten arc welded joints that carry the electrical signal from the detector element to the upper cable. Variation in the tensile or compressive loads, material strengths or weld strength could have combined with the random cumulative effects of radiation induced material fatigue to result in the open circuit failure.
Without the ability to perform a radiograph of the failed detector (due to the high radiation work evolution), the exact point of failure cannot be definitively determined. At this time, vendor analysis has not identified a systematic lot issue involving the WRNMs.
The 'C' detector had been in-core for approximately 6 years and the 'E' detector had been in core for approximately 4 years. Manufacturing information such as serial numbers, year of manufacture and installation dates were investigated resulting in no common concerns based on this information.
Corrective Actions
The 'C' and 'E' WRNMs were replaced in September 2009 during the P3R17 refueling outage.
The vendor manufacturing process is being reviewed for potential improvements.
Similar WRNMs associated with the 'C' and 'E' WRNM manufacturing batches will be evaluated.
Technical and manufacturing similarities between the WRNMs and the Local Power Range Monitors (LPRMs) and Traversing In-Core Probe system will be investigated.
Previous Similar Occurrences There were no previous LERs identified relating to inoperable WRNMs due to common cause concerns.
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