05000458/LER-2008-003
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | 06-27-2008 |
|---|---|
| Report date: | 08-26-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4582008003R00 - NRC Website | |
On June 27, 2008, it was determined that a safety-related reactor pressure instrument had been left out of the tolerance range allowed by Technical Specifications during the performance of a scheduled quarterly calibration on April 11, 2008. That condition existed until the next scheduled performance of that test on June 7th. The as-left setting of the instrument during the calibration on April 11th resulted in it being in an inoperable status that was not recognized. This condition existed for longer than the allowable outage time of 14 days in TS 3.3.4.2. This event is being reported in accordance with 10CFR50.73(a)(2)(i)(B) as operations prohibited by Technical Specifications.
INVESTIGATION AND IMMEDIATE ACTIONS
The affected instrument is a trip unit monitoring reactor pressure, and is part of the anticipated transient without scram (ATWS) circuitry. This instrumentation initiates a recirculation pump trip to add negative reactivity to mitigate the effects of an ATWS condition (i.e., reactor water level less than Level 2).
On April 11th, instrument technicians performed a channel functional test on the trip unit B21-N658A.
The as-found setting was low at 4.418 VDC. The calibration range is 4.547 + 0.02 VDC. It was re- calibrated to 4.541 VDC using a digital multi-meter (DMM).
On June 7th, technicians were performing the next scheduled channel function test on B21-N658A.
The as-found data was found out-of-specification high at 4.738 VDC, and was adjusted to 4.548 VDC using a different DMM than the April 11 test. The as-found value was outside of the Technical Specification allowable value, requiring the technicians to notify the Shift Manager and initiate a condition report.
The subsequent investigation of the June 7th test found that the magnitude of the as-found out-of- specification reading was approximately the same as the adjustment that was made on April 11th, but in the opposite direction. This discovery called into question the calibration of the two DMMs. On June 9th, both DMMs were sequestered and sent to the standards lab for calibration checks. The DMM used on April 11th was found out of calibration. The cause for that condition could not be determined.
The usage history of the out-of-calibration DMM was researched, and it was determined that there were no other cases where the operability of safety-related equipment could have been adversely affected.
CAUSAL ANALYSIS
Trip unit B21-N658A was inadvertently left in an out-of-tolerance condition on April 11th due'to a faulty DMM. At the next scheduled calibration, the condition was corrected since a different, correctly calibrated DMM was used.
At no point were the two DMMs used beyond their required calibration frequency. During both the trip unit calibrations, the technicians responded correctly to the as-found conditions.
Interim job expectations have been instituted regarding the discovery of out-of-tolerance settings during calibration checks. If the as-found setting is within Technical Specification allowable range, but outside the calibration tolerance, Operations and Maintenance supervision will be notified and a Condition Report will be initiated. If the as-found setting exceeds twice the allowable tolerance or is outside Technical Specification allowable range, a second piece of test equipment will be used to verify the finding, in addition to the above actions.
A revision to the procedures governing control of test equipment is being developed regarding its calibration frequency to minimize the possibility of out-of-tolerance settings. This action is being tracked in the station's corrective action program.
PRIOR OCCURRENCE EVALUATION
A review of Licensee Event Reports at River Bend Station in the last five years found no previous occurrence of a similar condition.
SAFETY SIGNIFICANCE
The ATWS circuitry initiates a recirculation pump trip to mitigate the effects of an ATWS event. The decrease in recirculation flow adds negative reactivity due to increased steam voiding in the core.
The ATWS pump trip occurs at low reactor water level (Level 2) to maintain level above the top of the core. The reduction of core flow reduces the neutron flux as well as reactor thermal power, therefore reducing the rate of coolant boil-off. The pump trip is credited in the ATWS analysis for peak vessel pressure, as documented in the River. Bend Station Technical Specifications and Updated Safety Analysis Report. That analysis shows that peak reactor pressure is maintained less than , the ASME Code limit of 1500 psig under ATWS conditions.' The ATWS circuitry consists of two independent trip systems, with two channels of reactor pressure and two channels of reactor water level in each trip system. Each trip system is a two-out-of-two logic for each function. Thus, either two water level or two pressure signals are needed to actuate a trip system. The outputs of the channels in a trip system are combined by a logic circuit so that either trip system will trip both recirculation pumps. The reported condition affected only one of the four reactor pressure instruments, and therefore affected only one trip system.
The miscalibration of the ATWS instrument could have potentially degraded the response to the ATWS event because of the increased core reactivity and corresponding increase in heat flux caused by a slight delay in the pump trip. The as-found setting on June 7th corresponded to a reactor pressure setpoint of 1214 psig, while the maximum pressure setpoint allowed by Technical Specifictions is 1165 psig. However, the resulting post-event peak pressure of approximately 1350 psig would have been well bounded by the maximum analytical limit of 1500 psig.
By design of the ATWS circuitry, the recirculation pumps are tripped after the time of peak neutron flux during the postulated event. For the event, the peak suppression pool temperature of 177F does not occur until approximately 25 minutes into the event. The as-found setting of the trip unit on June 7th could potentially have delayed the pump trip by a few seconds. Thus, the slight delay before the recirculation pump trip would have resulted in only a small change in heat load on the suppression pool, and pool temperature would have remained below the 185F acceptance limit.
This event was of minimal significance with regard to the health and safety of the public.
(NOTE: Energy Industry Component Identification codes are annotated as (**XX**).)