05000249/LER-2011-001
| Dresden Nuclear Power Station, Unit 3 | |
| Event date: | 01-15-2011 |
|---|---|
| Report date: | 06-13-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2492011001R01 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
Dresden Nuclear Power Station (DNPS) Unit 3 is a General Electric Company Boiling Water Reactor with a licensed maximum power level of 2957 megawatts thermal. The Energy Industry Identification System codes used in the text are identified as [XX].
A. Plant Conditions Prior to Event:
Unit: 03 Reactor Mode: 1 Event Date: 01-15-2011 Event Time: 1216 hours0.0141 days <br />0.338 hours <br />0.00201 weeks <br />4.62688e-4 months <br /> CST Mode Name: Power Operation Power Level: 100 percent
B. Description of Event:
On January 15, 2011, while performing Technical Specifications (TS) surveillance required control rod exercising on Dresden Nuclear Power Station (DNPS) Unit 3, control room personnel observed that at times, the rod block monitor (RBM) [JD] would indicate more local power range monitors (LPRMs) than expected.
The RBM count circuit appeared to not bypass LPRM inputs for some three LPRM string Control Rod Drives (CRDs) [AA] (a fourth LPRM string was included for several CRDs that apply three LPRM strings for signal determination). This situation was not present when a control rod was selected that had either two or four LPRM strings associated with it. Based on the information at the time, the operators concluded that the RBM remained operable.
Subsequent troubleshooting on January 21, 2011 of the RBM anomalies associated with CRDs with three associated LPRM string assignments, determined, that both RBM channels were inoperable. The troubleshooting revealed that the RBM upscale trip and the RBM INOP count circuit for LPRM inputs were non-conservative.
With both channels of the RBM inoperable, a channel is required to be placed in trip within one (1) hour in order to comply with TS 3.3.2.1 Condition B. However on January 15, 2011, the operators did not properly identify this inoperability. Therefore the required action was not completed within the completion time specified by the plant's TS. The failed card was replaced on January 21, 2011, and the system returned to an operable condition.
The RBM is designed to monitor local power using two to four LPRM strings that are in the local area while a CRD is being withdrawn. The number of LPRM strings utilized is based on CRD location within the core. The RBM is required to be operable in MODE 1 when reactor thermal power is greater than or equal to 30 percent and no peripheral control rod is selected per TS 3.3.2.1. The design of the system is to block CRD withdrawal during a Rod Withdrawal Error (RWE) to prevent exceeding the minimum critical power ratio (MCPR) safety limit. The RBM function is not credited by the RWE analysis and therefore the safety significance is minimal.
This condition is being reported in accordance with 10 CFR 50.73(a)(2)(i)(B), as an operation or condition which was prohibited by the plant's Technical Specifications.
C. Cause of Event:
A failed diode on a relay card for RBM 8 allowed LPRMs 48-17, 48-25, and 56-25 to be counted into the RBM circuitry along with the normally selected LPRMs for certain control rods that normally utilize only three LPRM strings. This condition affected both LPRM inputs into both RBM 7 and RBM 8. This rendered the RBM upscale trip and RBM INOP count circuit non-conservative with extra LPRM inputs being detected on scale that should not have been counted. This condition was unique to ten (10) CRDs. A subsequent causal investigation was performed to determine why operations personnel did not identify that the RBM was inoperable.
A knowledge deficiency related to the expected indication of the count circuit meter was identified as the apparent cause. The crew did not realize the effect the anomaly had on the operability of the RBM. They determined operability by power level rather than how the number of inputs based on rod selection was affecting the operability of the RBM. It was not recognized that the count circuit meter could have been utilized as an additional tool to confirm the anomaly was adversely affecting the operability of the RBM. This would have led the crew to realize that additional expertise was required to address the issue prior to continuing with rod exercising.
The causal investigation also revealed that a lack of risk awareness and procedure adherence contributed to the erroneous determination of operability for the RBM. If shift personnel would have used the proper procedure checklist to determine operability, it may have resulted in rod exercising being stopped and proper assistance engaged as necessary to ensure that immediate operability was thoroughly assessed.
D. Safety Analysis:
The design of the RBM system is to block CRD withdrawal during a RWE to prevent exceeding the MCPR safety limit. Since the RBM upscale rod block is not credited in the analysis of a rod withdrawal error, the safety significance of the failure of the relay card is minimal. Therefore, health and safety of the public was not compromised as a result of this condition.
E. Corrective Actions:
Following troubleshooting, the failed relay card was identified and replaced.
Appropriate shift personnel were coached on proper use of plant procedures.
Training Requests were initiated to perform a review of the event and the function of the count circuit meter with licensed personnel.
This event is being used as a Case Study for discussions with Operations Shift Managers.
F. Previous Occurrences:
A review of DNPS Licensee Event Reports (LERs) for the last three years did not identify any LERs associated with control rod block inoperabilities or failures.
G. Component Failure Data:
Manufacturer Description Part Number General Electric Relay Circuit Board 136B1351G001