05000269/LER-2014-002
| Oconee Nuclear Station, Unit 1 | |
| Event date: | 11-25-2014 |
|---|---|
| Report date: | 01-26-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| Initial Reporting | |
| ENS 50640 | 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition |
| 2692014002R00 - NRC Website | |
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2014 02 00
EVALUATION:
BACKGROUND
In October, 2014, AREVA INC. (AREVA), the fuel vendor for Oconee Nuclear Station (ONS), notified Duke Energy Nuclear Fuels Engineering Group that the initial fuel pellet temperatures used in the existing Loss of Coolant Accident (LOCA) analysis may be non-conservative, when fuel thermal conductivity degradation (TCD) is considered. Further analysis resulted in a November 25, 2014, letter to Duke Energy confirming the final evaluation results of the deficiency discovered in the fuel thermal conductivity model computer codes. The evaluation resulted in the predicted limiting Peak Cladding Temperature (PCT) increasing to 2280 degrees F, that exceeded the maximum allowable PCT of 2200 degrees F. The TCD deficiency had no adverse impacts on current plant operations or public health and safety because of interim restrictions previously implemented on October 20, 2014.
Fuel cladding failure during a postulated LOCA is limited by restricting the maximum linear heat rate (LHR) so that the PCT does not exceed 2200 degrees F. Peak cladding temperatures greater than 2200 degrees F can cause severe cladding failure by oxidation due to a Zircaloy water reaction. Other criteria must also be met (e.g., maximum cladding oxidation, maximum hydrogen generation, coolable geometry, and long term cooling), but peak cladding temperature is usually the most limiting.
On November 25, 2014, Duke Energy made an 8-hour non-emergency notification (NRC Event Notification EN 50640) under 10 CFR 50.72(b)(3)(ii)(B), "Unanalyzed Condition." The condition is being reported in this LER in accordance with 10 CFR 50.73(a)(2)(ii)(B) as an event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety.
On the date of the event, ONS Unit 1 was in MODE 6 "Refueling," and Units 2 and 3 were in MODE 1 at 100% power.
EVENT DESCRIPTION
This condition is being reported in accordance with 10 CFR 50.73(a)(2)(ii)(B) as an event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety.
On October 20, 2014, the fuel vendor (AREVA) for ONS verbally notified Duke Energy's Nuclear Fuels Engineering that the initial fuel pellet temperatures used in the existing LOCA analysis may be non-conservative when fuel TCD is considered. Specifically, when applied to the Large Break LOCA (LBLOCA) evaluation model (EM), the change resulted in a significant increase to the calculated PCT per 10 CFR 50.46(a)(3)(i). On October 20, 2014, Oconee implemented interim restrictions to use previously existing administrative excore imbalance limits if the full incore detectors are unavailable. This issue was entered into Oconee's corrective action program.
On October 23, 2014, Duke Energy received a letter notifying Duke Energy's Nuclear Fuels Engineering Group of the need for interim restrictions to support adequate LOCA peaking margins at the axial power imbalance operating limits specified in current Core Operating Limits Reports (COLR). This was a conservative recommendation by AREVA until final confirmation of results could be obtained. AREVA recommended immediate actions to limit LOCA LHR limits and to control the Oconee Units to appropriate evaluated axial imbalance limits administratively until further evaluations were completed. The interim restrictions implemented on October 20, 2014, ensured that the Units would not violate the reduced LHR limits specified by AREVA.
On November 25, 2014, Duke Energy received a follow-up letter from AREVA that confirmed the final evaluation results of the deficiency discovered in the thermal conductivity model computer codes. The evaluation resulted in the predicted limiting PCT increasing to 2280 degrees F, that exceeded the ONS maximum allowable PCT of 2200 degrees F in 10 CFR 50.46(b)(1). Based on the evaluation results, Duke Energy made an 8-hour non-emergency notification (NRC Event Notification EN 50640) under 10 CFR 50.72(b)(3)(ii)(B), "Unanalyzed Condition" at 1635 on November 25, 2014. Because of the recommended interim restrictions implemented on October 20, 2014, the deficiency had no adverse impacts on current plant operations or public health and safety.
In addition, on December 16, 2014, AREVA submitted a 10 CFR Part 21 "Notification for a Defect in LOCA Analyses for B&W Plants" (ML14351A308).
On December 17, 2014, Duke Energy submitted the required 30-Day report for Oconee Units 1, 2, and 3; "Estimated Impacts to Peak Cladding Temperature due to Fuel Pellet Thermal Conductivity Degradation," as required by 10 CFR 50.46 (ML14353A214). Subsequent to this submittal, the Oconee Units 1, 2 and 3 COLRs and the plant computer [ID] were updated with the new operational imbalance limits. With the updated operational imbalance limits in place, the interim restrictions to use administrative excore imbalance limits were no longer required and were terminated.
CAUSAL FACTORS
The apparent cause of the event was a deficiency in the AREVA fuel thermal conductivity model, in that the codes do not adequately represent the reduction in fuel thermal conductivity with burnup. AREVA is conducting a cause evaluation that will be reviewed by Duke Energy upon its completion.
CORRECTIVE ACTIONS
Immediate Corrective Action:
1. Implemented interim restrictions to use the administrative excore imbalance limits specified in the "Power Maneuvering Predictions" procedure.
Subsequent Corrective Actions:
1. The plant computer and the Core Operating Limits Reports (COLR) for each Unit were updated with new operational imbalance limits.
Planned Corrective Action:
1. In its December 17, 2014, 30-Day report for Oconee Units 1, 2, and 3 (ML14353A214), Duke Energy committed to performing a Large Break LOCA reanalysis.
SAFETY ANALYSIS
Prior to receipt of the AREVA model deficiency notification, the licensing basis value for the Oconee LBLOCA PCT was 1854 degrees F. The increase in initial fuel temperatures when TCD is appropriately accounted for does impact the Oconee LBLOCA calculations for PCT. In order to ensure the Oconee Large Break LOCA peak cladding temperature would be less than or equal to the peak cladding temperature previously reported when the initial fuel temperature uncertainty error correction is applied, AREVA recommended allowable LHR limit reductions applicable to Mk-B-HTP fuel in full-core configurations and LHR reductions for Mk-B-HTP fuel and Mk-B11 fuel in mixed-core configurations for previous operating cycles. Imposition of the LHR limit reductions ensures the peak cladding temperature is maintained at or below the previously reported value, which is well below the 10 CFR 50.46 acceptance criteria of 2200 degrees F. Therefore, the local oxidation and whole core hydrogen generation also remain within the 10 CFR 50.46 acceptance criteria for the LBLOCA scenarios. With the LHR limit reductions, the core geometry remains amenable to cooling and acceptable long-term cooling is unaffected by these changes.
The LBLOCA LHR limits provided by AREVA are used by Duke Energy to establish operational core power imbalance limits which are specified in the COLRs per ONS Technical Specification 3.2.2 for the current operating cycles. Duke Energy has determined that the LHR penalties did not impact the full-incore operational imbalance limits currently specified in the Core Operating Limits Reports for the current operating cycles. The LHR penalties did result in minor reductions to the imbalance alarm limits for the excore detectors and the backup incore detectors for the current operating cycles. A similar conclusion regarding minor reductions to the imbalance alarm limits for the excore detectors and the backup incore detectors was shown to be true for previous operating cycles. The minor reductions to the imbalance alarm limits specified in the cycle-specific COLRs were confirmed to be bounded by existing procedural administrative limits for excore imbalance of ±10% at 100% full power operation, and ±15% below 90% power. These procedural administrative limits helped to ensure the ONS Units would not exceed 10 CFR 50.46 acceptance criteria.
A review of historical measured power imbalances concluded that the Oconee units did not operate at a condition that would have resulted in 10 CFR 50.46 acceptance criteria being exceeded, if a LBLOCA would have occurred. The TCD issue does not affect the Oconee Small Break LOCA analyses.
ADDITIONAL INFORMATION
A review of the Oconee Nuclear Station LERs over the last five (5) years did not reveal any similar events with the same or similar causes.
A review of the Oconee Nuclear Station corrective action program over the last five (5) years identified eight (8) items related to LOCA analyses. A review found that while similar, none of the eight (8) issues involved the same underlying reason as the issue in this LER.
Note that the LOCA analysis issue in this LER applies generically to B&W PWRs with AREVA fuel.
Energy Industry Identification System (EllS) codes are identified in the text as [XX]. This event is not considered reportable under the INPO Consolidated Events System (ICES). The Equipment Performance and Information Exchange (EPIX) program question in Block 13 of the LER is answered "NO" based on the ICES review.
[NOTE: EPIX has been replaced by ICES.] No component was selected because no specific equipment failures occurred. There were no releases of radioactive materials, radiation exposures or personnel injuries associated with this event.