ML091100286
| ML091100286 | |
| Person / Time | |
|---|---|
| Site: | LaSalle, PROJ0728  |
| Issue date: | 04/16/2009 |
| From: | Gardner R AREVA NP |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| NRC:09:041 IR-09-002 | |
| Download: ML091100286 (4) | |
Text
A AREVA April 16, 2009 NRC:09:041 Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Interim Report of an Evaluation of a Deviation Pursuant to 10 CFR 21.21(a)(2)
AREVA NP Inc. has identified an issue related to fuel rod liftoff (crud plus oxide) measurements at LaSalle Units 1 and 2. The issue inivolves measurements of liftoff that indicate the potential for excessive tenacious crud build-up beyond that assumed in analyses supporting plant operating limits.
The following information is provided pursuant to the requirements of 10 CFR 21 to submit an interim report on issues for which the evaluation will not be completed within 60 days of discovery.
An interim report on the evaluation is attached, specifically; Interim Report No.09-002, "High Cladding Liftoff Measurements at LaSalle Reactors."
The AREVA NP customer potentially affected by this issue has been notified and will receive a copy of this interim report.
If you have any questions related to this interim report, please contact Mr. Alan B. Meginnis, Product Licensing Manager at 509-375-8266 or by e-mail at alan.meqinnis(areva.com.
Sincerely, Ronnie L. Gardner, Manager Corporate Regulatory Affairs AREVA NP Inc.
Enclosure cc:
H.D. Cruz Project 728 AREVA NP INC.
An AREVA and Siemens company 3315 Old Forest Road, P.O. Box 10935, Lynchburg, VA 24506-0935 Tel.: 434 832 3000 - Fax: 434 832 3840 - www.areva.com
Document Control Desk NRC:09:041 April 16, 2009 Page A-1 Interim Report 09-002
Subject:
Interim Report of an Evaluation of a Deviation Pursuant to 10 CFR 21.21 (a)(2)
Title:
High Cladding Liftoff Measurements at LaSalle Reactors Identification of Basic Activity:
LaSalle Units 1 and 2: Combined thickness of zirconium oxide corrosion film and coolant system deposits (crud) on fuel rods may exceed analyzed maximum.
Basic Activity Supplied By:
AREVA NP Inc.
Nature of Deviation:
Measurements obtained during poolside exams at the two LaSalle units show high total liftoff on fuel rod surfaces. Liftoff is the sum of the thicknesses of the zirconium oxide corrosion film and coolant system deposits (crud), and is typically measured using eddy current and/or profilometry techniques. Typical past and recent BWR measurements of liftoff at other plants have been low - with maximum values of less than 60 pm and with low amounts of crud.
Measurements on AREVA fuel at the LaSalle units are significantly higher. AREVA's fuel rod thermal-mechanical methodology requires explicit accounting for the maximum amount of oxidation along with additional consideration for the effect of crud. The fuel rod analyses for the LaSalle fuel do not sufficiently account for the high liftoff measurements since it was assumed that the oxidation and crud would be consistent with prior AREVA experience. In addition, the AREVA thermal-hydraulic analyses for LaSalle did not account for high liftoff.
Discovery Date:
The issue was determined to be a deviation on February 16, 2009.
Corrective Actions to Date:
Since the initial discovery of the issue, investigative effort was focused on the accuracy of the liftoff measurements and on the applicability of the coolant chemistry conditions during the cycles that produced the high liftoff values to current operation at the LaSalle units. A summary of these efforts along with other actions are provided below. First, some background is given on the measurements.
As part of a joint AREVA/Exelon/EPRI program to assess the impact of higher zinc injection levels, cladding liftoff (oxide plus crud) measurements were taken along with crud deposit samples. Measurements for the program were taken in 2007 and then in 2008 from fuel operating in both LaSalle units. The liftoff measurements were made using a dual coil EC (eddy current) device that measures the separation from the cladding metal surface by a non-
Document Control Desk NRC:09:041 April 16, 2009 Page A-2 conductive and non-permeable layer (i.e., oxide and crud). The application of the measurement method was complicated by zinc injection because the zinc and iron in the reactor water system can combine to produce a tenacious ferritic crud deposit that affects the EC measurements. A compensation technique had been developed earlier to correct the measurements for the ferromagnetic effect of the crud. The compensation was applied to both sets of data.
During the operability assessment following the deviation discovery, it was assumed that the 2007 liftoff measurements were correct. The 2008 data were initially discounted due to the assumption that the EC signals contained electrical noise that masks the true liftoff measurements. Also there was a clear lack of consistency between the 2008 EC measurements and the profilometry (rod diameter) measurements. Penalties on fuel thermal limits and fuel rod exposure limits were generated as a result of the assessment. Operation continues to be supported with the limitations summarized below.
" Required Margin to Operating Limit MCPR o U1C13, 8500 MWd/MTU - NEOC, Base Case, NSS or TSSS, ED 3% margin to MCPR limits o U2C13, BOC - 10,156 MWd/MTU, Base Case, NSS or TSSS E1 If P > 40%P, no margin requirement E If P < 40% P, 2% margin to MCPR limits
- Rod average burnup is limited to 57.3 MWd/kgU In the case of equipment out of service, more restrictive limitations may apply.
Following the issuance of the operability assessment on February 19, 2009, discussions were held with Exelon and EPRI. Both the 2007 and 2008 EC measurements were re-evaluated using multiple techniques. AREVA decided that many of the EC measurements could not be relied upon because of compensation difficulties. Due to questions on the interaction of the crud composition with the EC measurements, it was decided to investigate alternate methods to evaluate liftoff. The primary option was to derive liftoff indirectly from the profilometry data. This is done by calculating the amount of cladding'creepdown from the original diameter using AREVA's fuel rod thermal-mechanical methodology along with detailed fuel rod power histories.
The difference between the profilometry measurement and the diameter at creepdown is the calculated oxide plus crud layer thickness. Metal loss due to oxidation was also taken into account in calculating the layer thickness.
A detailed review was also conducted on the water chemistry conditions at the LaSalle plants.
Exelon informed AREVA that actions were taken to reduce the impact of zinc injection on fuel rod crud deposition by limiting the feedwater iron content. Therefore, crud levels for current operation (Cycle 13 for both units) are expected to be conservatively represented by the 2007 and 2008 liftoff measurements.
Currently, AREVA is working to refine the liftoff values from profilometry. Preliminary profilometry data reduction results indicate changes in liftoff with some rods still exhibiting similarly high peak liftoff values but the rod average liftoff may be significantly lower. For this reason, the existing operability assessment is believed to be conservative.
Document Control Desk NRC:09:041 April 16, 2009 Page A-3 No crud-related fuel failures have occurred at either plant during the time represented by the liftoff measurements. Visual examinations of the fuel during the 2007 and 2008 measurement campaigns did not reveal signs of excessive spallation. Based on these observations, AREVA has no reason to believe a substantial safety hazard exists. The uncertainty on the crud levels in current operation has led to evaluating the issue under the requirements of Part 21.
Evaluation Completion Schedule Date:
July 31, 2009