ML20206H455

Safety Evaluation Accepting Util 860520 Proposed Change Re Augmentation Factor Removal from Approved Calculational Methodology for Determination of Operating Limits of Fuel Cycle

ML20206H455
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Site:	Maine Yankee
Issue date:	06/20/1986
From:	Office of Nuclear Reactor Regulation
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NUDOCS 8606260084
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E **pa asog,j NUCLEAR REGULATORY COMMISSION UNITED STATES 1 I

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO THE AUGMENTATION FACTOR REMOVAL FROM APPROVED CALCULATIONAL METHODS i MAINE YANKEE ATOMIC POWER COMPANY MAINE YANKEE ATOMIC POWER STATION DOCKET N0. 50-309 l l

1.0 INTRODUCTION

l By letter dated May 20,1996 (Ref.1), Maine Yankee Atomic Power Company (MYAPCO) proposed that the axially dependent power peaking augmentation factors be removed from the presently approved calculational methods.

This proposed change is based on a reanalysis of fuel densification and clad collapse performed for modern PWR fuel rods by Combustion Engineering (CE) for EPRI (Ref. 2). The primary conclusions of Reference 2 are that i for modern PWR fuel rods, that is, fuel rods containing nondensifying 00 l fuel pellets in prepressurized fuel rods, cladding will not collapse in 2-10 years of reactor operation and augmentation factors caused by inter- l pellet gaps are insignificant relative to the uncertainty of power peak- 1 ing calculations or measurements.

l The staff has previously approved a similar proposal by Baltimore Gas  !

and Electric Company (BG&E) for its Calvert Cliffs Unit 1 (Ref. 3) and Unit 2 (Ref. 4).

The staff evaluation of the MYAPC0 proposal follows. ~

2.0 EVALUATION Fuel rods used in the fuel cycles of PWRs in the 1970-1973 period con-pellets in unpressurized fuel rods. Some of sisted of rods these fuel densifying exhibite U0,d large interpellet gaps and, in a number of cases, also exhibited collapse of the cladding. The staff and fuel ven-dors review of this fuel densification problem resulted in not only a ,

better understanding of irradiation (and temperature induced) fuel den-sification but to the inclusion of its principal effects in fuel perform-ance evaluations and operating limits determination for a fuel cycle.

The principal effects were the evaluation of the creep collapse of the cladding, of the increase in the average linear heat generation rate of the reactor caused by fuel pellet shrinkage in the fuel rods, and of the axially dependent power peaking augmentation factors caused by interpellet gap formation.

Since the early days of densifying 00, pellets in unpressurized fuel rods, a number of significant changes have becurred in PWR fuel to alleviate the fuel densification problem. The first change resulted in fuel made with densifying U0, pellets in prepressurized fuel rods. This transition fuel type was replaced by all the PWR fuel vendors with the current modern fuel made with nondensifying 00, pellets in prepressurized fuel rods. Nondensi-fying fuel pellets are those that, upon resintering, densify (increase in density) by less than 0.5% of theoretical density while densifying fuel pellets densify (increase in density) by approximately 3% of theoretical density.

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> i Combustion Engineering (CE), as part of a study (Ref. 2) performed for the Electric Power Research Institute (EPRI), has evaluated interpellet gap formation and clad collapse in modern PWR fuel rods (that is, nondensi-fying 002 pellets in prepressurized fuel rods). This evaluation included reported results on fuel manufactured by Westinghouse and Babcock and Wilcox as well as data from CE manufactured fuel. Based on the results of this evaluation, CE has revised its clad collapse methodology, CEPAN, to treat finite gaps by applying finite length correction factors to infinite gap calculations and reanalyzed the power peaking augmentation factor using interpellet gap statistics based on its modern PWR fuel. The inter-pellet gap statistics used gap data from early Palisades and Maine Yankee fuel rods to ensure conservative gap statistics.

The report concludes that, in modern PWR fuel, the maximum expected inter-pellet gap sizes will be less than 0.05 inches which is considerably less than the value established for the earlier type of densifying, unpres-surized fuel rods. The report also concludes that the interpellet gaps exhibit an essentially random axial distribution of gaps where previously an axially dependent interpellet gap size distribution was used. Based on the gap size and axial distribution factors for modern PWR fuel, CE per-formed an evaluation of the power peaking augmentation factor. CE conclu-ded that for realizable interpellet gaps the augmentation factor was insignificant compared to power peaking factor calculational and measure-ment uncertainties.

In the report CE made another important conclusion regarding its (and other fuel vendors) modern PWR fuel. This conclusion is that the time to creep collapse far exceeds the reactor residence time of any fuel.

MYAPC0 has also submitted a letter (Ref. 5) from EXXON Nuclear Company (ENC) in support of its proposed change. In the letter ENC states that it has concluded that interpellet gaps will not occur in its fuel based on a' careful evaluation of the conditions necessary to form such gaps. ENC also states that supporting calculations and post-irradiation examinations were contained in the ENC high burnup design report (Ref. 6).

3.0 CONCLUSION

S Based on the review of the information presented in the CE report prepared for EPRI on interpellet gap formation and clad collapse in modern PWR fuel rods (that is, nondensifying U0, pellets in prepressurized fuel rods), the staff concurs with MYAPCO's proposal to delete the axially-dependent, power peaking augmentation factor in its approved methodology for the deter-mination of operating limits of a fuel cycle. The staff's concurrence is based, in part, on supporting evidence presented by ENC, especially in its high burnup design report.

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4.0 REFERENCES

1. Letter (MN-86-69) dated May 20, 1986 from Maine Yankee Atomic '

Power Company to Director, Office of Nuclear Reactor Regulation. 3

2. "CEPAN Method of Analyzing Creep Collapse of Oval Cladding.

Volume 5: Evaluation of Interpellet Gap Formation and Clad Collapse in Modern PWR Fuel Rods", EPRI NP-3966-CCM, Volume 5, April 1985.

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3. Letter from D.' H. Jaffe (NRC) to A. E. Lundvall, Jr. (BG&E), d May 20, 1985. l .,

4. NRC Memorandum from L. S. Rubenstein to G. C. Lainas, " Evaluation I of Technical Specification Changes for Calvert Cliffs Unit 2 Cycle 7 (TAC # 59594)", October 18, 1985. *

5. Letter from J. B. Edgar (ENC) to R. T. Lee (MYAPCO), May 28, 1986.

6. " Qualification of EXXON Nuclear Fuel For Extended Burnup", -

XN-NF-82-06, June 1982.

Principal Contributor: D. Fieno Dated: June 20, 1986 4

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