ML20198D986
| ML20198D986 | |
| Person / Time | |
|---|---|
| Site: | Maine Yankee |
| Issue date: | 05/20/1986 |
| From: | Whittier G Maine Yankee |
| To: | Thadani A Office of Nuclear Reactor Regulation |
| References | |
| 7567L-HFJ, GDN-86-122, GDW-86-122, MN-86-69, NUDOCS 8605270172 | |
| Download: ML20198D986 (3) | |
Text
i MAIRE HARHEE 'A10MICPOWERCOMPARUe avaug,,"l,,?,?e g"gl h
(207) 623-3521 May 20, 1986 O
HN-86-69 GDH-86-122 Director of Nuclear Reactor Regulation United States Nuclear Regulatory Commission Washington, D. C.
20555 Attention:
Mr. Ashok C. Thadani, Director PHR Project Directorate #8 Division of Licensing
References:
(a) License No. DPR-36 (Docket No. 50-309)
Subject:
Augmentation Factor Removal Gentlemen:
Maine Yankee has determined that the use of axially dependent flux augmentation factors is no longer appropriate. He propose that the factor be removed from the presently approved operating limit determination calculations of KH/Ft limit margin to fuel centerline melt limit. As shown in Attachment A, the factor accounts for an effect that is no longer present in modern design PHR fuel rods used in the Maine Yankee Atomic Power Station. Removal of the augmentation factor by another licensee has been accepted by the NRC.
The first application of this change will be for the Cycle 10 Core Performance Analysis.
Cycle 10 is scheduled for startup in April, 1987. He request your review of this change by July 15, 1986 so that our analysis can be performed as scheduled.
Very truly yours, MAINE YANKEE ATOMIC POWER COMPANY
-)J& nV Y
G. D. Whittier, Manager Nuclear Engineering and Licensing GDH/bjp Attachment cc: Dr. Thomas E. Hurley Mr. Pat Sears
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Mr. Cornelius F. Holden Oh 7567L-HFJ 8605270172 860520 DR ADOCK 0500 9
M AINE YANKEE ATOMIC FOWER COMMY g
ATTACHMENT A The presently approved method for determining the limit for KH/Ft margin to fuel centerline melt employs an Augmentation Factor, Reference (a).
The factor accounts for an effect that is no longer present in modern design PWR fuel rods used in the Maine Yankee Atomic Power Station.
Removal of the augmentation factor has been previously proposed by another licensee, Reference (b). The NRC staff has reviewed the issue, and an SER has been issued on the subject, Reference (c), approving its removal.
The factor was based on a model that accounted for the formation and distribution of axial gaps in the fuel pellet column caused by the combir,ed effects of a large amount of pellet densification and clad creepdown. The gaps were formed if the fuel pellet column experienced pellet-clad " lockup".
Axial gaps as large as 0.4 inches were seen in fuel in Maine Yankee Cycle 1 and as large as 4.0 inches were seen in another plant in the early 1970's, Reference (d). The axial gap caused a decrease in the local neutron absorption resulting in an increase in the power peaking of the adjacent rods. The effect was further enhanced by the complete collapse of the cladding into the resulting fuel pellet column gap which increased the amount of moderator present in the locality. The augmentation factor is defined as the ratio of peak augmented power (due to axial gaps) to peak unaugmented power.
The modern PHR fuel now used in Maine Yankee is different from the fuel upon which the experimental observations and model were based in several important areas. The fuel pellets are fabricated in a manner which reduces the propensity to densify during irradiation in the reactor. Modern fuel is also prepressurized to inhibit the incidence of clad creepdown during the early irradiation period wherein fuel pellets could experience a small amount of densification.
The combination of these changes diminishes the magnitude and incidence of gap formation by preventing pellet-clad " lockup".
Based on these differences, the two fuel vendors that have supplied fuel assemblies to Maine Yankee have provided justification fo.r removal of the augmentation factor, References (d) and (e).
The Maine Yankee fuel vendors have also performed extensive measurements of irradiated fuel rods to quantify the amount of axial gap formation that may occur in current lines. Combustion Engineering has found that the axial gaps are typically less than 0.05 inches in the fuel it is presently fabricating, Reference (d).
Exxon has not found gaps larger than 0.060 inches (the limit of detection for the equipment it is employing), Reference (f).
The fact that the gaps are very small and also randomly distributed results in the calculation of axial gap augmentation factors that are very small.
Typical factors for a gap as large as 0.1 inches are less than 1.004.
Factors for the fuel currently in the Maine Yankee Atomic Power Station as described above are insignificant relative to the uncertainties used to determine operating limits.
The factor, therefore, can be set to 1.0 for use in all analysis and licensing calculations.
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MAINE YANKEE ATOMIC POWER COMPANY ATTACHMENT A (CONTINUED)
In summary, the conclusion of References (d) and (e) is that modern prepressurized fuel rods loaded with non-densifying fuel pellets are resistant to the formation of fuel pellet stack gaps and, therefore, the calculated augmentation factor is negligibly small. No new data has been developed which would challenge the bases for this conclusion.
Furthermore, the fuel rod manufacturing processes are the same as that used when minimal fuel pellet column gaps were observed or, if changed, not changed in a way that would adversely affect the conclusion that use of the augmentation factors is not required.
Based upon these evaluations and conclusions, we intend to delete the axially dependent flux augmentation factors from the safety limit determination calculations for the Maine Yankee Atomic Power Station.
References:
(a) " Safety Evaluation by the Office of Nuclear Reactor Regulation Supporting Amendment No. 29 to Facility Operating License No. DPR-36, Maine Yankee Atomic Power Company, Maine Yankee Atomic Power Station, Docket No.
50-309."
(b) Letter, A. E. Lundvall, Jr. (BG&E) to J. R. Miller (USNRC), "Calvert Cliffs Nuclear Power Plant Unit Nos. 1 & 2, Docket Nos. 50-317 & 50-318, Request for Amendment", December 31, 1984.
(c) " Safety Evaluation by the Office of Nuclear Reactor Regulation Related to Amendment No.104 to Facility Operating License No. DPR-53, Baltimore Gas and Electric Company Calvert Cliffs Nuclear Power Plant Unit No. 1, Docket No. 50-317."
(d) H. M. Adams, et al. "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,VS, April, 1985.
(e) T. J. Helbling (Exxon) to R. T. Yee (MYAPCo), Letter transmitting memo justifying Augmentation Factor removal.
TJH:085-86, dated April 8, 1986.
(f) XN-NF-82-06(P) Revision 1, " Qualification of Exxon Nuclear Fuel for Extended Burnup", June, 1982.
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