ML20127N003

Provides Addl Info Re ECCS & LOCA Analyses,In Response to NRC .Operation W/Control Rod Restrictions for Rest of Cycle 17 Assures Compliance w/10CFR50.46.Meeting to Discuss Changes to App K Model for Cycle 18 Requested

ML20127N003
Person / Time
Site:	Yankee Rowe
Issue date:	06/26/1985
From:	Papanic G YANKEE ATOMIC ELECTRIC CO.
To:	Zwolinski J Office of Nuclear Reactor Regulation
References
FYR-85-69, NUDOCS 8507010402
Download: ML20127N003 (4)

Text

,

Te'ephone (8 ") 8 ' -8 '00 YANKEE ATOMIC ELECTRIC COMPANY TM0( 710-380-7619

^:-" 2.C.2.1 1671 Worcester Road, Framingham, Massachusetts 01701 FYR 85-69 i .s tLLu ,/

June 26, 1985 United States Nuclear Regulatory Commission Washington, DC 20555 Attention: Mr. John A. Zwolinski, Chief Operating Reactors Brench No. 5 Division of Licensing

References:

(a) License No. DPR-3 (Docket No. 50-29)

(b) Letter, J. A. Zwolinski, USNRC to J. A. Kay (YAEC), dated May 22, 1985, " Confirmation of ECCS Codes" (c) Letter, USNRC to YAEC, dated December 4,1975, " Amendment to Facility Operating License No. DPR-3" (d) Letter, USNRC to YAEC, dated June 5, 1984, " Amendment No. 82 to Facility Operating License No. DPR-3"

Subject:

LOCA Analyses

Dear Sir:

This letter provides Yankee's response to the staff's request in Reference (b) for additional information regarding ECCS performance analyses.

Specifically at issue is the matter of an approved evaluation model being used to evaluate ECCS performance which adequately addresses a " range of power distribution shapes and peaking factors representing power distributions that may occur over the core lifetime" as is required by Section I. A. of Appendix K to 10CFR Part 50. Additionally, the adequacy of Yankee LOCA-related Technical Specifications are questioned since maximum linear heat generation rates are not modified as a function of core elevation. Before answering the specific questions put forth by the staff in Reference (b), we wish to raise the following points.

Since December 4, 1975, YAEC has performed ECCS performance calculations for Yankee using an NRC-approved pressurized water reactor ECCS evaluation model. The original approval of this model was received in December 1975

[ Reference (c)]. The model has been modified several times since then and all changen have been reviewed and approved by the NRC. The latest application of the model was for Core 17 and was approved for this application in Reference (d).

/

OD 0507010402 050626 /1 I

)0 PDR ADOCK 05000029 P PDR

United States Nuclear Regulatory Commission June 26, 1985 Attention: Mr. John A. Zwolinski, Chief Page 2 The question of sensitivity to axial power shapes was addressed in 1975 l by reference to a study performed by Exxon Nuclear Inc., for the HB Robinson i Plant in Topical Report KN-75-41A, Supplement 3. This study concluded that a '

l chopped-cosine axial power shape was most limiting. The staff now questions the applicability of this study to Yankee and has therefore requested that we answer the questions in the May 22, 1985 letter.

We have re-examined the Exxon study and if top-skewed axial power shapes are taken as a single issue, they are more limiting than are cosine power shapes. This does not necessarily imply that our cosine-based methodology does not conform to 10CFR50, Appendix K. To account for any shape anomalies due to the permitted control rod movement, the measured linear heat generation rates have applied to them factors for control insertion, F , and I for xenon redistribution, Fxe. These two factors, although not containing any spatial dependency, combine to add as much as 13% at end-of-cycle to the measured linear heat generation rate, which is then compared to the LOCA limit (other uncertainty factors are also applied).

Thus, operation with a singular-type LOCA limit at Yankee is intended to incorporate the presence of any shape anomalies and does maintain the plant within compliance to Appendix K with the new groundrules assuming a rodded core. The LOCA-related Technical Specifications, as such, have been utilized for quite some time. In 1976, the entire Technical Specification package was altered in accordance with the Westinghouse Standardized Technical Specification format. These new plant Technical Specifications were reviewed and subsequently approved by the NRC in 1977. The basic form of the LOCA LHGR Technical Specification remained unchanged in the process of adopting the standardized Technical Specification format.

The following information, as requested by the staff in Reference (b), is provided below:

1. Given that the axial shape study performed by Exxon Nuclear in KN-75-41A, Supplement 3 no longer has staff concurrence; current Yankee LOCA analysis only partially fulfills the requirements of Section I. A of Appendix K to 10CFR Part 50. A break spectrum analysis and a burnup sensitivity study is performed for Yankee.

The break spectrum analysis is performed at Beginning-of-Cycle (BOC) '

conditions where fresh fuel conditions are most ilmiting.

Chopped-cosine axial power distributions, which exist at BOC, are employed along with design radial peaking factors pluJ

uncertainties. Since LHCR limits achieved in the break spectrum l analysis are not sufficient for full-power operation, a burnup

sensitivity analysis is performed on the limiting break size. In this burnup study, the allowable linear heat generation rate for both fresh and once-burnt fuel is determined at various cycle l

, burnups . The variation in hot rod radial peaking factors is l

accounted for in the burnup study. Chopped-cosine axial power shapes are assumed and no accounting is made for the presence of xenon-induced transient axial power shapes.

United States Nuclear Regulatory Commission June 26, 1985 Attention: Mr. John A. Zwolinski, Chief Page 3

2. The current Yankee Technical Specifications provide maximum allowable linear heat generation rates for fresh and exposed fuel types as a function of cycle average burnup. These itmits contain no spatial dependency.

Since the demonstration of compliance required by Questions 1) and 2) of Reference (b) cannot be fully shown, Yankee has taken action to assure compliance for the remainder of the current cycle. Yankee's future plans and the actions taken for Cycle 17 are described in answer to Questions 3) and 4) of Reference (b).

3. Yankee will perform LOCA analyses for Cycle 18 which fully conform to the requirements of Appendix K to 10CFR50. At this time, the full scope of that analysis has not been determined, since improvements to Yankee LOCA methods will be required to properly address top-skewed axial power shapes. These improvements, primarily in the area of reflood heat transfer, are necessary to regain lost margin to LOCA limits experienced in assuming the more limiting axial power shape. Yankee personnel will request a meeting with the staff on the matter of model improvement by early August.

At this time, we intend to perform the new LOCA analyses within the confines of the current Technical Specification format for LHGR limits. We will perform our burnup sensitivity studies with worst-predicted, top-peaked axial power distributions. LHGR limits thus generated will encompass all power distributions that may occur over core lifetime. Since the assumed axial power shapes will account for xenon-induced transients, the portion of the Technical Specifications dealing with the xenon redistribution factor will either be altered or eliminated. In addition, a statistical approach to applying uncertainty factors to measured LHGRs will also be pursued. In the event that acceptable LHCRs are not attainable by the above approach, a more complex LHGR Technical Specification will have to be generated, which accounts for the specific variation in LOCA limit with core height.

LOCA limits based upon chopped-cosine axial power shapes have a(ready been completed for Cycle 18. Yankee will submit these limits in the Core Performance Analysis Report, which will be submitted to the staff in August 1985. The cosine-based LOCA limits meet all Appendix K requirements to a cycle average burnup of approximately 4,000 mwd /Mtu. At this point, either a new LOCA analysis will be submitted, or control rod restrictions, similar to those imposed in Cycle 17, will be imposed.

4. When informed of the Exxon problems, Yankee was near the End-of-Cycle (EOC) 17 operation. After evaluating the effects of top-skewed axial power shapes, the NRC was informed and Yankee took action to limit movement of control rod Group C between 80-84 inches to maintain the plant within the bounds of the current analyses.

These rod limits were in effect through 11,000 mwd /Mtu. At this point, it was determined that unrestricted control rod maneuvering 1

. O

.o-United States Nuclear Regulatory Commission June 26, 1985 Attention: Mr. John'A. Zwolinski, Chief Page 4 within this band could result in a top-peaked axial power profile and a new restriction to a 80-83 inch band was initiated. The 80-83 )

inch band maintains the current analysis through 12,500 mwd /Mtu,

- Which is near the end-of-full power core operation. The end-of-full power operation is expected to occur in August.

LOCA evaluations were performed to substantiate the above conclusion. The high-power, exposed fuel (most limiting) was evaluated with the worst axial power shape produced with the above rod restrictions. The results showed that full-power operation met the acceptance criteria in 10CFR50.46.

The control rod restrictions for the coastdown phase of operation are still being evaluated. However, the current analysis will be

. maintained for this phase and it is expected that the plant will coast down to a predetermined power level at which time the control rods will be withdrawn in such a manner as to minimize xenon-induced power redistribution. When the control rods are fully withdrawn, there is no xenon peaking and the current analysis will be maintained until the refueling in mid-october.

During the first 4,000 mwd /Mtu of Cycle 18 operation, the predicted axial power distributions are closely approximated by a chopped-cosine power distribution. After 4,000 mwd /Mtu Core Average Burnup (CAB), either new analyses will be in place for Cycle 18 or rod restrictions will be imposed.

Summary Based upon the above, Yankee concludes that operation with the control rod restrictions for the remainder of cycle 17 assures compliance to 10CFR50.46. A meeting is requested with the staff to discuss potential 4 changes to our Appendix K model for Cycle 18.

We trust this information is satisfactory; however, if you have any questions, pleasa contact us.

Very truly yours, YANKEE ATOMIC ELECTRIC COMPANY G. Pa ic, Jr.

Senior Project Engineer Licensing 3

GP/hja 4

- > , - , .m.%w.-,.,.-,.--,.,_,_. ,,-.--.,,,..c.,--, ..n,, .2m, . . ~ ,_ _ .,,_,,w,.,,. .,,---.,,,,ww-,.,~,..,--,,e - - . - , , . - - , , , . -