ML20027E801
| ML20027E801 | |
| Person / Time | |
|---|---|
| Site: | Maine Yankee |
| Issue date: | 10/20/1982 |
| From: | Knopp L Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20027E795 | List: |
| References | |
| NUDOCS 8211160189 | |
| Download: ML20027E801 (3) | |
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UNITED STATES
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SAFETY EVALUATION OFFICE OF NUCLEAR REACTOR REGULATION MAINE YANKEE CYCLE 7 CEA MODIFICATIONS MAINE YANKEE ATOMIC POWER COMPANY OfCKET NO. 50-309 1.0 Introduction By letter from J. H. Garrity of the Maine Yankee Atomic Power Company (MYAPC) to R. A. Clark (NRC) dated August 6,1982, MYAPC proposed control element assembly (CEA) bank modifications and changes to the analytical methods to be used in the Maine Yankee Cycle 7 reload core.
2.0 Evaluation 2.1 CEA Modifications In order to achieve the required increased shutdown margin for the ster.m line break transient, the scram reactivity requirements for Cycle 7 have increased.
Therefore, MYAPC has proposed to achieve this by replacing the part-strength (one or two B C fingers) Bank 5 CEAs with full-strength 4
(five B C fingers) CEAs.
Bank 5 is the lead regulating bank and is the 4
only bank inserted above about 70 percent of rated power. Also, it is proposed to add four part-strength CEAs with two active B C fingers to 4
Bank 5 in former part-length CEA locations for better local power distribution control. These four CEA locations are non-scrammable and do not contribute to the available scram reactivity.
Additional information.was provided in the Maine Yankee Cycle 7 Core Performance Analysis reload report YAEC-1324 d1ited September 1982 which enabled us to complete our evaluation of the CEA modifications. Because of the significant increase in reactivity worth of CEA Bank 5, reanalyses of the dropped, withdrawn, and ejected CEA events were presented. These events were reviewed and found to be acceptable with respect to initial assumptions, methods, and consequences. A more detailed evaluation will be presented in our Safety Evaluation Report for the Maine Yankee Cycle 7 reload.
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2 2.2 Methodology Modifications MYAPC has also proposed changes to the licensing methodet,, dealing with the physics parameters used as input to the CEA ejection analysis.
These changes are based on the assumption that the CEA ejection analysis method employed for Maine Yankee since Cycle 3 contains conservatisms based on both analytical model assumptions and selection of input parameters which can be justifiably reduced.
The core average Doppler defect curve has been typically calculated for the unrodded condition. Therefore, the local power weighting effects have been minimal with a 25 percent uncertainty conservatively applied to the defect curve.
For Cycle 7, and beyond, MYAPC has proposed that a core average Doppler defect curve with local power weighting based on the explicit pre-ejected power shape be applied for each of the ejected CEA cases. We find this proposed change acceptable since the core average Doppler defect curve thus derived is more representative of the given conditions than the previously derived unredded curve.
The core average Doppler defect uncertainty which has typically been included for transient analysis application is 25 percent. This un-certainty accounts for items such as uncertainties in neutron cross sections, isotopic burnups, and power distributions.
It also accounts for uncertainties in the local Doppler reactivity weighting during the course of a transient due to changing conditions. This latter effect is a significant factor in limiting the adverse effects of the transient.
For Cycle 7, MYAPC has proposed that the uncertainty applied to the core average Doppler defect be reduced from 25 percenj to 15 percent in application to CEA ejection transients only. Based on the results from higher-order calculations performed by Ccmbustion Engineering for the Maine Yankee FSAR and on space-time control rod ejection analyses performed by other PWR vendors, we find this reduction acceptable.
e Using the above-mentioned two changes to the CEA ejection analysis methodology, reanalyses-of the Cycle 7 CEA ejection event were performed for both HFP and HZP initial conditions. We have evaluated the analyses and find the assumptions, calculational techniques, and consequences acceptable. Since the calculations resulted in peak fuel enthalpfes less than 280 cal /gm, prompt fuel rupture with consequent rapid heat transfer to the coolant from finely dispersed molten UO was assumed not 2
to occur.
3.0 Conclusion We have reviewed the proposed CEA modifications and the associated CEA ejection analysis methodology modifications for Maine Yankee Cycle 7 operation and find these proposed changes' acceptable. A more detailed evaluation of these changes as well as their impact on the Mair.e Yankee Technical Specifications will be presented in our Safety Evaluation Report for Cycle 7 operation.
Dated:
October 20, 1982 Principal Contributor:
Larry Kopp
.