ML20141M932

Safety Evaluation Supporting Amends 94 & 84 to Licenses DPR-39 & DPR-48,respectively

ML20141M932
Person / Time
Site:	Zion  File:ZionSolutions icon.png
Issue date:	02/19/1986
From:	Office of Nuclear Reactor Regulation
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ML20141M924	List: ML20141M923 ML20141M930 ML20141M932
References
NUDOCS 8603030067
Download: ML20141M932 (6)

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[ g NUCLEAR REGULATORY COMMISSION j WASHINGTON. D. C. 20555

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGU ,

RELATED TO AMENDMENT NO. 94 TO FACILITY OPERATING LICENSE.NO. OPR-39 Ap AMENDMENT N0.

84 TO FACILITY OPERATING LICENSE NO. DPR-48 C0f'MONWEALTH EDISON COMPANY ZION NUCLEAR POWER STATION, UNITS 1 AND 2 DOCKET NOS. 50-295 AND 50-304 a- _ INTRODUCTION

, By letter from P.C. LeBlond to H. R. Denton dated July 12, 1985, Comonweal th

!.. Edison, r.equested an amendment to Facility Operating 1.icenses Nos DPR-48, Appendix A, Section 5.0, Design Features. . DPR-39 and c The requested changes would raise the allowable enrichment to approximately 3.7 weight percent (

the Spent Fuel Pool and 4.0 w/o for the New Fuel Vault.

No physical changes are being made to the Spent Fuel Pool, the New Fuel Vault,Theor their rac .

enrichment Zion reactors. increases are needed to implement 18 month operating c

. The submittal includes analyses of the effects of the stated enrichments on the criticality of both the new and spent fuel racks at t Zion Power Station, and justification of the adequacy os used to perform the analyses.

of the design m ANALYSIS METHODS The analyses of the enrichment changes for the Zion fuel storage ra k c s were performed using the KENO-IV Monte Carlo computer code for reactivity determination with neutron cross sections generated by the AMPX code package.

A 218 energy group cross section library is generated from ENDF/B IV -

These codes have been benchmarked against a set of 27 r ments in critical expe the range of. pellet diameters, water-to-fuel ratios and ments U-235 enrich that encompass the Zion design.

This benchmarking led to the conclusion that the calculational model is capable o' determining {he multinli ti factor (keff) of the new and spent fue: ca on probability at a 95% confidence level. ucks to within 0.0032 M Cith a 955 8603030067 860219 The benchmarking data is sufficiently PDR ADOCK 05000295 ""

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' diverse to establish that the method bias and uncertainty will apply to rack conditions which include strong neutron absorbers, large water gaps and low moderator densities. These codes have been used for a number of similar licensing actions in the past and were found acceptable. Based upon this, and our review of the methods for the present licensing action, we find the methods acceptable for the Zion analysis. ' 2 NEW FUEL STORAGE RACK ANALYSIS The criticality of fuel assemblies in the new fuel storage rack is prevented by maintaining a minimum separation of 21 inches between assemblies.

Although ;

new fuel is normally stored in a dry configuration, the NRC acceptance criteria for new fuel storage are that there is a 95% probability at a 95%

. confidence level (including uncertainties) that k,ff of the fuel assembly

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- array will be:

(1) no greater than 0.95 when fully loaded and flooded with unborat'ed' water and (2) no greater than 0.98 under conditions of optimum moderation if higher reactivities can be attained at achievable rroderation conditions other than full density unborated water.

In addition to the calculational method uncertainty mentioned previously, uncertainties and biases due to mechanical tolerances such as stainless steel thickness, cell inner diameter, center-to-center spacing, and asymmetric assembly position are included either by using worst case initial corjditions or by performing sensitivity studies to obtain the appropriate values. Credit is taken for the neutron absorption in the full length stainless steel angle frons at the corners of each fuel assembly.

Using these methods and assumptions, the nominal k,ff of the new fuel storage racks fully flooded with unborated water is calculated as 0.9010. The fuel is assumed to be the Westinghouse 15x15 0FA fuel assembly design at a U-235 enrichment of 4.0 weight percent. The OFA 15x15 fuel design is the most reactive of the 15x15 designs. Adding the appropriate 95/9'5 pro!ghility/

confidence uncertainties and biases yields a value of 0.9086 for*t$e multiplication factor. This meets our acceptance criterion of 0.95. ~ An U

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• analysis of " optimum moderation" at low water densities, but with a finite model of tlje actual Zion fresh fuel storage vault rather then the infinite a7 ray model for the normal storage calculation, yielded a k 0.9. This is acceptable because our criterion for the optimumeff Of 1ess than moderation case is that k eff tshould be less than 0.98.

In addition, events such as the inadvertent drop of an assembly between the outside periphery of the rack and the pit wall would not cause a criticality accident because of the assumption of the doubic contingency principle. This states that it is unnecessary to assume two ur1*kely, independent, concurrent

' events to ensure protection against a criticality accident. Therefore, for '

- accidents such as this, the absence of water in the new fuel storage pit can be assumed since assuming its presence would be a second unlikely event.

Without water, any postulated assembly drop accident would result in a k,ff value Very much less than our acceptance criterion of 0.95. We therefore conclude that the analysis supporting storage of Westinghouse (OFA) 15x15 fuel with enrichments no greater than 4.0 weight percent is acceptable.

SPENT FUEL. STORAGE RACK ANALYSIS

' The criticality of fuel assemblies in the spent fuel storage rack is prevented by maintaining a minimum separation of 10.35 inches between assemblies and by .

inserting the neutron absorber, Boral, between assemblies. Although spent fuel is.nomally stored in borated pool water containing approximately 2000 ppm b,oron, the NRC acceptance criterion for spent fuel storage is that there is a 95% probability at a 95% confidence level (including uncertainties) that k eff of the fuel assembly array will be less than 0.95 under all conditions,

' including when fully flooded with unborated water. The licensee incorrectly states that the criteria allow keffI0.98 under accident conditions (as for new fuel storage), but this is not the case.

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In addition to the calculational method uncertainty mentioned previously, uncertainties and biases due to mechanical tolerances, thermal conditions, and B4 C particTe self-shielding are included either by using worst case initial conditions or by performing sensitivity studias to obtain the appropriate values.

Credit is taken for the neutron absorption in full length structural

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materials and in solid materials added specifically for neutrcn abso'rptien.

ilowever, for conservatism, the minimum poison loading is assu: red in these cases.

Using these methods and assumptions, the nominal k eff of the spent fuel racks fully flooded with unborated water is calculated as 0.9387.

The fuel is p assumed to be Westinghouse U-235. The temperature of ti 15x15 0FA at a enrichment of 3.7 weight percent

- reactivity. water is taken as that which yields the largest

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A conservative value of 1.0 gm/cc is used for the density of the

-water.- Adding the appropriate 95/95 probability / confidence uncertainties and biases fie'1ds a value of 0.9481 for the multiplication factor.

acceptance criterion of 0.95. Thi3 meets our Postulated events such as the inadvertent drop of an assembly between the outside periphery of the rack and the pool wall would not cause a criticility accident because of the assumption of the double contingency principle.

In other words, for accident conditions, the presence of soluble baron in the storage pool can be assumed and would result in a k I value very much less than our acceptance criterion of 0.95. We, the."efore, eff concluded that fuel assemblies of the Westinghouse (0FA) 15x15 design.havin '

enrichments'no fuel pool.

greater than 3.7 weight percent may be stored in the Zion spent L

FINDINGS Based en our review, we conclude that the storage racks meet the requirements of General Design Criterion 62 as regards criticality. Also, we conclude that any number of Westinghouse (0FA) 15x15 fuel assemblies of maximum enrichment no greater than 3.7 weight percent U-235 may be stored in t'he speg fuel racks of Zion Nuclear Power Station and no greater than 4.0 weight perce$t U-235 be stored in the fresh fuel racks.

considerations: These conclusions are based on'th'e following

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State-of-the-art calculational methods which have been verified by comparison with experiment have been used.

2. Conservative assumptions have been made about the enrichment of the fuel to be stored and the pool conditions.

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3. Credible accidents have been considered.

4. Suitable uncertainties have been considered in arriving at the final value of the multiplication factor.

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The final effective multiplication factor value meets our acceptance criterion.

We also conclude, that the proposed changes to Section 5 of the Zion Units 1 and 2 Tbchnical Specifications adequately account for the fuel enrichment increase and are, therefore, acceptable.

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. I ENVIRONMENTAL CONSIDERATION These amendments involve a change in the installation or use of the facilities components located within the restricted areas as defin'e d in 10 CFR 20. The staff has detennined that these amendments involve no significent increase in the amounts, and no significant change in the types, of any effluents that may be released offsite and that there is no significant increase in individual or cumulative occupational radiation

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exposure. The Comission has previously issued a proposed finding that

. these amendments involve no significant hazards consideration and there has been no public coment on such finding. Accordingly, these amendments meet the eligibility criteria for categorical exclusion set forth in 10 CFR Sec 51.22(c)(9). Pursuant to 10 CFR 51.22(b) no environmental impact statement or envirenaental assessment need be prepared in connection with the issuance of these amendments.

i CONCULSION We have concluded, based on the considerations discussed above, that:

(1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, i

and (2) such activities will be conducted in compliance with the l Comission's regulations and the issuance of these amendments will not be inimical to the common defense and security or to the health and safety of the public.

Dated: February 19, 1986 PRINCIPAL CONTRIBUTOR:

M. Dunenfeld l

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