ML20082M904
| ML20082M904 | |
| Person / Time | |
|---|---|
| Site: | Seabrook  |
| Issue date: | 08/27/1991 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20082M898 | List: |
| References | |
| NUDOCS 9109060087 | |
| Download: ML20082M904 (5) | |
Text
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UNITED STATES 3 3.q n
.i NUCLEAR REGULATORY COMMISSION
- .oi WASHINGTON, D.C. 20666
%..v j SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION SUPPORTitlG AMENDMENT NO.6 TO FACILITY OPERATlHG LICENSE NO. HPF-86 PU_BL_lcSEMIC,E,00ftPAt4YOFNEWHAMPSHIRE SEABROOK STATION, UNIT N0_. 1 DOCKET NO. 50-443
).0 glTRODUCTION By letter dated Parch 18, 1991, the licensee, New Hampshire Yankee (NHY) requested changes to the Seabrook Station Technical Specifications (TS). The proposed changes would increase the maximum enrichment of reload fuel assemblies authorized by TS 5.3.1 to 5.0 weight percent (w/o) U-235 from the current 3.5 w/o U-235 limitation, in addition, the proposed changes would add two new TS, 3.9.13 (Spent Fuel Asseinbly Storage) and 3.9.14 (New Fuel Assembly Storage) and their assetiated bases. These new TS specify the Limiting Conditions for Operation (LCO) and Surveillance Requiremerts (SR) associated with the storage of fuel assemblies in the sper.t fuel pool storage racks and in the new (unirradiated) fuel storage vault. The staff's safety evaluation of the criticality aspects of these proposed changes fo11cws.
2.0 DALUATION The Seabrook -new and spent fuel storace racks were previously analyzed for the storage of standard Westinghouse 17xr/ fuel assemblies with enrichments up to 3.5 w/o U-235. The current analysis considers the storage of Westinghouse 17x17 assemblies with enrichments up to 5.0 w/o U-235.
The criticality analysis was performed with the KENO-Va tionte Carlo computer code, the CASMO-3 integral tt ansport theory code, and the SIMULATE-3 nodal diffusion theory code. The use of KEt:0-Va and CASMO-3 for fuel storage criticality analysis has been validated by comparison to fuel storage critical experiments. The use of SIMULATE-3 for fuel storage burnup credit criticality analysis has been validated by comparison to reactor criticals, measured asser.bly burnops, fuel storage criticals, and flux trap criticals. These benchmarts indicate that the analytical methods adequately reprodace the experimental values.
The staff finds these methods and models to be acceptable.
The design basis for preventing criticality outside the reactor is that, confidence level (95/95 probability / confidence) probability at a 95 percent including uncertainties, there is a 95 percent that the effective multiplica-tion f actor (L-cff) of the fuel assembly array will be no greater than 0.95 when fully roderated by unborated water and no greater than 0.98 when moderated by reduced density hydrogenous material such as foam or mist (optimum modera-tier).
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The criticality analyses were performed with several assumptions which tend to maximize the rack reactivity.
For the spent fuel racks, these include:
(1) Unborated pool water at 68'F.
t (2) Racks of infinite extent with no radial or axial leakage.
(3) Neutron absorption by spacer grids is neglected.
For the new fuel vault, these include:
(1) Unborated water uniformly throughout vault and between fuel rods, varied l
from flooded to dry.
(2) Racks are a three-dimensional array with reflection from floor and walls.
l (3) Neutron absorption by spacer grids is neglected.
I In addition, the reactivity effects of mechanical uncertainties due to center-to-center spacing, canister envelope, Boraflex width and thickness, Boraflex boron loading, fuel stack density, and fuel enrichment, as well as burnup and methodology biases and uncertainties have been incorporated at a 95/95 proba-t bility/ confidence level. This meets the staff's acceptance criteria.
l For the spent fuel storage racks, curves of k-eff versus initial enrichment show that an infinite array of fresh fuel with maximum enrichment of 3.75 w/o U-235 results in a L-eff of 0.95.
The storage of fuel with greater enrichment requires either that it have a burn 4p greater than some value which is dependent on initial enrichment (reduced effective enrichment) or that it be stored in a checkerboard pattern in the racks (increased effective spacing).
From a reactivity standpoint, fresh fuel enriched to 3.75 w/o U-235 is equivalent to fuel with an initial enrichment of 5.0 w/o U-235 which has achieved a burnup of approximately 9 GWd/MTU.
From calculations of k-eff versus burnup, the minimum assembly burnup necessary to meet the k-eff no greater than 0.95 criterion for initial enrichments up to 5.0 w/o U-235 was obtained. These enrichment /burnup combinations were then used to define the single unit (infinite array) burnup credit acceptance criterion. This criterion defines two regions: a region of acceptable burnup and enrichment for placement in the spent fuel storage racks and a region of unacceptable burnup and enrichment. The fuel assemblies with characteristics in the region of 3
unacceptability can be made acceptable by checkerboarding this fuel with empty locations or with fuel of lower enrichment and/or higher burnup. This is shown in TS Figure 3.9-1 (enclosed) in which three fuel types are defined according to initial enrichment and burnup. Type 1 fuel may be stored anywhere in the racks.
Type 2 fuel must not be stored next to type 3 fuel, Type 3 fuel must be stored in a checkerboard pattern next to either type 1 fuel or empty locations, it is possible to postulate events which could lead to an increase in storage rack reactivity.
For example, the introduction of the checkerboard loading pattern means that misloading 5.0 w/o U-235 fuel in any location must be considered, The limiting case is the loading of such fuel throughout the entire spent fuel racks.
In such misloading events, credit may be taken for the presence of soluble boron (2000 ppm) in the pool water and k-eff is well i
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j 3
below the acceptance criterion of 0.95.
Other accidents such as an assembly on top of the racks or inadvertently positioned next to the sides of the racks likewise show acceptable results with 2000 ppm of soluble boron in the pool l
water. The licensee bas confirmed that plant procedures require a weekly verification of a minimum boron concentration of 2000 ppm in the spent fuel pool water. The staff considers this to be an appropriate means for allowing credit for boron in accident situations.
The new fuel storage vault is a temporary storage area for fresh, unirradiated fuel. The fresh fuel is normally stored in air (dry) and moderator can only be i
introduced by abnormal situations such as fires, which require fire fighting foam or water mist. Therefore, the criticality of the vault was studied as a function of moderator density with particular emphasis on conditions of low i
density (0.1 to 0.05 g/cc) or optimum moderetion. The results show that fuel only up to 3.675 w/o U-235 can be allowed in the fully loaded (90 assemblies) vault at optimum moderation conditions (0.05 g/cc of water) and meet the acceptance criterion of k-eff no greater than 0.98.
Therefore, in order to allow storage of fresh fuel enriched to as much as 5.0 w/o U-235, restrictions must be placed on allowed storage patterns.
In order to meet the optimum moderation acceptance criterion, criticality calculations have shown that every i
other central column location must be empty for enrichments between 3.675 and i
5.0 w/o U-235, thus allowing a total of only 81 assemblies.
This storage restriction, which has been incorporated into the plant TS, also guarantees that the k-eff of the vault will be below the 0.95 acceptance criterion in the fully flooded condition.
It is, therefore, acceptable.
[
The licensce has proposed changes to Sections 5.3 and 5.6 of the Seabrook TS l
and has proposed additional TS to Section 3.9.
Section 5.3 has been amended to allow reload fuel to have a maximum enrichment of 5.0 w/o U-235.
However, each t
reload core design, of course, will be evaluated for any enrichment to confirm that the cycle core design adheres to the limits that exist in the accident onalyses and TS.
Section 5.6 has been modified to specify that the uncertainty in the calculational methods and mechanical tolerances for the criticality calculations are at the 95/95 probability / confidence level and to add a i
description of the new fuel storage vault.
The proposed addition of two new TS, 3.9.13 (Spent Fuel Assembly Storage) and 3.9.14 (New Fuel Assembly Storage) specify the Limiting Conditions for Operation and Surveillance Requirements i
associated with the storage of fuel assemblies in the spent fuel pool and in the new fuel storage vault. The proposed changes are consistent with the criticality analyses provided and are, therefore, acceptable.
l Based on the above review and evaluation, the staff concludes that fuel i
assemblies having initial enrichments up to 5.0 w/o U-235 may be safely stored in the fresh and spent fuel racks if the requirements of the TS are met.
This i
conclusion is based on the following:
(1) Analyses were performed with well established methods that were properly
- verified, t
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1* !.
(2) Conservative input assumptions were used.
(3) The analysis results meet the NRC criteria for acceptance with respect to k-eff.
(4) The consequences of limiting accidents are acceptable.
(5) The proposed TS are consistent with the analyses provided.
3.0 STATE CONSUL,T,ATION in accordance with the Commission's regulations, the New Harrpshire and Massachusetts State officials were notified of the proposed issuance of the amendment. The State officials had no comments.
4.0 ENVIRONMEt1TAL CONSIDERATION Pursuant to 10 CFR 51.21, 51.32, and 51.35, en enviror. mental assessment and finding of no significant impact have been prepared and published in the Federal Register on August 27, 1991 (56 FR 42365). Accordingly, based upon the environmeEtal assessment, the Commission has determined that the issuance of this amendment will r.ot have a significant impact on the quality of the human environment.
5.0 CONCLUSION
The Commission has concluded, based on the considerations d4 cussed above, that:
(1) there is reasonable assurante that the health ano safety of the public will r,ot be endangered by operation in the proposed manner, (2) such activities will te conducted in complionce with the Co;nmission's regulations, and (3) the issuance of the amendment will r.ot be inimical to the common deftnse and security or to the health and safety of the public.
Principal Contributor:
L. Kopp Date:
August 27, 1991 1
ENCLOSURE TO SER n
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1 can be stored anywhere 2 rnust not be stored next to 3 3 rnust be stored nset to 1 or empty heations
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20-Type 1 15 yp
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Type 2
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Type 3 n_
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4 2.5 2.75 3.0 3.25 3.5 3.75 4.0 4.25 4.5 4.75 5.0 inrtial Ennchment (w/o U235)
Figure 3.9-1 Fuel Assembly Burnup vs. Initial Enrichment For Spent Fuel Assembly Storage
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