ML20045H556
| ML20045H556 | |
| Person / Time | |
|---|---|
| Site: | Callaway  |
| Issue date: | 07/07/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20045H555 | List: |
| References | |
| NUDOCS 9307200376 | |
| Download: ML20045H556 (4) | |
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WASHINGTON. D.C. 20555-0001 NUCLEAR REGULATORY COMMISSION SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT N0. 82 TO FACILITY OPERATING LICENSE NO. NPF-30 UNION ELECTRIC COMPANY CALLAWAY PLANT. UNIT 1 DOCKET NO. 50-483
1.0 INTRODUCTION
By application for license amendment dated June 12, 1992, and clarifying information dated August 31, 1992, Union Electric Company (the licensee) requested changes to the Technical Specification (TS) Section 3/4.9.12, " Spent Fuel Assembly Storage," for the Callaway Plant, Unit 1.
The amendment would replace Figure 3.9.1, a graph of fuel assembly cumulative exposure (MWD /MTU) versus fuel assembly initial enrichment. The change would reflect an increase in the maximum initial U-235 enrichment for storage in Region 2 of the spent fuel pool.
The current maximum allowable enrichment of 4.25 w/o U-235 would be increased to 4.45 w/o U-235.
During the next refueling outage, Refuel 6, fuel with a maximum initial enrichment of 4.40 w/o U-235 will need to be discharged to, the spent fuel pool-Region 2.
The spent fuel pool consists of two regions, I and 2.
Region 1 is designed to store unirradiated fuel assemblies and a full core-off-loading.
Region' 2 is designed to store irradiated fuel assemblies.
The current and future fuel which. Region 2 needs to be able to store consists of three types:
- 1) Westinghouse Standard Fuel Assemblies, 2) Optimized fuel Assemblies, and 3)
Westinghouse Vantage 5 fuel Assemblies (V-5).
l The staff review addresses the criticality aspects of Region 2.
Since the licensee changed methodologies to perform the criticality calculations, the new methodology was also reviewed.
2.0 EVALUATION l
2.1 CRITICALITY ANALYSIS The spent fuel pool incorporates the maximum density rack concept.
For Region 2, spent fuel assemblies are stored in a three out of four configuration.
The current bounding exposure curves have been calculated up to a maximum initial enrichment of 4.25 w/o U-235..The methodology used to support these curves is described in Section 9.lA of the Callaway Final Safety Analysis Report (FSAR). A different methodology is used to support the exposure curves for proposed extension of the maximum initial enrichment to 4.45 w/o U-235.
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However, the method used in both cases is the well known reactivity equivalence method.
This involves selecting a conservative spent fuel pool effective multiplication constant (k-effective) which is used to generate the exposure versus fuel assembly initial enrichment curves.
Since the calculations are performed for an infinite array in all directions, the calculated k-effective is actually the infinite multiplication factor (k-infinity) of the storage racks.
The licensee has performed criticality calculations for Region 2 using the CASM0/GRPDQ codes and NITAWL/ KENO-V.a in the SCALE-4 code package. The CASMO-3 code is capable of performing burnup calculations using a 40 energy group library based on data from ENDF/B-4, and is used to generate the exposure versus initial enrichment curve.
The NITAWL code and the Monte Carlo theory code, KENO-V.a, are used to verify the CASM0/GRPDQ criticality calculations using the 27 energy group master SCALE library, and to generate.a conservative reference k-effective used for determining the exposure versus initial enrichment curve.
A conservative k-effective is found after determining the sum total of uncertainties and biases in the reactivity dut to the method bias, the calculational uncertainty, the uncertainties in the storage rack geometry, and a 5 percent uncertainty due to burnup. Since the V-5 fuel and the Optimized fuel have identical design parameters for criticality considerations, the criticality calculations were only performed for V-5 fuel assemblies and the Standard Fuel Assemblies.
A conservative k-effective of 0.9250, not including the sum total of uncertainties and biases, was chosen for each fuel type. An equivalent zero burnup enrichment which yields the same spent fuel pool k-effective as the CASMO burnup calculations was done with the KENO-V.a program.
From this verification calculation, a conservative reference k-effective of 0.921, not including the total reactivity deviation, was chosen for determining the exposure versus enrichment curves.
The design basis for preventing criticality in the spent fuel pool. is that there is q 95 percent probability at a 95 percent confidence level (95/95 probability / confidence) that the effective multiplication factor (k-effective), including uncertainties, will be no greater than 0.95 under unborated moderator conditions.
The licensee has shown that the maximum k-effective for Region 2 was calculated to be 0.9480, including all appropriate uncertainties.
This meets the staff acceptance criterion, and is, therefore, acceptable.
2.2 METHODOLOGY Since' methodologies have been changed to perform the spent fuel pool criticality calculations, the licensee has performed a benchmark analysis for CASMO-3 and NITAWL/ KENO-V.a.
The benchmark analysis includes benchmarking for criticality, depletion, and isotopic inventory calculations. CASMO-3 and NITAWL/ KENO-V.a criticality benchmarking consisted of the set of the Babcock &
Wilcox Critical Experiments, which used 2.46 w/o enriched fuel pins, and the Battelle Northwest Laboratory Critical Experiments, which used an enrichment
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of 4.306 w/o, to simulate light water reactor fuel storage conditions.
CASM0-3 depletion and isotopic inventory calculations were validated against the Yankee Rowe Core I isotopic benchmarks.
The criticality calculations had been performed for Region 2 using the spent fuel pool coolant temperature of 90 degrees F which yielded the largest reactivity for conservatism.
The reactivity versus spent fuel pool temper-ature curve shows a reactivity peak at 90 degrees F.
The Region 2 k-effective versus temperature curve in the FSAR, however, shows an increasing k-effective with temperature over the same range.
In response to a staff question con-cerning this, the licensee evaluated the dissimilarity between the curves of the reactivity with temperature, and attributed the discrepancies to the use of different codes and methodologies which have slight differences in 1) PDQ model geometries, 2) cross section data (trends with temperature), and 3) cross section generation models.
The CASMO code has been used extensively for spent fuel pool criticality calculations in the nuclear industry, and the staff finds the provided CASM0/GRPDQ benchmark results to be reasonable.
The NITAWL and KENO-V.a codes have also been extensively applied to spent fuel pool criticality calculations in the nuclear industry, and the staff finds the provided NITAWL/ KENO-V.a benchmark results to be reasonable. Thus, the staff finds the methodology used in the criticality analysis acceptable.
2.3 ACCIDENT ANALYSIS Certain postulated events which could lead to a storage rack reactivity increase were evaluated. Asymmetric positioning of a fuel element in the cells has been shown to yield results equal to or more conservative than symmetrically positioned fuel assemblies. A dropped fuel assembly will be sufficiently separated from the active fuel height of the assemblies in the rack such that there will be no storage rack reactivity increase.
Conditions which would result in an increase in reactivity, such as placing a fuel element outside or adjacent to the rack or placing a fuel element in the wrong position, were also evaluated.
Application of the double contingency principle of ANSI N16.1-1975 allows credit to be taken for the 2000 ppm of soluble boron in the spent fuel pool.
The double contingency principle states i
that an evaluation is not required to assume two unlikely, independent concurrent events to provide for protection against a criticality accident.
In both cases considered above, the evaluation showed that the spent fuel pool would stay subcritical by at least 5%.
This satisfies the NRC acceptance criterion of k-effective no greater than 0.95.
Additionally, the water box, in which no fuel is stored in the three out of four configuration, has been provided with a lead-in guide which prevents insertion of fuel assemblies into 1
the box.
The staff has reviewed the submittal for the spent fuel assembly storage technical specification change and determined that the methodology used was l
acceptable.
Figure 3.9-1 will be used to determine if a fuel assembly with an initial U-235 w/o enrichment up to 4.45 enrichment that has undergone a
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determined exposure can be stored in Region 2 of the storage racks.
The review has concluded that the spent fuel pool criticality analysis is in accordance with staff positions, and that storage of Callaway Plant fuel assemblies in Region 2 using the proposed limiting exposure curves is acceptable.
3.0 STATE CONSULTATION
In accordance with the Commission's regulations, the Missouri State official was notified of the proposed issuance of the amendment. The State official had no comments.
4.0 ENVIRONMENTAL CONSIDERATION
Pursuant to 10 CFR 51.21, 51.32, and 51.35, an environmental as.sessment and finding of no significant impact has been prepared and published in the Federal Register on July 7, 1993 (58 FR 36482). Accordingly, based upon the environmental assessment, the Commission has determined that the issuance of this amendment will not have a significant effect on the quality of the human environment.
5.0 CONCLUSION
The staff has 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, (2) such activities will be conducted in compliance with the Commission's' regulations, and (3) the issuance of this amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributors:
D. O'Neal L. R. Wharton Date:
July 7, 1993 I
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