ML20133G950
| ML20133G950 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 01/14/1996 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20133G944 | List: |
| References | |
| NUDOCS 9701160235 | |
| Download: ML20133G950 (5) | |
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UNITED STATES 4
j-j NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 2066H001
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO. 178 T0 i
FACILITY OPERATING LICENSE NO. NPF-6 J
ENTERGY OPERATIONS. INC.
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ARKANSAS NUCLEAR ONE. UNIT NO. 2 DOCKET NO. 50-368
1.0 INTRODUCTION
In a letter of August 23, 1996, Entergy Operations, Inc. (E01), requested
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changes to the Arkansas Nuclear One, Unit 2 (ANO-2) Technical Specifications (TS) to reflect an increase in the U-235 enrichment of fuel stored in the 4
fresh fuel storage racks or the spent fuel storage racks from 4.1 weight percent (w/o) U-235 to 5.0 w/o U-235.
The staff's evaluation of the criticality aspects of the proposed enrichment i
increase follows.
2.0 EVALUATION The analysis of the reactivity effects of fuel storage in the ANO-2 fresh and spent fuel racks was performed with the SCALE-4 code package which included the BONAMI-S code, the NITAWL-S code, and the three-dimensional Monte Carlo code, KENO-Va.
Since the KENO-Va code package does not have burnup j
capability, depletion analyses and the determination of small reactivity increments due to manufacturing tolerances were made with the two-dimensional transport theory code, CASM0-3. SIMULATE-3, a three-dimensional nodal simulator was also used to provide data for the evaluation of burnup i
distribution and spectral history effects. The SCALE-4 system used in the i
reactivity analysis has been benchmarked against experimental data for fuel assemblies similar to those for which the ANO-2 racks are designed and has been found to adequately reproduce the critical values. These experimental data are sufficiently diverse to establish that the method bias and uncertainty will apply to rack conditions which include close proximity storage and strong neutron absorbers. The staff concluded that the analytical j
method was acceptable and capable of predicting the reactivity of the ANO-2 storage racks.
The NRC acceptance criterion for preventing criticality outside the reactor, including uncertainties, assumes a 95% probability at a 95% confidence level (95/95 probability / confidence) that the effective neutron multiplication factor (k,,,) of the fuel assembly array will be no greater than 0.95.
This k,,, limit applies to both the fresh and spent fuel racks under all conditions, except for the fresh fuel rack under low water density (optimum moderation) conditions, where the k,,, limit is 0.98.
4 9701160235 970114 PDR ADOCK 05000368 P
1 For the fresh fuel racks, the analyses conservatively assumed the most reactive fuel type in use or stored at ANO-2 and a U-235 enrichment of 5.0 w/o over the entire length of each fuel rod. The criticality analyses evaluated the effects of varying moderator density and showed that the optimum moderation condition occurs at a density of 0.065 gm/cc and results in a k,,,
of 0.976, including uncertainties at a 95/95 probability / confidence level.
Since k,,deration conditions is met.is less than 0.98, the acceptance criterion for criticality optimum mo For the fully flooded accident scenario, the 95/95 k,,, is 0.916 and meets the acceptance criterion of 0.95.
The ANO-2 spent fuel storage pool is categorized into two regions, referred to as Region 1 and Region 2.
The Region 1 storage racks contain Boraflex panels held in place by a stainless steel wrapper plate. The Region 2 racks contain no neutron absorbers.
For the nominal storage cell design, the moderator was assumed to be pure water at a density of 1.0 gm/cc and a temperature of 68'F which conservatively bounds the range of normal pool water temperatures.
Uncertainties due to tolerances in U-235 enrichment and density, fuel inner and outer diameter, guide tube thickness, stainless steel thickness, and assembly position were accounted for including a method bias and uncertainty.
These uncertainties were appropriately determined at least at the 95/95 probability / confidence level.
In addition, an allowance for uncertainty in depletion history and isotopic calculations for those cases where burnup credit was used, were included. These biases and uncertainties met the previously stated NRC requirements and were, therefore, acceptable.
In the Region I calculations, additional assumptions were made to consider the increase in reactivity due to Boraflex gap or shrinkage.
The results of reported blackness tests performed on other Westinghouse racks were used to determine a maximum shrinkage of 4.1%.
Three scenarios were considered to determine the most conservative assumption:
- 1) all panels have gaps, 2) all panels have end shrinkage, and 3) 65% of the panels have gaps and 35% have end shrinkage. The location and size of the gaps and/or end shrinkage were based on Boraflex test results reported by the Electric Power and Research Institute (EPRI) and were acceptable.
The most limiting configuration for the ANO-2 rack design was the case where all Boraflex panels have end shrinkage. Also, the minimum design B-10 loading and physical dimensions were assumed as well as a 4.1% shrinkage in panel width. These were acceptable conservative assumptions based on existing industry-wide test results.
In response to NRC Generic Letter 96-04, "Boraflex Degradation in Spent Fuel Pool Storage Racks,"
the licensee stated that pool silica levels indicated some Boraflex degradation due to water ingress may be occurring. However, since the ANO-2 configuration only allows minimal water flow around the Boraflex panels, substantial degradation due to water erosion is not immediately expected.
The licensee's analysis using the acceptable methods discussed above has shown that fuel assemblies with enrichments up to 5.0 w/o U-235 can be stored in Region 1 as long as each assembly is adjacent to two water holes or located diagonally from four water holes. This configuration is called " Configuration A".
The calculated 95/95 k,,, value for this configuration is 0.938.
. Credit for fuel assembly burnup was used to allow storage of assemblies with enrichments up to 5.0 w/o U-235 in an arrangement utilizing all of the rack cells. This is called " Configuration B",
and the allowable burnup versus initial enrichment (in terms of average U-235 loading per unit length) is shown in TS Figure 3.9.2.
(The upper limit of 0.614108 gm U-235 per inch is equivalent to 5.0 w/o U-235). The calculated 95/95 k,,, value for this configuration is 0.942.
There are three allowed storage configurations for Region 2 which are called A, B and C.
Configuration A utilizes a checkerboard array to allow storage of assemblies with enrichments up to 5.0 w/o U-235.
Each assembly must be adjacent to four water holes. The calculated 95/95 k,,, for this configuration is 0.926.
For Configurations B and C in Region 2, credit for assembly burnup is used to allow storage of assemblies enriched to 5.0 w/o U-235.
In Configuration B, fuel assemblies meeting the Configuration B burnup versus initial U-235 loading curve shown in TS Figure 3.9.2 can either be stored adjacent to two water holes or in diagonal locations from the four water holes.
The calculated 95/95 k,,, value for Configuration B is 0.944.
For Configuration C in Region 2, any assembly meeting the burnup requirements shown in TS Figure 3.9.2 may be stored in any Region 2 rack location. The 95/95 k,,, value is 0.9498.
Since the Region 1 racks are not separated from the Region 2 racks by additional water spacing, calculations were performed to determine if any limits should exist for the region 1 cells at the Region 2 interface. These calculations show that 5.0 w/o assemblies should be restricted from storage in the first row of Region 1 cells at the Region 1 - Region 2 rack interface.
Region 1 Configuration B assemblies may be placed in the first row.
Therefore, no restrictions are placed on the storage of Region 1 assemblies which meet the burnup versus initial U-235 loading requirements of Configuration B (Region 1).
Criticality analyses were also performed for the fuel transfer upender and the containment temporary storage rack. These analyses indicated that two assemblies with enrichments up to 5.0 w/o U-235 could be transported using the fuel upender while maintaining k less than the 0.95 limit assuming no credit for soluble boron. AssembTieswithenrichmentsupto5.0w/oU-235 could be stored in the containment temporary storage racks while maintaining k,,, less than 0.95 for both normal and accident conditions.
Althoughthek,h,limitofeitherthefreshfuelorthespentfuelstorage racks will not exceeded under abnormal storage conditions, it is possible to postulate events, such as flooding the dry fuel storage racks or the inadvertent misloading of an assembly in the spent fuel storage racks with an unacceptable burnup and enrichment combination causing an increase in reactivity.
Flooding of the fresh fuel racks with full density rack or optimum moderation conditions would allow the limiting k,,, of 0.95 and 0.98,
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respectively to be met.
For the spent fuel pool accidents, credit was taken j
for the presence of soluble boron in the pool water, which is assured by TS 3.9.12.c. This is because the staff does not require the assumption of two unlikely, indeper. dent, concurrent events to ensure protection against a i
criticality accident (Double Contingency Principle). The reduction in k caused by the boron more than offsets the reactivity addition caused by,,,
credible accidents.
The following TS changes have been proposed as a result of the requested enrichment increase. Based on the above evaluation, the staff finds these j
changes acceptable as well as the associated Bases changes.
- 1) TS 3.9.12.a is being revised to allow fuel assemblies containing enrichments of up to 5.0 w/o U-235 to be stored in the spent fuel pool.
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- 2) TS 3.9.12.b and Figure 3.9.2 are being revised to provide alternate storage configurations in the spent fuel pool in order to accommodate the higher enrichment fuel.
- 3) TS 5.6.1.2 is being revised to allow fuel assemblies containing a maximum i
U-235 enrichment of up to 5.0 w/o to be stored in the fresh fuel storage l
racks.-
Although the licensee did not address a specific higher fuel burnup value in i
this amendment, the staff evaluated the consequences of operation at a i
bounding value (60,000 MWD /T) because the licensee's reference to the use of highly enriched fuel (up to 5.0 weight percent U-235). The fuel handling j
accident doses associated with extended burnup in the SER for Amendment i
No. 111 for ANO-2, dated November 27, 1990, bounds this request. The staff i
concludes that the radiological consequences associated with this accident are within the acceptance criteria set forth in 10 CFR Part 100 and the control room operator dose criteria specified in GD-19 of Appendix A, to 10 CFR j
Part 50 and are acceptable.
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3.0 STATE CONSULTATION
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In accordance with the Commission's regulations, the Arkansas State official was notified of the proposed issuance of the amendment. The State official j
had no comments.
4.0 ENVIR0lfiENTAL CONSIDERATION The amendment changes a requirement with respect to installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20 and changes surveillance requirements. The NRC staff has determined that the amendment involves no significant 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 l
occupational radiation exposure. The Commission has previously issued a pro-posed finding that the amendment involves no significant hazards
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consideration, and there has been no public comment on such finding (61 FR 52964). Accordingly, the amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9).
Pursuant to 10 CFR 51.22(b) no environmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendment.
5.0 (QHCLUSION Based on the review described above, the staff finds the criticality aspects of the proposed increase in the fuel enrichment limit of fuel that can be stored in the ANO-2 fresh and spent fuel pool storage racks are acceptable and meet the requirements of General Design Criterion 62 for the prevention of criticality in fuel storage and handling. The proposed TS changes correctly j
state the evaluated enrichment, burnup, and storage configuration requirements and are acceptable.
The Commission has concluded, based on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of the i
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 the amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributor:
Larry Kopp Date: January 14, 1997