ML17264A146
| ML17264A146 | |
| Person / Time | |
|---|---|
| Site: | Ginna  |
| Issue date: | 08/30/1995 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML17264A145 | List: |
| References | |
| NUDOCS 9509060279 | |
| Download: ML17264A146 (6) | |
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UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON> D.C. 2055&4001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATING TO PROPOS D CRI ICA ITY ANA YSIS OF THE GI N
W A D S NT FUE RACKS CONSOLIDATED ROD STORAGE CANISTERS ROCHESTER GAS AND ELECTRIC CORPORATION R.E.GINNA NUCLEAR POWER PLANT DOCKET NO. 50-244
1.0 INTRODUCTION
By letter dated Hay 5,
- 1995, Rochester Gas and Electric Corporation (RGSIE) submitted proposed engineering calculations to increase the allowable Uranium-235 (U-235) enrichment of fuel to be stored in the new and spent fuel storage facilities of the Ginna plant.
The proposed changes would allow for the storage of fuel with an enrichment not to exceed a nominal 5.0 weight percent (w/o) U-235 in the new (fresh) and spent fuel storage racks.
- 2. 0 EVALUATION The analysis of the reactivity effects of fuel storage in the new and spent fuel storage racks was performed w'ith the three-dimensional multi-group Honte Carlo KENO-5a (computer code),
using neutron cross sections generated by the NITAWL (computer code) package from the 227 energy group library.
Since the KENO-5a code package does not have depletion capability, burnup analyses were performed with the two-dimensional transpo} t theory, PHOENIX (computer code).
PHOENIX was also used to determine the reactivity effects of material and manufacturing tolerances.
These codes are widely used for the analysis of fuel rack reactivity and have been benchmarked against results from numerous critical experiments.
These experiments simulate the Ginna fuel storage racks as realistically as possible with respect to parameters important to reactivity such as enrichment, assembly
- spacing, and absorber worth.
The intercomparison between two independent methods of analysis (KENO-5a and PHOENIX) also provides an acceptable technique for validating calculational methods for nuclear criticality safety.
To minimize the statistical uncertainty of the KENO-5a reactivity calculations, 270,000 neutron histories were typically accumulated in each calculation.
Experience has shown that this number of histories is quite sufficient to assure convergence of KENO-5a reactivity calculations.
Based on the above, the NRC staff concludes that the analysis methods used are acceptable and capable of predicting the reactivity of the Ginna new and spent fuel storage racks with a high degree of confidence.
The fresh fuel storage vault is intended for the receipt and storage of fresh fuel under dry (air) conditions.
However, to assure the criticality safety 9509060279 950830~
PDR ADQCK 05000244 P
PDR t Enclosure
under normal and accident conditions and to conform to the requirements of General Design Criterion 62 for the prevention of criticality in fuel storage and handling, two separate criteria must be satisfied as defined in NRC Standard Review Plan (SRP), Section 9.1. 1.
These criteria state that the maximum reactivity of the fully loaded fuel racks shall not exceed a k <<
(effective multiplication factor) of 0.95 if fully flooded with unborated water or a k,<< of 0.98 assuming the optimum hypothetical low density moderation (e.g.,
fog or foam).
The maximum calculated reactivity must include a margin for uncertainties in reactivity calculations and in manufacturing tolerances such that the true k,<< will not exceed the calculated maximum value at a 95X probability, 95X confidence level (95/95).
The maximum k << for a fully loaded vault of Westinghouse Optimized Fuel Assembly (OFA) fuel enriched to 5.0 w/o U-235 was calculated to be 0.9146 under fully flooded conditions.
For the hypothetical low-density optimum moderation condition, the maximum calculated k,<< was 0.6666 at a moderator density of approximately 6X of full density for a fully loaded vault of OFA fuel.
The calculations included a calculational bias and uncertainty derived from benchmark calculations, as well as uncertainties due to KENO-5a statistics, cell wall thickness and fuel enrichment at the 95/95 probability/confidence level.
The results conform to the acceptance criteria of SRP 9. 1. 1 and are, therefore, acceptable.
The storage racks in the spent fuel pool are divided into two regions.
Region I contains 351 stainless steel storage cells spaced
&.43 inches apart and contains no Boraflex or other neutron absorber.
Region 2 consists of 840 storage cells and contains 0.075-inch thick Bor aflex panels.
The cells are also stainless steel arranged on a 8.43-inch center-to-center spacing.
The spent fuel racks are normally fully flooded by borated water.
However, to meet the criterion stated in Section 9. 1.2 of the NRC SRP, k << must not exceed 0.95 with the racks fully loaded with fuel of the higfiest anticipated reactivity and flooded with unborated water at a temperature corresponding to the highest reactivity.
The maximum calculated reactivity must include a
margin for uncertainties in reactivity calculations and in manufacturing tolerances such that the true k,<< will not exceed 0.95 at a 95/95 probability/confidence level.
Initial calculations for Region I have shown that OFA fuel was the most reactive type.
The spent fuel storage racks in Region I were evaluated for 4.0 w/o U-235 enriched fuel moderated by pure water at 68' with a density of 1.0 gm/cc.
The fuel assemblies were arranged in a two-out-of-four checkerboard pattern.
For the nominal storage cell design in Region I, uncertainties due to tolerances in fuel enrichment and density, fuel pellet dishing, storage cell I.D. and pitch, and stainless steel thickness were accounted for as well as eccentric fuel positioning.
These uncertainties were appropriately determined at the 95/95 probability/confidence level.
In addition, calculational and methodology biases and uncertainties due to benchmarking and water temperature range were included.
The resulting k,<<
was 0.9487, meeting the 0.95 acceptance criterion.
As an alternative method for determining the acceptability of fuel storage in the Region I racks, the k (infinite multiplication factor) of a nominal fresh 4.0 w/o U-235 fuel assembly in the Ginna core geometry was determined to be 1.458.
Therefore, fuel with a reference k
no greater than 1.458 can be stored in a checkerboard configuration in Region 1 and meet the 0.95 rack reactivity acceptance criterion.
To enable the storage of fuel assemblies with nominal enrichments greater than 4.0 w/o U-235, the concept of reactivity equivalencing was used.
In this technique, which has been previously approved by the NRC, credit is taken for the reactivity decrease due to the integral fuel burnable absorber (IFBA) material coated on the outside of the UOz (Uranium oxide) pellet.
Based on these calculations, the reactivity of the fuel rack array, when checkerboarded with fuel assemblies enriched to 5.0 w/o U-235 with each containing 64 IFBA
- rods, was found to be equivalent to the rack reactivity when checkerboarded with 4.0 w/o fuel with no IFBA rods.
The calculation assumed the standard IFBA patterns used by Westinghouse with the minimum standard loading of 1.675 mg/inch of Boron-10 per rod.
Since the worth of individual IFBA rods can change depending on position within the fuel assembly, additional margin was included in the IFBA requirement to account for this.
In addition, the IFBA requirements also include a
10X margin on the total number of IFBA rods for 5.0 w/o enriched assemblies to account for calculational uncertainties.
The staff concludes that the IFBA requirement calculations contain sufficient conservatism to account for manufacturing and calculational uncertainties.
The Region 2 spent fuel storage racks were analyzed for storage of Westinghouse 14x14 OFA fuel assemblies with nominal enrichments up to 1.95 w/o U-235, and Westinghouse 14x14 STD (standard) assemblies with nominal enrichments up to 1.85 w/o U-235.
The same initial assumptions, biases and uncertainties as used for the Region 1 analyses were included, except for the effects of Boraflex shrinkage and gaps.
Since the Region 2 racks contain Boraflex, the reactivity calculations also considered the effects of Boraflex shrinkage and gap formation.
All Boraflex panels were modeled with 4X shrinkage.
Five different scenarios were examined ranging from all of the Boraflex panels experiencing random gap formation to all of the panels experiencing shrinkage from the bottom end.
Since the bottom end results in more active fuel exposure than the top end, the latter assumption is conservative.
The worst-case assumption was where 75X of the panels experience non-uniform shrinkage (random gaps) and the remaining 25X of the panels experience uniform shrinkage (pull-back) from the bottom end.
This scenario was used to perform the criticality analysis for Region 2.
Based on the results of blackness testing performed at other storage facilities, and on upper bound values recommended by Electric Power Research Institute (EPRI),
the staff concurs that these assumptions bound the current measured data and future development of additional shrinkage and gaps.
The final Region 2
- design, when fully loaded with Westinghouse OFA fuel enriched to 1.95 w/o U-235, resulted in a k,<< of 0.9469 when combined with all known uncertainties.
This meets the staff's criterion of k,<< no greater than 0.95 including all uncertainties at the 95/95 probability/confidence level and is, therefore, acceptable.
In order to store Westinghouse 14xl4 OFA as well as Westinghouse STD and Exxon 14x14 assemblies with enrichments
.up to 5.0 w/o U-235, the concept of burnup credit reactivity equivalencing was used.
This is predicated upon the reactivity decrease associated with fuel depletion and has been previously accepted by the staff for spent fuel storage analysis.
For burnup credit, a
series of reactivity calculations are performed to generate a set of initial enrichment versus fuel assembly discharge burnup ordered pairs which all yield an equivalent k,<< less than 0.95 when stored in the spent fuel storage racks.
The results indicate that a fresh OFA 1.95 w/o enriched fuel assembly yields the same rack reactivity as an initially enriched 5.0 w/o OFA depleted to approximately 36,200 NWD/HTU.
Since the Westinghouse 14x14 STD is more reactive than the Westinghouse 14xl4 OFA fuel at the low enrichment limit for the Region 2 rack, a separate burnup credit curve was determined for the STD fuel.
In addition, since the Exxon 14x14 fuel is less reactive than the Westinghouse OFA, the Westinghouse 14xl4 OFA burnup credit curve may be used for the Exxon 14x14 fuel.
A reactivity uncertainty of 0.0121 iR (reactivity increase) was applied to the burnup credit curves.
This is consistent with current practice and is acceptable.
To allow for possible future storage of Ginna fuel rods in consolidated rod storage canisters (CRSC),
analyses were performed to determine the acceptable range of the numbei of consolidated rods.
The fuel rods were assumed to be i randomly dispersed in the canister and the same uncertainties and biases used for the Region 2 rack analysis were applied.
The results indicate that an acceptable range of the number of consolidated rods is no greater than 144 or no less than 256 rods.
The storage of a canister which contains between 144 and 256 consolidated rods is not acceptable.
Host abnormal storage conditions will not result in an increase in the k,<< of the racks.
However, it is possible to postulate
- events, such as the misloading of an assembly with an enrichment and burnup (or IFBA) combination outside of the acceptable area or pool temperatures exceeding 180 'F in Region 1 (heatup event) or decreasing below 50 'F in Region 2 (cooldown event),
which could lead to an increase in reactivity.
However, for such events, credit may be taken for the presence of boron in the pool water required by proposed improved Technical Specification 3.7. 12 (current Technical Specification 5.4),
since the staff does not require the assumption of two unlikely, independent, concurrent events to ensure protection against a criticality accident (double contingency principle).
The reduction in k,<< caused by 300 ppm of boron is sufficient to mitigate the worst postulated accident in any pool region.
Therefore, the staff criterion of k,<< no greater than 0.95 for any postulated accident is met.
- 3. 0 CONCLUSION Based on the review described
- above, the NRC staff finds the criticality aspects of the proposed enrichment increase to the Ginna new 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.
Although the above-mentioned fuel is acceptable for storage in the Ginna fuel storage
- racks, evaluations of reload core designs (using any enrichment) will be necessary, to be performed on a cycle by cycle basis, as part of the reload safety evaluation process.
Each reload design is to be evaluated to confirm that the cycle core design adheres to the limits that exist in the accident analyses and Technical Specifications to ensure, that reactor operation is acceptable.
Principal Contributor:
Laurence Kopp Date:
August 30, f995
Dr. Robert C. Hecredy R.E. Ginna Nuclear Power Plant CC:
Thomas A. Moslak, Senior Resident Inspector R.E. Ginna Plant U.S. Nuclear Regulatory Commission 1503 Lake Road
- Ontario, NY 14519 Regional Administrator, Region I U.S. Nuclear Regulatory Commission 475 Allendale Road King of Prussia, PA 19406 Hr. F. William Valentino, President New York State
- Energy, Research, and Development Authority 2 Rockefeller Plaza
- Albany, NY 12223-1253 Charlie Donaldson, Esq.
Assistant Attorney General New York Department of Law 120 Broadway New York, NY 10271 Nicholas S. Reynolds Winston 8 Strawn 1400 L St.
N.W.
Washington, DC 20005-3502 Hs. Thelma Wideman
- Director, Wayne County Emergency Management Office Wayne County Emergency Operations Center 7336 Route 31
- Lyons, NY 14489 Hs. Mary Louise Meisenzahl Administrator, Honroe County Office of Emergency Preparedness ill West Fall Road, Room ll Rochester, NY 14620