ML20236S525
| ML20236S525 | |
| Person / Time | |
|---|---|
| Site: | Vogtle  |
| Issue date: | 11/20/1987 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20236S511 | List: |
| References | |
| NUDOCS 8711250235 | |
| Download: ML20236S525 (6) | |
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{{#Wiki_filter:- __ ~ ) ENCLOSURE SAFETY EVALUATION ON SPENT FUEL P00L RACK DESIGN i Id V0GTLE ELECTRIC GENERATING PLANT, UNIT 1
1.0 INTRODUCTION
By Amendment 22 to the FSAR, the applicant (now licensee) indicated that a h different spent fuel rack design would be used at Vogtle Unit 1. The aopli-cant proposed to replace the original Unit 1 spent fuel pool racks having 938 fuel atoembly storage locations on a center-to-center spacing of 13.0 in, with two new high-density freestanding racks having 288 fuel assembly storage locations, or approximately 1.43 cores. Each rack consists of 144 fuel assembly cells arranged in a 12-by-12 configuration, with a center-to-center spacing of 10.6 in, between fuel assemblies. Additional fuel racks with different configurations may be added to the Unit I spent fuel pool in the future. Any additions would have to be approved by the staff. In SSER 2, the staff noted that the spent fuel storage facility would be an open item until the applicant provided more detailed design information. In particular, the staff stated that the applicant would have to provide the details of the rack design, including a criticality analysis, seismic analysis, materials compatibility analysis, and fuel handling accident analysis. The applicant provided most of the requested information in Amendment 5 to the FSAR which allowed the staff to resolve all issues regarding the design of the racks except for the seismic analysis in SSEP 4. By letter dated November 28, 1986, the staff requested that the applicant provide additional information regarding the seismic analysis of the racks. By letter dated December 29, 1986, the applicant requested a schedular exemption to regulations that apply to spent fuel pool racks. In SSER 5, the staff determined that pursuant to 10 CFR 50.12(a)(1) the schedular exemption to 10 CFR 50.34(bll2)1 as it pertains to GDC 2, 61, and 62 of Appendix A to-10 CFR 50 is authorized by law, will not present an undue risk to the public health and safety, and is consistent with the common defense and security. Accordingly, this schedular exemption was included in Vogtle Unit I license nos. NPF-61 dated January 16, 1987, and NPF-68 dated March 16, 1987, (low power and full power) for the time period before the racks contain irradiated fuel. The licensee responded to the staff's November 28, 1986, request for additional information by letter dated January 21, 1987. On the basis of the staff's review of this submittal, the staff requested additional information by letter dated April 7, 1987. The licensee responded to this request by letter dated May 22, 1987, as supplemented by letter dated July 20, 1987. The licensee further provided the spent fuel rack design report by letter dated September 29, 1987. The discussion that follows is the staff's evaluation of the licensee's submittals dated January 21, May 22, July 20, and September 29, 1987. 1 P _ _ _ _ _ _ _ _ _ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ~ - - ' - ~ ~ ~ - ~ ~
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2.0 DESCRIPTION
AND EVALUATION 1 The spent fuel pool is constructed of reinforced concrete and lined with 1/4 inch stainless steel to ensure against leakage. The spent fuel pool is located ~ within the seismic Category I fuel handling building. The pool is approximately 50 feet long and 33.5 feet wide and 41 feet deep. The spent fuel will be stored in two high density racks. Each rack consists of 144 fuel assembly cells arranged in a.12-by-12 configuration.. The cell is made from 14 gauge (0.075 inch thick) type 304 stainless steel. The nominal inside dimension of the cell -is 8.8 inches square, and the length from the top of the fuel seating surface to the top of the cell funnel is approximately 168.25 inches. Storage cells are fastened and supported by two grid assemblies, also made from type 304 stainless steel, which are located at the top and bottom elevations of the racks. The racks are assembled in a checkerboard pattern with a 10.6 inch center-to-center spacing. The racks are freestanding, neither anchored to the floor nor braced to the pool wall. Because the racks rest freely on the pool floor, it is necessary to determine that during seismic events the racks do not impact each other nor the walls, and are capable of maintaining their integrity. Thus, displacement and stress calculations of the racks are required. Analyses of the racks were performed on the Westinghouse Electric Computer Analysis (WECAN) Code. It is a general purpose finite element code with capabilities of performing static, dynamic, linear, and nonlinear analyses. The WECAN code has been audited and no errors or discrepancies were found by the NRC Vendor Program' Branch as discussed in a report dated January 7,1985 (G. Zech, NRC to J. Gallagher, Westinghouse). Effective structural properties of an average fuel cell within the rack assembly were obtained using a' three-dimensional linear structural model that represents the rack assembly. The mathematical model is shown in Figure 1. These struc-tural properties were then used in a two-dimensional nonlinear seismic model to perform seismic calculations. The mathematical model is shown in Figure 2. In addition to' the structural properties, hydrodynamic mass of the fuel, the gap between the fuel and cell, the support pad boundary condition of the freestanding rack, and the assumed. coefficient of friction between the support pad and pool floor were also input to the WECAN code. A coefficient of friction equal to 0.2 was assumed to obtain the maximum sliding distance of the base of a rack. A co-efficient of friction equal to 0.8 was assumed to obtain the maximum load in the rack and maximum structural deflection of the rack. Based on experimental test data on water-lubricated stainless steels, E. Rabinowicz concluded in " Friction Coefficients of Water-Lubricated Stainless Steels for a Spent Fuel Rack Facility," dated November 5, 1976, that a design based on friction coefficient values be-tween 0.2 and 0.8 should cover all eventualities. Therefore, the range of coefficients of friction between 0.2 and 0.8 used by the licensee is reasonable. The maximum loads thus obtained were then input to a three-dimensional structural model to obtain local stresses in the rack. The licensee indicated that the maximum single rack sliding displacement was 0.036 inches, and the maximum lateral deflection at the top of the rack was 0.1 inches. Because the clearance between the two racks is approximately 4
- inches and the minimum clearance between the racks and pool walls is 4.25 inches, it is apparent that the racks will not impact each other and the walls, because the actual clearances between the rack-to-rack and rack-to-wall are much greater than the distance due to rack sliding or deflection calculated for the safe shutdown earthquake (SSE) condition. The licensee has provided analysis results of stresses due to the interactions between the rack and pool floor and between the fuel and cell within a rack for both operating basis earthquake (0BE) and SSE conditions. The results indicated that the maximum stresses in the liner plate and concrete floor are less than allowable stress values for the rack and pool floor interaction and that the maximum acceleration on the fuel assembly was 3.0 g. In response to the staff request as to what g levels the fuel assembly had been qualified for, the . licensee stated in the submittal of July 20, 1987, that the actual maximum test load was 20 g. Therefore, the integrity of the fuel assembly will be maintained during earthquakes. The licensee has calculated the maximum thermal stress in the rack as 4000 psi by assuming that one cell was loaded with hot fuel while the surrounding cells remained empty. When this maximum thermal stress is combined with stresses associated with seismic and dead weight, the licensee stated that the total stress, thus obtained, was still below the allowable stress value. Therefore, the integrity of the rack will be maintained during thermal and seismic conditions. During a July 15, 1987, telephone conference call, the staff questioned the validity of using 25 percent of critical damping for the fuel grid and asked for documentation. The licensee responded in the July 20, 1987, submittal that the grid damping value of 25 percent had been developed from data presented in the WCAP 9401-P-A, whicn was approved by the staff by letter dated May 7,1981. The staff considers that the damping question has been answered and resolved. The licensee has also performed an analysis of dropping a fuel assembly straight through a storage location, and the results indicated that the pool liner would not be perforated. The licensee did not perfonn the first and second types of fuel assemF y accident analyses as specified in the NRC position paper, " Operating Technolor Position for Review and Acceptance of Spent Fuel Storage and Handling Applica',ons," dated April 14, 1978, as modified on January 18, 1979. However, the l'.ensee stated that, with the 2000 ppm boron in the pool water, there would be r deformation that could reasonably be achieved by the drop of a fuel as';mbly that would cause the criticality acceptance criterion of 0.95 to be r.ceeded. The staff has reviewed this issue and agrees with the licensee. iherefore, the staff considers that the licensee has adequately addressed the fuel assembly accident analysis requirements. The licensee stated that the cask crane was designed in such a way that it could not pass over the spent fuel pools and, therefore, this would preclude the movement of heavy loads over the spent fuel pools. The staff considers that this design is acceptable. The licensee stated that the methodology used for the Vogtle Unit I spent fuel pool analysis is the same as that which had been used for McGuire, Turkey Point, Peach Bottom, and Palisades and that the models used in the analysis are similar s ~
l 4. .) to those which had been used for these:four plants, and that the methodology and model had been reviewed and found acceptable by the staff for the referenced plants. On October 8 and 9,1986, the staff performed an audit at the Westing-house, Pensacola, Florida facility for the spent fuel rack design of Palisades .f ~ and was satisfied with the general methodology and mathematical models used in the analysis. The staff approval of the general methodology and analysis models was documented:in the Palisades SER included in the issuance of Amendment 105 dated July 24, 1987.
3.0 CONCLUSION
l Based on the review of the_submittals by the licensee, the staff has concluded I that the licensee has adequately and satisfactorily addressed all the issues related to the seismic adequacy of the spent fuel pool rack design at Vogtle Unit ~1. The total weight of 288 spent fuel assemblies, as currently proposed, is less than the total weight of 938 spent fuel assemblies,'which was previously licensed. Therefore, the capability of the spent fuel pool ' structure, which has been designed and licensed for supporting 938 spent fuel assemblies, is more than adequate for supporting 288 spent fuel assemblies. The capability of the spent fuel pool structure should be re-analyzed when the total weight of spent fuel and racks exceeds the total weight of its previous design. Based on the staff's acceptance of the spent fuel pool rack design submitted by the licensee, the staff has concluded that the schedular exemption included in the Unit 1 full power license is no longer required. l 1. L l I
I /N f\\ \\ N Top Cell to es s N N PX Cen Weld-l g N-N n S / t - - / ( s s ( s N N S ' N N \\ S / t ~ ~ ~ e s s s ' ~ ( s N N S i .e L \\ s N Cell to Cell Weld / t ~ ~ - p g N ( E ' p N Q ' m m - s /\\ \\ / s E i h I N / t ~ ~. fs s s ' - - ,k( \\ M b Base Plate y Structure Figure 1 Structural Model of Typical Puel Rack i
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y y FUEL-TO-CELL / 4' d' ( GAP ELEMENT / M) / v n w ( = V N / { ' It) L S, / SUPPORT PAD ~ (k) l e / (>-- / '~~ / - - / k' [ ... y / / _.lllf///////////////j Figure 2 Nonlinear Seismic Model _ - - - _ _ - _ _ _ _ _ _ _ _ _ _ _ _}}