ML20114D824
| ML20114D824 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 08/25/1992 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20114D821 | List: |
| References | |
| NUDOCS 9209090380 | |
| Download: ML20114D824 (5) | |
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! " 3,, I NUCLE AR REGULATORY COMMISSION n
WASHING T ON, D. C. 205$5 SF ETY EV_ALVATION BY THE OFF_LC1,0F NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT N05. 43 AND 32.TQ FACILITY OPERATING LICENSE NOS. NPF-76 AND NPF-80 F,dS_ TON LIGHTING & POWER COMPANY lm,(ET NOS. 50-498 AND 50-499 SOUTH TEXAS P70 JECT UNITS 1 AND 2
1.0 INTRODUCTION
By application dated May 26, 1992, as supplemented by letter dat 2d June 3, 1992, Houston Lighting & Power Company (HL&P), et. al., (the licensee) requested changes to the Technical Specifications (Appendix A to Facility Operating License Nos. NPF-76 and NPF-80) for the South Te a Project (STP)
Units 1 and 2.
The proposed changes would reflect the revised criticality analyses and rack utilization schemes for the spent fuel storage racks in the STP units.
The spent fuel storage pool contains two rack types. The Region 1 racks use Boraficx panels in a removable stainless steel box to absorb neutrons and are currently licensed to accept fresh fuel with a maximum U-235 enrichment of 4.5 weight percent (w/o). The Region 2 racks are fabricated by trapping Boraflex panels betweet. the cell walls.
The Region 2 racks are currently licensed to take credit for the burnup of discharged fuel to allow e higher storage density.
The original criticality analyses for the STP fuel storage racks were performed by Pickard, Lowe, & Garrick (PLG), under subcontract to U.S. Tool &
Die. A subsequent reanalysis by Westinghouse, which was endertaken to investigate the reactivity effects of possible Boraflex gaps, indicated that l
the original analysis overestimated the maximum fuel enrichment of the Reg'.cn 1 racks and that the current limit of 4.5 w/o U-235 should actually be 4.0 w/o. This finding was partly re:porsible for the issuance of NRC Information Notice 92-21 and Supplement 1, " Spent Fuel Pool Reactivity Calculations."
Therefore, HL&P has submitted TS changes to corrett the error and allow for c more effective utilization of the' available storage racks.
The criticality I
aspects cf these proposed changes are evaluated below.
1 The staff, after telephone conversations with the licensee, has made several editorial changes to the original proposed technical specification (TS) revisions. Table of Contents, page xv, was revised to incorporate the changes to the B2ses related to minimum spent fuel pool boron concent*ations; the page numbers in Sections 5.6 and 5.7 were revised to reflect the adoitional pages; and the section number of 5.6.1 was added to CRITICAllTY to be consistent with l
I 9209090380 920825 l
POR ADOCK 05000478 P
9
..s e the standard format.
The editorial changes were minor in nature and did not affect the scope of the proposed amendment or the initial no significant hazards consideration determination published in the Federal Reaister (57 FR 32572).
- 2.0 EVALVATION The ousign basis for preventing criticality in a stcrage rack is that, including uncertainties, there is a 95 percent probabil ity at a 95 percent confidence level (95/95 probability / confider.ce) that the cifective neutron multiplication factor, k-eff, of the fuel assembly array in the racks will be no greater than 0.95 under all conditions. Criticality of fuel assemblies in a fuel storage rack is prevented by the design of the rack which limits fuel assembly interaction.
This is accomolished in the sip spent fuel storage racks by fixing the minimum separat.cn between fuel assemblies and inserting neutron poison (Boraflex) between fuel assemblies.
In addition, the concent of reactivity aquivalencing is used.
Reactivity equivalencing has been apprcved by the NRC for numerous fuel storage facilities and is predict,ted upon the reactivity decrease associated with fuel depletion or the addition of integral _ fuel burnable absorber (IFBA) fuel rods.
IFBAs consist of neutron absorbing materia; applied as a thin zirconium diboride (ZrB ) coating on the 2
outside of the U0 fuel pellet.
2 The reanalysis cf the reactivity effects of fuel storage in Region 1 and 2 was performed with the KENO Va Monte Carlo computer code with neutron cross sections generated b.y the AMPX code-package from the 227 energy group ENDF/B-V library. Since the KENO Va code package does not have depletion capability, burnup analyses were performed with the two-dimensional transport theory code, PHOENIX. These codes are widely used for the analysis of fuel rack reactivity and burnup and have been benchmarked against results from numerous critical experiments.
The staff concludes that the analysis methods used are acceptable.
Regions 1 and 2 ofitie STP spent fuel racks were analyzed for both closely packed configurations and for checkerboard configurations.
Fuel assembliec may be-stored in every location for the close packed configuration. The checkerboard configurations involve alternating fresh fuel with 1::: enriched fuel or water holes and are described more fully below.
For the Region I close packed storage configuration, the KENO Va model assumed minimum center-to-center fue' assembly spacings, minimum Boraflex material dimensions, maximun stainless steel thicknesses, and symetrically placed fuel assemblies.
These assumptions conservatively give a worst case calculated k-eff.
In addition, U-235 enrichment was assumed to be 4.05 w/o which conservatively accounts for enrichment manufacturing variability. The resulting k-eff was 0.9359, includirg all uncertainties at a 95/95 probability / confidence level. Therefore, the NRC acceptance criterion of k-eff no greater than 0.95'is mat for Region 1 close packed storage of fuel a::semblies enriched to a maximum ncminal 4.0 w/o U-235. This is_ categorized
- as Category 2 fuel.
l-I
i 5torage of close packed fuel assemblies with nominal enrichments greater than 4.0 w/o U-235 in Region I was also,nalyzed by means of reactivity equivalencing in which reactivity credit was taken for assembly burnup or for the presence of IFBAs in fuel assemblies.
For the reactivity decrease associated with fuel depletion, a series of reactivity calculations were performed to generate a set of enrichment-fuel assembly discharge burnup ordered pairs which yield the equivalent k-eff when the fuel is stored in the racks. The results are shown in Figure 5.6-1 of the STP TS, which shows that the reactivity of a rack containing fuel that is initially enriched to 5.0 w/o U-235 and achieves a burnup of 5400 MWD /MTU is equivalent to the rentivity of a rack containing fresh unirradiated fuel with nominal U-235 enrichment of 4.0 l
w/o.
As mentioned previously, this fuel resulted in a maximun k-eff of 0.9359 and is, therefore, acceptable.
Fuel which meets the minimum 'ournup as shown in Figure 5.6-1 is also categorized as Category 2 fuel, Reactivity equivalencing was also determined based upon the reacti"ity decrease associated with the presence of IFBAs.
In this scheme, a series of reactivity calculations were performed to generate a set of enrichment-fuel assembly IFBA content ordered pairs which yield the same equivalent k-eff when the fuel is stored in the Region 1 racks.
Figure 5.6-2 of the STP TS shows the minimum wmber of IFBA pins required in a fuel assembly, as a function o; initial enrichment, for close packed storage in Region 1.
The curve shows reactivity of the speint fuel rack array when filled with 4.0 w/o fuel that m
iFBA pins is equivalent to the reactivity of the rack when filled with with 5.0 w/o fuel with 80 IFBA pins.
Therefore, fuel which meets the minimum IFBA content shown in Figure 5.6-2 also results in an acceptable k-eff equal to 0.9359 and is categorized as Category 2 fuel.
The infinite multiplication factor, k-inf, was used as an alternative method for determining the acceptability of fuel a<sembly storage in Region 1.
The fuel array model was based on a unit assembly configuration, infinite in lateral and axial extent, for a nominal fresh 4.0 w/o assembly in the STP reactor core geometry with unborated water at a temperature of 68* F.
The resulting k-inf was 1.445. Therefore, fuel which has a reference k-inf of less than or equal to 1.445 in the core geometry will result in c maximum spent fuel rack reactivity of less than 0.95 (k-eff equal to 0.9359) when stored in a close packed configuration.
This fuel is ilso categorized as Category 2 fuel.
Storage of fresh fuel of nominal enrichment of 5.0 w/o U-235 in a two-out-of-four checkerboard array alternating with depleted assemblies which meet a burnup credit criterion was analyzed for Region 1.
The same worst case consideration of mechanical and material tolerances assumed for tiie close packed analysis was used. As shown in Figure 5.6-3 of the STP TS, the L
depleted fuel is assumed to have a reactivity equivalent to a fresh assembly l
with an initial nominal enrichment of 2.8 w/o U-235 and is cateo;rized as l-Category 3 fuel.
This minimum burnup curve starts at 2.8 w/c at 0 MWD /HTU and ends at-5.0 w/o at 17,500 MWD /MTV. The maximum k-eff for the storage of frcsh 5.0 w/o and depleted fuel assemblies, meeting the minimum burnup requirements shown in Figure 5.6-3, in a checkerboard pattern was 0.9252, including all l
Y
. appropriate uncertainties at a_95/95 probability / confidence level. This configurrtion meets the 0.95 reactivity criterion and is, therefore, acceptable.
Since the Category 3 fuel has a lower reactivity than the Category 2 fuel, Category 3 fuel can also be stored in legion 1 in a close packed configuration.
Since the Ragion 2 racks have a smaller center-to-ce.a r spacing between fuel assemblies, close packed storage in Region 2 requires a lower enrichment than for Region 1 in orde) to meet the 0.95 criticality criterion.
As for the Region I calculations, the KEN 0 Va model for Region 2 assumed minimum center-to-center fuel assembly spacings. miM. em Boraflex material dimensions, maximum stainless steel thicknesses, and symmetrically placed fuel assemblies.
The U-235 enrichment was held to the nominal value since the enrichment tolerance is incorporated into the minimum burnup requirements curve shown in Figure 5.6-4 of the STP TS. Un'.cradiated fuel enriched to a nominal 1.7 w/o U-235 resulted in a 95/9E k-eff of 0.9412. Therefore, the a,:ceptance criterion for criticality is met for Region 2 close packed storage of fuel assemblies enriched '.o a nominal 1.7 w/o U-235.
Reactivity equivalencing for burnup credit is shown in Figure 5.6-4 from which it is seen that the spent fuel rack reactivity with fuel enriched to 5.0 w/c U-235 which has achieved a minimum burnup of 42,000 MWD /MTU is equivalent to the rack reactivity of fresh 1.7 w/o enriched fuel. This is categorized as Category 4 fuel. Therefore, Category 4 fuel can also be stored in a close packed configuration in both Region 1 and Region 2.
The calculated maximum k-eff fw fresh fue' vith nominal enrichment of 5.0 w/o U-235 in a checkerboard pattern alternating with empty wner holes in Region 2 was 0.8790, including uncertainties at a 95/95 level. This configuration also meets the criticality acceptance criterion of 0.95 and is acceptable. The same worst case mechanical and material tolerances assumptions were used as in the close packed analysis with the fuel at a maximum enrichment of 5.05 w/o U-235 to account for enrichmeat manufacturing variability.
Since the neutron poison material used in the STP fuel storage racks consists
'of Boraflex panels between the cells, an analysis was performed to determine the reactivity effects of radiMicn induced shrinkage and gap formation in these panels.
The results show tnat the criticality acceptance criterion of 0.95 will still be met even if the ends of the Boraflex panels shrink up to 8.75 inches in Region 1 and up to 7.50 inches in Region 2.
In addition, the-l L
development of gaps of up to 3.75 and 3.00 inches in length at the midplane of every panel were found not to result in a violation of the 0.95 k-eff criterien in Region 1 and Region 2, respectively. Since gaps would not be expected to occur in every panel or precisely at the midplane, these gap size and location assumptions are conservative. A Bordlex surveillance program is followed at STP to detect any Boraflex degradation and to institute epropriate corrective actions if degradation of the panels is indicated.
Most abnormai storage conditions will not result in an increase in the k-eff of the racks. However, it is possible to postulate events, such as the inadvertent misloading of an assembly into a position for which the l
l l
I-l 1
4
-5 restrictions on enrichment, bui cip, or IFBA content are not met or dropping an assemble adjacent to a rack module, which coulo lead to an increase in reactivity. However, for such events credit may be taken for the presence of at least 700 ppm of boron in the pool water required whenever fuel is stored in the spent fuel racks by TS 3.9.13 since the staff does not require th_
assumption of two unlikely, independent, concurrent events to ensure protection against a criticality accident (Double Contingency Principle). The reduction in k-eff caused by the boron more than offsets the reactivity addition caused oy credible accidents.
In sunwaary, the staff has reviewed the proposed 15 changes as well as the Updated Final Safety Analysis Report (UFSAR) changes which were developed based an the reanalysis of the criticality aspects and the rack utilization schemes for the STP spent fuel storage racks.
Based on the above safety evaluation, these proposed changes were found to be acceptable.
3.0 STATE CONSULTATION
In accordance with the Commission's regulations, the Texas t tate Official was notified of the proposed issuance of the amendment.
The State official had no comments.
4.0 ENVIRONMENTAL CONSIDERATION
The amenunent 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.
The NRC staff has determinea that the amendment involves no significant irarease in the amounts, and no significaat change in the types, of any effluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. The Commission has previously issued a proposed finding that the amendment involves no significant hazards consideration and there has been no public comment on such finding (57 FR 32572). 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 prepered in connection with the issuance of the amendment.
5.0- CONCLUSION The Commission 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, and (2) such activitics 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:
L. Kopp (NRR/SRXB)
Date:
August 25, 1992 l
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