ML20053E462
| ML20053E462 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 05/25/1982 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20053E459 | List: |
| References | |
| NUDOCS 8206080187 | |
| Download: ML20053E462 (13) | |
Text
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e UNITEt> STATES
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!T NUCLEAR REGULATORY COMMISSION o
/... E WASHINOTON, D. C. 20555 f
DESIGI TED ORIGIN 1I.
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Csrtified By
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATIO SUPPORTING AMENDMENT N0. 32 TO FACILITY OPERATING LICENSE NO. NPF-6 ARKANSAS POWER & LIGHT COMPANY ARKANSAS NUCLEAR ONE, UNIT N0. 2 DOCKET NO. 50-368 Introduction In Supplement No.1 to the Safety Evaluation Report (SER) (Ref. 9) for the issuance of the ANO-2 operating license we stated the following:
ANO-2 is the lead plant with Combustion Engineering 16x16 fuel; therefore, there is no data base for direct evaluation of rod bowing as a function of burnup. Consequently, rod bow measurements on 14x14 fuel have been extrapolated, by the staff, to 16x16 fuel with methods which are generally conservative. This extrapolation was based on methods described in the staff's revised interim evaluation for rod bowing and combines the Combustion Engineering, Inc., data on the effect of rod bow on departure from nucleate boiling with rod bow magnitude versus exposure.
Credit has been given for thermal margin due to a multiplier of 1.05 on the hot channel enthalpy rise used to account for pitch teouction due to manufacturing tolerances. The resultant in departure from nucleate boiling ratio j
due to rod bow is given by:
Departure From Nuclear Boiling Burnup*
Ratio Penalty (points)**
0-2.1 0
2.1-5 4.0 5-10 5.9 10-15
, ~ 8.8 15-20 11.4 20-25 13.6 25-30 15.6 30-35 8206080187 820525 17,4 PDR ADOCK 05000 P
- In units of Giga watt days per metric ton of uranium.
- Points subtracted from a departure from nucleate boining ratie value. For example, a penalty of 4.0 points subtracted from 1.34 would result in a penalized value of 1.30.
1
The licensee proposed in the Reload Report submittals dated February 20, 1981 and March 5, 1981 for Cycle 2 operation to delete TS 4.2.4.4 on the basis that a two percent rod bow penalty factor was already accounted for in the minimum DNBR trip limit value of 1.24.
The position was based on the generic consideration of both fuel and poison rod bowing in Combus-tion Engineering Nuclear Steam Supply Systems which is documented in the topical report CENPD-225 (Ref. 1), " Fuel and Poison Rod Bowing,"
and the accompanying Supplements 1, 2, and 3 (Refs. 2, 3 and 4).
The staff evaluated the licensee's proposal and reported its findings in Section 2.1.3 of the SER enclosed with Amendment No. 24 to the license wherein it is stated:
"The staff has not yet approved the CENPD-225 report. Accordingly it is the staff positicn that the rod bow compensation currently specified in Technical Specification 4.2.4.4 shall remain applicable for initial Cycle 2 operation.
Evaluation The staff has retained a contractor,,The Brookhaven National Laboratories (BNL) to review the CENPD-225 reports. This review is nearing completion and BNL has prepared a preliminary conclusion on thei.' evaluation of the CENPD-225 re;
's and CE response (Refs. 11 and 12) to questions generated during BNL's review. We have subsequently received the BNL response (Ref. 13), which concluded that all but 2 questions generated during the review have been satisfactorily resolved. The two issues, as stated in the BNL review, are described and evaluated in the following paragraphs.
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Rod Bowing Statistical Methodology "The statistical methodology is considered incomplete because it accounts for bowing on only 2 of the 8 fuel rods that surround the core's hot rod.
The accounting for all rods in the neighborhood of the hot rod would increase the calculated. fuel rod bowing DNBR penalty by a factor of about 2."
Fuel Assembly Bowing "The widening of the designed inter-assembly gap increases the local neutron moderation and results in an increase in the power of fuel assembly peripheral rods.
Significant assembly bowing on the order of several hundreds of mils has been measured out-of-pile, and an g enalty may be warranted."
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We have reviewed these issues and agree with the technical merits of each.
However, because of mitigating effects that are described below, we have detemined that these 2 issues are sufficiently accommodated in, and hence resolved for, the ANO-2 safety analysis.
Rod Bowino Statistical Methodology With regard to the first issue on the DNBR statistical methodology, the esNnce of this concern is that the CENPD-225 method used to calculate the t
recfiction in DNBR on a fuel rod due to the bowing of its neighbo'rs is one diepnsional and not two dimensional.
Hence, the CE method considers the efftact due to bowing on only the 2 rods adjacent to and in the same plane as theIcentral rod of interest and, therefore, does not account for the remiining 6 adjacent rods (third and farther nearest neighbors are believed to Itave no significant influence). As stated above, BHL has found that a two ? dimensional model would predict an increase in the 'CE proposed DNBR pene]ty by a factor of about 2.
The CE proposed penalty (shown in the revised TS 4'.2.4.4) applicable to the ANO-2 fuel d6 sign, increases with burnup up to a value of about 5.3Y. at a burnup of 50 GWD/MTV. However, there are several DNBR conservatisms in the CE bowing analysis that together constitute a margin sufficient to offset the underestimated penalty; therefore the value
- t of the penalty proposed by CE is appropriate for the ANO-2 plant. Some of a
these conservatisms are:
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1.
Combustion Engineering has analyzed the worst span bow for each fuel assembly used to obtain the closure correlation coefficients.
In many cases, the worst span is in the lowemost region of the j
fuel assembly, where minimum DNBR is not likely to occur.
2.
The DNBR penalty will increase with burnup because of the associated reduction in gap spacing.
Conversely,, nuclear peaking tends to decrease with burnup.
Combustion Engineering has con-servatively not accounted for this fuel depletion effect.
3.
The DNB experiments, which employed a displaced rod and which were designed to assess the effect of bowing of one specific rod, had generalized bowing (though small) throughout on all of the other simulated fuel rods. This bowing was attributable to 2 factors:
(a) the simulated fuel rods were not manufactured perfectly straight and (b) when power was applied to the ferromagnetic cartridge inserts, magnetic forces between rods were induced thus creating widespread bowing of small magnitudes.
- Hence, the DMB experiments and the respective analyses of the DNBR penalties are not strictly applicable to only situations involving one large bow.
Rather, these penalties are more applicable to actual and more probable inpile situations and associated analyses involving a large bow in j
a field of several lesser bows.
Consequently, this aspect, though unquantifiable, will partially compensate for the use of a 2 rather than an 8 bowed-rod DNBR penalty calculation in the CENPD-225 methodology.
4.
There is modeling conservatism in the treatment of reduction-in-DNBR as a function of gap closure. As shown in Figure 1, the proposed CE licensing curve (depicted by the solid linear line) bounds the expected behavior (hypothesized by the dashed curve).
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5.
Experimental techniques used to measure rod bowing yield projected gap closures (i.e., those components of the actual gap closures that are parallel to the face of the fuel assembly).
The use of projected gap closure is conservative because the magnitude of a projected gap closure is always greater than or equal to the magnitude of the actual gap closure.
6.
The CE augmentation factor of Fr (radial peaking. factor) was assumed to be equal to the augmentation factor for Fg (total peaking factor).
Actually, the augmentation factor on F should be the statistical r
average of the heat generation augmentation factors of the 4 fuel rods which comprise the coolant channel and thus must be less than the augmentation factor for any one fuel rod.
7.
There are 2' effects that rod bowing has on DNBR.
The first is to alter the local flow area, and this effect.f s relatively small.
However, the second effect, which can be quite significant, is the perturbation of the fluid boundary layer on the hot rod.
From partial-closure DNB tests, we know that this effect is nil for closures less than 50%. The probability of having more than ime large gap closure of greater than 50% in one central region (3x3 array) of interest is low, and certainly when such a low probability is convoluted w'ith the additional small probability that one of the core's hot rods is present in this central region of interest, is then even lowerm Consequently, the construction of an 8-bowed rod DNBR penalty analysis would not likely yield a significant impact when actal gap closures are used. This is clearly true for the 4 second-nearest neighbor rods, because they are 2.5 times farther from the central rod than the nearest neighbor reds. Therefore, the probability of having the gap spacing to a second-nearest neighbor rod being less than the gap spacing to a nearest-neighbor rod is negligible.
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8.
All CE calculations were performed assuming no baron concentration in the coolant. Thus, CE maximized the rod bow augmentation factors on F and F by using the most negative moderator temperature coefficient.
l 9.
An area reduction in a coolant channel will result in a reduction in' the heat generated in that coolant channel.
Combustion Engineering took no credit for this phenomenon.
Instead, thi channel heat generation rate was conservatively assumed to increase by the magnitude of-the augmentation factor on F.r 10.
Cladding creepdown increases the nominal rod-to-rod spacing.
This phenomenon was not modeled in the CE analysis.
Fuel. Assembly Bowing Out-of-pile inspections (Refs.14,15, and 16) at several plants have detected large fuel assembly bowing on the order of several hundreds of mil s.
Such large assembly bowing is an order of magnitude greater than that of fuel rod bowing and can primarily affect both DNB and LOCA margins of peripheral fuel rods.
The DNBR of peripheral rods is significantly higher than that of interior rods of equal power. This is because peripheral rods (a) have no adjacent unheated surfaces (i.e., CEA guide tuEies) to cause a reduction in DNBR and (b) are subjected to greater cooling even when assemblies bow to contact because of the minimum inter-assembly rod spacing' affurded by the grid straps and spring offsets. Consequently, the interior fuel rods, which are essentially unaffected by fuel assembly bowing, will remain the most limiting (that is, with respect to DNB).
The impact of assembly bowing on the LOCA margin arises due to the increased local neutron moderation and concurrent power increase that accompanies the widening of the inter-assembly gap.
Consequently, assembly bowing is mostly influential on peripheral rods.
For the CE NSSS design, the power limiting rods are located next to CEAs and not on the fuel assembly periphery.
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In order to investigate the effect of assembly bowing on peripheral rod power, CE perforned sensitivity calculations.
In these calculations,
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CE employed a time-dependent assembly bow model and assessed the effects out to an inter-assembly gap spacing greater than that experimentally observed in CE designed fuel.
It was found for the maximum gap spacing assumed that the location of the most peaked rod ('i.e., with respect to the average power density) moved to a peripheral location, but that the power density in the peak peripheral rod was about the. power density in the peak interior rod if assembly bowing had not been present.
We desired to extend the CE analysis to yet a greater inter-assembly gap spacing; hence, the NRC staff extrapolated the CE results out to a gap of 800 mils (about 4 times the normal spacing). We have found that the new power density in the peaked peripheral rod was only 5% greater than that found in the CE analysis, and likewise 5% greater than that assumed in the general CE rod bowing analyses, which do not account for assembly bowing.
Nevertheless, we recognize several conservatisms that we believe to be of sufficient magnitude to individually, or certainly collectively, offset the detrimental effect of assembly bowing. Those conservatisms are:
1.
See conservatisms numbers 2 and 8 listed in the previous section on Statistical Methodology.
2.
The CE sensitivity analysis conservatively assumed that the fuel assemblies were unzoned; however, actual assemblies are zoned and have a lower enrichment in the corner regions.
3.
Assembly bow measurementshave been made out-of-pile under relatively unrestrained conditions.
In pile, there are physical constraints imposed on the assembly by the upper and lower core plates as well as neighboring assemblies or the core shroud. Th~e 'effect of these' restraints
-on assembly bowing is presently unquantified, though it probably is significant.
Based on our review of these offsetting conservatisms we conclude that no Fg penalty is required to account for fuel assembly bowing.
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We have reviewed the BNL preliminary evaluation of the CE fuel rod bowing analysis as described in CENPD-225 topical report, its supplements, and responses to questions generated during BNL's review. I~n response to the licensee's supplementai request dated May 3, 1982, for change to the Technical Specifications we have concluded that the fuel rod bowing DNBR penalty currently in the TS may be amended to reflect the values of the penalty proposed by the licensee (i.e. CENPD-225 Supplement 3 values). We have determined that the penalty values proposed in the licensee's May 3, 1982 submittal are consistent with the those in CENPD-225 Supplement 3 and are acceptable.
At the present time a rod bow penalty of two percent is included in the Core Protection Cilculator System software.
Impleme'ntation' of penalties beyond two percent is expected to be accomplished by modifying the value of the addressable constant for the power uncertainty factor, BERR1, according to the following formula which was also discussed in reference 18, page 37.
BERR1 = 1.065 x [1 + (RB + C - 2) x D/1003 B
where RB is the rod bow compensation (percent of DNBR) corresponding to the maximum fuel burnup of the limiting fuel batch; C (percent of DNBR) is any additional compensation to the DNBR limit; B is the uncertainty compensation directly affecting BERR1; D is the absolute value of the most negative derivative from the response to 492.66.
For information and clarification purposes it is noted that the DNBR trip limit value specified in the Technical Specifications and contained within the CPCS programming is 1.24.
This value includes the rod bowing compensa-tion of two percent on DNBR. For reasons which are stated in detail 8
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l in reference 18 the staff review as reported in reference 18 resulted in i
the conclusion that the approved DNBR limit should be 1.26. As stated on pages 36 and 37 of reference 18 the difference between 1.24 and 1.26 in accounted for by increasing the minimum value of BERR1 from 1.055 to 1.065. The value of 1.065 is included in the Technical Specifications Table 2.2-1.
Environmental Consideration We have determined that the amendment does not authorized a change in effluent types or total amounts nor an increase in power level and will not result in any significant environmental impact. Having made this determination, we have further concluded that the amendment involves an action which is insig6ificant from the staddpoint of environmental impact and, pursuant to 10 CFR 951.5(d)(4), that an environmental impact appraisal need not be prepared in connection with the inssuance of this amendment.
Conclusion We have concluded, based on the considerations discussed above, that:
(1) because the amendment does not involve a significant increase in the probability or consequences of accidents previously considered and does not involve a significant decrease in a safety margin, the amendment does not involve a significant hazards consideration, (2) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, and (3) such activities will be conducted in compliance with the Commission's 9
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i regulations and the issuance of this amendment will not be inimical to the common defense and security or to the health and safety of the public.
Date: May 25, 1982 Princi. pal contributors to this SER were D. Powers and Y. H. Hsii, CPB.
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References For Safety Evaluation 1.
" Fuel and Poison Rod Bowing," CE Report CENPD-225, October 1976.
2.
" Fuel and Poison Rod Bowing," CE Report CENPD-225, Supplement 1, February 1977.
3.
" Fuel and Poison Rod Bowing," CE Report CENPD-225, Supplement 2, June 1978.
4.
" Fuel and Poison Rod Bowing," CE Report CENPD-225, Supplement 3,-
June 1979.
5.
Letter from D. Trimble ( AP&LCo) to R. A. Clark (NRC),
Subject:
Cycle 2 Reload Report, February 20, 1981.
6.
Memorandum from D. F. Ross and D. G. Eisenhut (NRC) to D. 8. Vassallo and K. R. Goller, " Interim Safety Evaluation Report on the Effects of Fuel Rod Bowing in Thermal Margin Calculations for Light Water Reactors," December 8, 1976.
7.
Memorandum from D. F. Ross ar.d D. C. Eisenhut (NRC)'to D. B. Vassallo and K. R. Goller, " Revised Interim Safety Evaluation Report on the Effects of Fuel Rod Bowing in Thermal Margin Calculations for Light Water Reactors," February 16, 1977.
8.
Memorandum from R. O. fieyer (NRC) to D. F. Ross, " Revised Coefficients for Interim Rod Bowing Analysis," March 2,1978.
9.
" Staff Evaluation Report Related to Operation of Arkansas Nuclear One, Unit 2," NRC report NUREG-0308, Supplement No. 1, p. 4-1, June 1978.
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10.
Letter, J. R. Marshall, AP&L Co. to R. A. Clark, NRC, dated May 3, 1982 providing proposed penalty on DNBR to account for rdd bowing effects.
11.
Letter from A. E. Scherer (CE) to R. L. Tedesco (NRC)
Subject:
Responses to First Round Questions on CENPD-225, " Fuel and Poison Rod Bowing," Number LD-81-073, October 23, 1981.
12.
Letter from A. E. Scherer (CE) to J. R. ftiller (NRC),
Subject:
Revised Responses to Second Round Questions on CENPD-225-P, Number LD-82-021, Februsry 19, 1982.
13.
Letter from J. Carew (SNL) to D. A. Powers (NRC),
Subject:
Fuel Rod Bowing Topical Report CENPD-225, February 26, 1982.
14.
" Interim Report: Surry Unit 2 End-of-Cycle 2 Onsite Fuel Examination I
of 17x17 Demonstration Assemblies After One Cycle of Exposure,"
Westinghouse report WCAP-8873, January 1978.
15.
" Pool Side Examination of PWR Demonstration Fuel Assemblies and Creep Specimens: End-of-Cycle 2, Babcock & Wilcox report LRC-4733-5, August 1978.
16.
" Examination of Calvert Cliffs 1: Test Fuel Assembly Af ter Cycle 3,"
CE/EPRI Report RP 586-1, September 1979.
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17.
Imendment No. 24 to License No. NPF-6 and accompanying safety evaluation, issued June 19, 1981.
18.
Amendment No. 26 to License No. NPF-6 and accompanying safety evaluation, issued July 21, 1981.
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Figure 1 DNBR PENALTY DEPENDENCE ON R0D BOW contact C
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GAP CLOSURE --->
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expected behavior x
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