ML20056D935
| ML20056D935 | |
| Person / Time | |
|---|---|
| Issue date: | 01/11/1991 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20056D931 | List: |
| References | |
| NUDOCS 9308190028 | |
| Download: ML20056D935 (11) | |
Text
{{#Wiki_filter:.. ( ENCLOSURE SAFETY EVALUATION FOR THE TOPICAL REPORT " FUEL CHANNEL BOW ASSESSMENT"
1.0 INTRODUCTION
By letter to the NRC staff (Ref. 1) the General Electric Company (GE) has submitted its assessment of the impact of fuel channel bowing on thermal margins for plants with GE fuel. The submittal includes a determination of the channel deflection due to bowing, the resulting effect on local power peaking, and a determination of-the increase in operating limit minimum The critical power ratio (MCPR) required to insure adequate thermal margins. assessment concludes that the plant specific reduction in thermal margin 1 varies from negligible to a few percent. The submittal includes a method for accounting for the effects of channel bow which provides a best estimate MCPR The estimated uncertainty is prediction as well as an uncertainty estimate. found to be much smaller than the existing liCPR uncertainty allowance and is neglected. GE plans to account for the reduction in MCPR'at operating plants by adjusting-the R-factor values in the process computer data bank. The MCPR adjustment will be based on a prediction of core average bow using an empirical irradiation growth model that has been qualified by comparison to channel bow measurements, Section 3.5 of this evaluation. It is also assumed that the fuel channels are not reused and the methodology is only applicable to the first fuel channel lifetime. Brookhaven National Laboratory (BNL) has been the staff consultant in this review under f!N No. A-3868. 9308190028 910133 -{DR TOPRP EMVGENE PDR
i 1' 2 2.0
SUMMARY
OF THE FUEL CHAfifiEL BOW ASSESSMENT The GE channel bow assessment includes a quantitative evaluation of the impact -l of increased channel-to-channel spacing together with a method for accounting for the reduction in thermal margin. The principal elements in this assess-i ment are the determination of: (1) the expected mean and standard deviation of 4 the fuel channel displacements, (2) the sensitivity of the local pin powers to increased channel-to-channel spacing, and (3) the process computer bundle The GE q R-factor adjustments required to insure adequate thermal margin. evaluation of each of these factors is summarized in the following. 2.1 Determination of Fuel Channel Bowing (1) The GE evaluation attributes the fuel channel bowing to three sources: initial bow -B, (2) stress relaxation bow with irradiation - delta B5R, and g The stress j (3) bowing due to irradiation growth of zircaloy - delta B IR. relaxation bow is assumed to be random and averages to zero over the core, is taken while the radial channel bowing due to irradiation growth delta Big to be a linear function of the strain difference between the channel walls The core average bow, which is used to determine the defining the water gap. bundle R-f actor adjustment, is determined by a full core Monte Carlo procedure in which the bow for each channel is varied randomly about its predicted value by an amount determined by the uncertainty in the initial, stress relaxation and irradiation growth bow components. This calculation results in a substantial core average channel bow for the D-lattice (with the wider water gaps) than that for the C-lattice. The Monte Carlo calculation also calculates a standard deviation in the core average bow due to the input uncertainties. The core-average bcw is assumed to have a sinusoidal axial shape with a zero deflection at the top and bottom of the core, q k r
3 2.2 Calculation of the Increase in Local Power Peakin? = The effect of the increased channel-to-channel water gaps that result from channel bowing is calculated with the GE 1attice physics code. The primary assumption in these calculations is that the increase in powe" peaking is determined by the average bow E of the four bundles surrounding the control This assumption was validated by three calculations in which the cell blade. average bow B was fixed: (1)asinglebowedbundle,(2)twodiagonalbowed bundles,and(3)twoadjacentbowedbundles. While the bundle displacement geometry varied in these calculations, the pin power increases were very similar supporting the sole dependence on B. The GE analysis then assumes that the power peaking calculations can be performed using a symmetric GE perturbation in which all four bundles are displaced by the same amount. has performed sensitivity calculations that indicate that (to a good approxi-mation) the percent rod power increase is a linear function of the water gap In addition, the GE calculations indicate that the sensitivity to thickness. bowing is a function of bundle exposure and void fraction and is independent of I lattice design and enrichment. The calculations presented indicate an increase in corner red power for a completely symmetric standard deviation in the water gaps. 2.3 Impact of Fuel Rod Bowino on MCPR The impact of channel bowing on the BWR thermal margin calculation is incor-porated as a bundle adjustment applied directly to the R-factor used in the GEXL correlation. The fractional adjustment depends on bundle exposure and void fraction and is linear in the core-average bow 6. The adjustment depends on the rod location in the bundle but is independent of the bundle fuel design. z The adjustment is used to convert the core-average bcw 3 into a bundle dependent R-factor increase and also to determine the R-factor' uncertainty introduced by the spatial /model variation of the bowing - q. The GE assess-ment concludes that the R factor uncertainty is very small compared to existing R-f actor uncertainty allewance and may be neglected.
4
3.0 TECHNICAL EVALUATION
The GE assessment of the effect of channel bowing on the determination of the critical power ratio and the monitoring of the core power distribution is given in Reference 1. The initial review of this assessment resulted in a The sries of questions which were transmitted to GE in References 2 and 3. evaluation of the GE channel bowing assessment of Reference I and the responses of References 4 and 5 is sumarized in the following. 3.1 Octermination of Channel Bow As the fast neutron fluence exposure to the Zircaloy fuel channels increases with fuel burnup, the channels undergo irradiation growth and are deflected from their nominal core positions. The increased growth of the channel walls adjacenttothenarrowwatergaps(regionsofrelativelyhighfastflux)tends to deficct the fuel channels diagonally toward the narrow narrow gap and away from the wide-wide gap. Fuelchannelsinfastfluxgradients(e.g.,onthe coreperiphery)alsoexperiencechanneldeflection,however,thisbowingmay be either diagonal or parallel to the channel faces. The GE bowing methodology assumes that the channel displacement is diagonal, away from the control rod in order to account for this simplification an additional R-factor gaps. uncertainty allowance is included in the MCPR evaluation (Response-19, Ref. 4). The distribution of channel bow (mean and standard deviation) depends on the cycle core loading. This dependence results from (1) the dependence of the channelstrainonthebundlefluenceandinitialbowand(2)thegeometrical dependence of the bowing on the arrangement of the fuel bundles in the core. In the GE methodology the effect of channel bowing is evaluated statistically in terms of the core average bow and the standard deviation about this average -q These statistical parameters are determined using a Monte Carlo technique. In this procedure the fuel bundle channel bow is varied randomly basedon(1)allowablefabricationtolerances(whichdeterminetheinitialbow and the zircaloy growth via the channel texture factor) and (2) the uncertainty in the bowing model prediction. In addition, using a cycle specific reload 4 .m
s 9 batch fraction and reload batch average discharge exposure, the bundle loading in each four bundle cell is varied randomly based on cell loading probabilities. The bundle-specific prior operating history is also varied based on design and operating practice. Separate Monte Carlo analyses are performed for the D-lattice plants which have larger control rod water gaps and increased bowing. The use of generic (1) bundle loading probabilities and (2) bundle operating histories to determine the core-average bow I and standard deviation g for a specific reactor cycle introduces an additional uncertainty into the calcula-tion of the effects of channel bowing. In Response-2 of Reference 5 GE has indicated that for Monte Carlo trials, drawn from the same generic probabilities, the plant-to-plant variation in core average bow 6 and its associated uncertainties was found to be negligible compared to plant specific variations. The added uncertainty in addressing a typical rather than a plant specific core leading is negligible compared to the uncertainty allowance already associated with calculating the MCPR safety limit. That is, channel box bow uncertainties factored into the statistical calculation of the R-factor uncertainties plus MCPR uncertainties, make " typical" channel box bow uncertainties seem negligibit in comparison. Consequently, the GE methodology for determining the core average channel box bow and the standard deviation accounts for all significant effects anri provides an acceptable estimate of the true calculational uncertainty, t 3.2 Effect of Fuel Channel Dow on t.ocal Power Peaking The ef fect of channel bowing, and the resulting increased water gaps, on rod The calculation power peaking is calculated with the GE lattice physics code. The models the fuel bundles surrounding the control channel water gaps. boundary conditions imposed on the outer boundary of this cell affect the The sensitivity of the power peaking to changes in the water gap thickness. GE method determines these sensitivities using reflecting boundary The comparison of reficcting and periodic boundary conditions conditions.
6 ,? * ' provided in Response-3 (Ref. 4) indicates that the peaking sensitivities are in approximate agreement, except for the single bundle displacement which is An additional underpredicted, using reflective boundary conditions. uncertainty allowance is included in the MCPR methodology to account for the effcet of these assumed boundary conditions (Response-19. Ref. 4). In the bowing analysis the four fuel bundles included in the calculational cell are not the actual neighboring bundles that occur in the cycle core However, the power peaking sensitivity to bowing is believed to be loading. independent of the enrichment and exposure distributions of the three neighboringfuelbundles(Response-19,Ref.4). The determination of the power sensitivity to channel box bowing is made in a j conservative manner, using approved GE methods, and includes an adequate l allowance for known uncertainties and is therefore acceptable. 3.3 Effect of fuel Channel Bowing on the CPR The decrease in critical power ratio due to channel bowing is determined by an 1 exposure and void dependent correlation which is linear in the cell average channelbowB. In Figures 20.1 and 20.2 of Response 20 (Ref. 4) GE has provided the dependence of the individual rod powers and increase in R-factor 1 on channel bowing for typical bundle designs. The dependence of both the rod powers and the R-f actor is, to a good approximation, linear for bowing up to I 60 mils, which is greater than the channel offset expected during normal DWR i operation. The slope of the R-factor curves is used to determine the tincar for each red in the bundle as a function of cxposure and void sensitivity Aj A fuel design independent generic value for A) (E. V) is determined fraction. for each rod location in the bundle. The uncertainty in R factor resulting from the use of the generic, rather than a fuel dependent, sensitivity is less An allowance for this simplification is included in the i than 0.25 porcent. R-f actor uncertainty determination (Response-19 Ref. 4), i .t
I 7 a It is important to note that the simple linear dependence of the local power peaking and R-factor on the cell-average bowT assumes that the limiting HCPR j j locationdoesnotoccurinthebundle(inthefour-bundlecell)havingthe j largest bow (Response-12, Ref. 4).. This could occur in the case of a fresh fuel rod bundle contained in a reused second-lifetime channel with_large bow. However,inResponse-23(Ref.4)GEhasindicatedthattheproposedchannel bowing MCPR evaluation methodology is not applicable to second-lifetime fuel ] channels. t in most cases the fuel rod located in the corner of the fuel bundle experiences j the' largest increase in power. However, since this rod is not necessarily l limiting the R-factor sensitivity Aj(E.Y)iscalculatedforallrodsinthe j
- bundle, j
The core fuel loadings for certsin plants include fuel from multiple fuel j in this case, the determination of the core HCPR requires the-calculation of the bowing-penalty for non GE fuel. GE has indicated in-f
- vendors, Response 1 (Ref. 5) that (1) all rods bundles in the core will be GE fuel
] l designs and (2) '.n each application the utility. will confirm that the nominal i (unbowed) dimensions of the GE and non GE fuel channels are identical. Consequently, the calculated R-factor sensitivities Aj(E,Y)areapplicable. l to all' fuel bundles in the core. 4 The channel bowing displacement for the non GE fuel channels is required for the determination of the core average channel bow. GE has indicated in Response 1 (Ref. 5) that in the absence of bowing data for non GE channels, it will assume the performance of the GE and non GE channels to be equivalent and will apply.the GE bowing correlations to all. channels. The acceptability of-i this procedure will depend on the design and performance of the specific ] non-GE channels, and justification and appropriate' uncertainty allowance i should be provided in each reload application. j 1 it is concluded that, with the limitation to first lifetime cha'nnels and the-j provision for non GE fuel channels, the calculation of the reduction in CPR_ j due to channel bowing is acceptable. 'j [
8 7 . :.. p 3.. ,e. 3.4 Fuel Misleading /Misorientation Both the average channel bow B and the sensitivity of the R-factor to channel The may be affected by a fuel misleading or miserientation. bowing Aj sensitivity A, however, is to a good approximation determined by the location y of the fuel rod in the bundle and is independent of the fuel bundle In addition, the fuel channels are generally bowed so that the orientation. control rod water gap is increased and, consequently, a miserientation will result in a reduced water gap adjacent to the rotated high enriched rods and an increase in CPR. In the fuel misloading the maximum delta CPR penalty occurs when a highly exposed low-powered fuel bundle is replaced by a fresh high-powered bundle. Assuming the misloaded bundle is not contained in a second-lifetime channel, the new fuel bundle will have minimum bow and the associated CPR bowing penalty will be less than the value determined using the standard bowing analysis (Response 3 Ref. 5). The analysis cf fuel misleading /miserientation is therefore considered acceptable. 3.5 Evaluation of CE Channel Box Bow Data General Electric (GE) provided the HRC staff with channel box bow data :s a function of burnup. This data was compared with other channel box bow data obtained from such sources as ANF, EPRI, KW, and the Swedish Regulatory Authority. The evaluation of the GE data with other inhouse data is given below. 4 The bow data provided to HRC by GE was data with burtup, ranging from 0 to 50 j GWd/HTu Also, it was assumed in this evaluation, that the statistically cvaluated correlation utilized by GE included the adi!ition of two times the standard deviation, (.e., Bow (mils) = / Bow min / + 2 r i
9 Where / Bow min / refers to the mean value, i.e., average bow of the channel away frca the control blade, and sigma is the standard deviatien. To aid the NRC staff in evaluating this data, the staff correlated all of the This bow data was bow data available to it from fuel vendors and licensees. usually in the form of scatter plots of channel bow as a function of burnup. The data from all The staff developed limiting curves for each data source. of the sources were plotted as limiting curves. This provided the NRC staff with a visual representative of all the limiting bow data as a function of exposure and in a graphical manner. The results were very favorable. The plots indicate that there is very good agreement among all the data especially within the constraint of the single bundle lifetime, which is taken to be below approximately (40-50) GWd/MTu. That is, all of the data were consistent in magnitude and trends. In f'act, from 0 to approximately 50 GWD/MTu, channel bowing can be taken to be approximately linear. Beyond 50 GWD/MTu, data from all sources is sparce, consequently, viable data comparison is very difficult. As a result, based on As the small number of data points available, one can only point to a trend. such, beyond 50 GWD/MTu, channel bowing as a function of exposure is not well characterized. However, the rate of bowing with exposure does appear to have increased. Based on the evaluation discussed above, we conclude that the GE data used in the analysis of channel bow rs a function of exposure for single bundle lifetire channel boxes are acceptable. 4.0 TECHNICAL POSITION The General Electric assessment of the effects of channel bowing, including the determination of critical power and power distribution monitoring, as des:ribed in References 1, 3, and 5 has been reviewed in detail. It is concluded that the proposed methodology is acceptable for fuel channel bowing
10 i". analyses and for referencing in reload licensing applications with the A~ following conditions: (1) The methodology for determining the effect of channel bowing is not applicable to second-lifetime fuel channels (Section 3.3 and Section 3.4). Additional justification and appropriate uncertainty allowances sheuld be (2) provided for each application of the GE procedures and correlations to thedeterminationofthechannelbowingofnon-GEfuelchannels(Section 3.3).
5.0 REFERENCES
Letter, J. S. Charnley (GE) to R. C. Jones, Jr. (NRC), " Fuel Channel Bow 1. Assessment," November 15, 1989. Letter, Robert C. Jones (NRC) to J. S. Charnley (GE), " Request for 2. Additional Information Regarding Fuel Channel Bow Assessment," March 30, 1990. Letter, J. S. Charnley (GE) to R. C. Jones, Jr. (NRC), " Responses to 3. i Channel Bow Questions," May 3, 1990. L i Letter,RobertC. Jones (NRC)toJ.S. Charnley (GE),"Requestfor i 4. Additienal Information Regarding the Letter MFN085-99, ' Fuel Channel Bow s i Assessment,'" June 6, 1990, 1 Letter, J. S. Charnley (GE) to R. C. Jones, Jr. (NRC), " Responses to. 5. Channel Bow Questions," September 26, 1990.
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