ML20029B364
| ML20029B364 | |
| Person / Time | |
|---|---|
| Site: | LaSalle  |
| Issue date: | 02/28/1991 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20029B362 | List: |
| References | |
| IEB-90-002, IEB-90-2, NUDOCS 9103070108 | |
| Download: ML20029B364 (5) | |
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO FUEL CHANNEt EVALUATION FOR CYCLE 5 COMMONWEALTH EDISON COMPANY LASALLE UNIT _1 DOCKET NO. 50-373 1.0 INTR _0 DUCTION Reference 1, which presented Commonwealth Edison Company's (CECO) response to NRC Bulletin No. 90-02 for its Boiling Water Reactor stations, indicated that although CECO no longer places irradiated fuel channel boxes on new/ fresh fuel assemblies, previous channel box management practices included the reuse of channel boxes.
As a result, the channel boxes from the LaSalle County Station initial cycle discharge batch were placed on the fresh fuel assemblies that were loaded in LaSalle 1 Cycle 3 (as discussed in Reference 1).
These channel boxes had received a single cycle of irradiation, yielding channel box exposures from 4 to 12 GWD/STU, prior to their placement on the LaSalle 1, Cycle 3 and 4 reloads.
The licensee points out that although these residual channel boxes will accumulate their fourth cycle of irradiation during Cycle 5, because LaSalle County Station is a C-Lattice plant with uniform water gaps around the assemblies, there is less channel box bow as a function of exposure and a smaller i.mpact on local peaking (and hence critical power margins) relative to comparable D-Lattice plants.
NRC inhouse data, collected from various sources, supports this conclusion.
This Safety Evaluation covers the staff review of the CECO strategy for reuse of channel boxes in the upcoming cycle, Cycle 5.
2.0 EVALVATION 2.1 Channel Bor dow Analysis Methodolog CECO incorporated General Electric's generic channel box bow methodology for determining the R-Factor adjustments for LaSalle 1, Cycle 5.
The GE methodology has been approved by the NRC staff for determining fuel channel box bowing l
impact on the Minimum Critical Power Ratio (MCPR) and for referencing in reload i
licensing applications for both C-Lattice and 0-Lattice plants (Reference 4).
The approval was subject to application of this methodology to first bundle lifetime only.
The GE met ~odology can also be used to predict channel box bow based on cxposure L story.
In so doing the methodology takes into account the effects t
I of initial as manufactured channel box bow, the changes in bow due to stress l
relaxation under irradiation, and the changes in channel box bow due to growth l
of Zircaloy.
This data is derived from traccing the location and exposure (histories) of numerous GE and non-GE channel boxes.
The current data base history extends to approximately $5 GWD/MTV, i
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9103070100 910220 PDR ADOCK 05000373 P
The licensee pointed out that GE has tracked all the used channel boxes scheduled for installation in LaSalle 1, Cycle 5, and that it is this data that GE utilized in their analysis to calculate the channel box bow and the corresponding impact on the MCPR limits for the upcoming cycle, Cycle 5.
Since all the used channel boxes histories, maximum projected channel box bow and expected exposure (approximately 48 GWD/MTV), for LaSalle 1 Cycle 5 are enveloped :)y GE's data base, GE's methodology is, therefore, applicable to the LaSalle 1, Cycle 5 core configuration.
However, rather than use GE's core average bow tt ictermine the R-Factors, Ceco requested GE to conduct a cell average calcuiation taking into account the additional impact of the exposure history of all the reused channel boxes.
This analysis determined the average channel box bow in four-bundle cells, it should be pointed out that although the effect of individual channel box bow within a cell is mportant, it is the cumulative bow from all four channels that will im>act local peaking of the seripheral pins in an assembly. Also, this approac~ is more represer.tative tien the GE generic methodology as it accurately reflects the operating history of each channel in the four-bundle cell throughout its residence in the core and calculates the resulting channel box bow in each cell.
In conducting the cycle specific analysis for LaSalle 1, Cycle 5, GE selected 29 four bundle cells based on a octant symmetric core configuration and on individual reused chennel box exposures.
This selection process ensured that pottntially limiting cells which contained reustd channel boxes, were evaluated dircctly or by a representative octant symmetric cell configuration.
None of the 224 reused channel boxes were placed on limiting bundle in a four-bundle cell.
Cells located at or neer the core periphery were not included in the evaluation due to a large degree of margin in these low power regions.
Ceco's inhouse analysis, showed (by using GE approved computer codes) that these cells have at least 301 margin to the MCPR Operating limit at the most limiting point in the cycle.
2.2 Cell Selection in arriving at the 29 selected cells mentioned above, GE examined the histories and the ranges of exposures for the reused channels in each octant symetric cell containing one or more reused channel box.
If the histories and exposures of these cells were comparable and if their respective channel box bow was similar, then only one representative cell was chosen for evaluation.
If their histories and exposures were not comparable, each cell would have been evaluated, thus, ensuring that the effects of reused channel boxes with low and high exposures were taken into account. This method of choosing the cells to be evaluated will bound the channel box bow due to differential irradiated growth since the fluence and fluence gradient effects existing in the reused channel boxes will be taken into account. That is, since it is known that fluence accumulates directly proportional to exposure, the fluence effect is bounded by the consideration of the reused chennel boxes with high exposure; whereas, the fluence gradient effect is bounded by the consideration of the reused channel boxes with lower exposure. This lower power exposure is indicative of an operating history in a low power zone, a peripheral core location where the fluence gradient is very large.
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i 3-In conducting this channel box bow analysis for LaSalle 1, Cycle 5, GE makes the assumption that the limiting MCPR in the core occurs at the sant cycir exposure as the niaximum channel box exposure.
The minimum MCPR mergin to the MCPR Operating Limit actually occurs at mid-cycle, while the maximum channel box exposure occurs at the end of the cycle. Therefore applying the impact of the maximum channel box exposure to all cycle exposures,is constrvative.
2.3 Channel Box Bow Analysis Results in evaluating the 29 four-bundle cell locations, GE made use of the histories of the channel boxes in each cell in order to detemine the cell average bow.
Sixteen (16) out of the 29 cell were placed at non-control cell locations, while thirteen (13) of the cells were control-cell locations.
The calculated average bow for the non-control-cells is 25 mils away from the control rod, and the calculated average bow for the control cells is 52 mils 6way from the control rod.
GE's evaluation: of the non-control-cell locations suggests the use of cell average bow of 35 mils for the calculation of the corresponding R factors.
The projected channel box bow for the evaluated non-control cell locattuns for LaSalle 1, Cycle 5 indicated that two of the evaluated cells have a projected end of Cycle 5 average channel box bow which slightly exceeds the recomended bow value for the R factor adjustment.
However, these cells heving an average bow of 38 mils and 36 mils respectivcly, are located on the periphery of the core and thus, will not become limiting throughout the cycle due to their relatively low radial powers.
The most limiting assemblies in these cells were found to have a n.inimum margin of 35% and 25% to the MCPR Operating Limit respectively. The remaining non-control-cell locations have an end of Cycle 5 predicted channel box bow ranging from 13 to 32 mils. Consequently GE showed that the use of' 35 mil bcw for the calculation of the R-factors for,the non-control-cell locations will adequately account for the channel box bow during Cycle 5.
GE's results indicate that the control-cell locations rcsulted in a slightly higher degree of channel box bow predictions than those of non-control-cell locations at the end of Cycle 5.
This is to be expected since the channels in j
a control-cell location will be exposed to an increased flux gradient due to the insertion of the control blade during norra h
operation. GE's analysis of the LaSalle 1, Cycle 5 indicater sugh the control-cell locations have a slightly higher average er Jhese same cells demon-l strate greater margin to the MCpR limit tho -
atrol-cell locations.
This is because four relatively high exposur, aquently low power) bundles are loaded in these cells as part of te
.1 Cycle 5 control cell core management strategy.
GE recomended to CECO the use of cell average bow of 55 mils for the calcula-tion of the R-foctors for the control-cell locations for Cycle 5.
However, CECO proposes to use a more conservative end of cycle value of 65 mils.
This approach will bound the projected channel box bow for all the control-cells except for one control cell which has a predicted average channel box bow value of 81 mils. Addressing this one particular cell, CECO decided to replace these four channel boxes with new channel boxes prior to installation in Cycle 5.
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Finally, GE analysis shows that for Cycle 5 the minimum margin to the MCPR Operating Limit is 16% for all the control-c, ell locations (including those located in the central, high powered regions of the core).
3.0 MCPR SAFETY LlHIT EVALUATION As part of its cycle specific analysis for LaSalle 1 Cycle 5 Ceco requested GE to evaluate the impact of the channel box bow on the MCPR Safety Limit due to increased measurenet uncertaintles. GE evaluated the spread in the channel box bow data and concluded that this spread is within the tolerance used in tne generic methodology (Reference 4).
It appears that the use of the single cycle exposed channel boxes for LaSalle 1, Cycle 5 reload will be adequately accounted for by an adjustment to the R-factors. The licensee has accepted the recommendations made by GE to adjust the R factur for the non-control-cells by assuming a channel box bow of 35 mils, and adjusting the L-f 6ctor fu the control-ce.t 4 assuming a channel box bow of 65 mils.
The GE R-factor uncertainty outlined in Rcference 4 also assumes a channel box bow uncertainty which covers the entire range of the calculated values for both the control-cells and non-control-cell bundles for LaS611e 1 Cycle 5.
Since the uncertainties calculeted for LaSalle 1, Cycle 5 are within the Reference 4 database, GE concluded that no adjustment to the HCPR Safety Limit is required to ensure fuel cladding integrity.
Finally, as indicated in Reference 1, CECO has discontinued the previcus practice of channeling fresh fuel with previously irradiated channels and is committed to assuring that any residual reused channels will have no impact on safety. Ceco's ultinate goal is to phase out the reuse of used channel boxes completely in future cycles.
4.0 CONCLUSION
Based on the above evaluation the NRC staff has concluded that the licensee's Cycle 5 reload design with reused channel boxes and the methods used to account 1or the channel box bow impact on the cure operating lin.its is acceptable, because the data used and the methodology provide reasonable assurance that the thermal margin to the CpR safety limit is maintained.
If in future cycles channel box reuse is continued, further review and prior approval by the NRC staff will be required.
5.0 REFERENCES
1.
Letter from M.H. Richter, CECO, to NRC Response to NRC Bulletin 90-02,
" Channel Bow Analysis for Dresden 2 " April 26,1990.
2.
Conference call on November 26, 1990 between CECO (11. Richter et al.)
and NRC (B. Siegel, L. Phillips and A. Attard).
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t 3.
Letter from M.H. Richter, CECO. to NRC, "LaSalle County Station Unit 1 i
Fuel Channel Evaluation," January 7, 1991.
4.
Letter, P.M.-Marriot (GE) to T.E. Murley (NRC), " Fuel Channel Bow,"
August 22, 1989.
Principal Contributor:
A. Attard Date:
February 28, 1991 5
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