ML20205T118
| ML20205T118 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 04/16/1999 |
| From: | Christian D VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 99-132, NUDOCS 9904270151 | |
| Download: ML20205T118 (20) | |
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April 16, 1999 U. S. Nuclear Regulatory Commission Serial No.99-132 Attention: Document Control Desk NLOS/ETS:R0 Washington, D.C. 20555 Docket Nos. 50-338 50-339 License Nos. NPF-4 NPF-7 Gentlemen:
VIRGINIA ELECTRIC AND POWER COMPANY NORTH ANNA POWER STATION UNITS 1 AND 2 PROPOSED IRRADIATION OF FUEL RODS BEYOND CURRENT LEAD ROD BURNUP LIMIT Virginia Electric and Power Company (Virginia Power) plans to irradiate a small number of fuel rods to end of life rod average burnups ranging from about 65,000 to 73,000 MWD /MTU. Irradiation of these rods is intended to provide data on fuel and material performance that will support industry goals of extending the current fuel bumup limits, and will provide data to address Nuclear Regulatory Commission (NRC) questions related to fuel performance behavior at higher burnups. These fuel rods have already been irradiated for three cycles, and currently have cumulative rod average burnups ranging
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from approximately 49,000 to 55,000 MWD /MTU. The fuel rods will replace some of the
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original fuel rods in a once-burned fuel assembly, which will then be irradiated for one additional cycle in North Anna Unit 2.
As detailed in Attachment 1, the use of these fuel rods will be fully evaluated as part of our normal reload design process, and it is expected that all design criteria will be satisfied.
h Although the proposed irradiation of this limited number of fuel rods to high burnup does not require any Technical Specifications changes, a specific safety evaluation will be performed for the condition consistent with the existing core reload program to ensure that no unreviewed safety question exists as defined by 10 CFR 50.59.
A licensing base commitment on rod burnup limits affects the implementation of this proposed program. In a letter dated December 14,1993, the NRC identified a 60,000 MWD /MTU lead rod burnup limit for Virginia Power. Since the high burnup fuel rods will operate to burnup levels exceeding this limit, NRC approval is requested prior to the implementation of this program.
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r The fuel assembly containing the rods that will be irradiated to high burnup is scheduled
'to be used in North Anna Unit 2, Cycle 14, which will begin operation in October 1999. To support the core reload design schedule for this cycle, we request NRC concurrence with this irradiation program by the end of June 1999.
By means of this letter, we also wish to document a clarification of terminology used by i
Virginia Power with regard to reconstituted fuel assemblies at North Anna Units 1 and 2.
Our Technical Specifications change request (Serial No.93-706, dated November 19, 1993) to allow the use of reconstituted fuel assemblies at North Anna Units 1 and 2, and the NRC's Safety Evaluation Report (dated August 9,1994) approving this change, described reconstituted assemblies as assemblies in which failed fuel rods have been replaced with solid stainless steel or zirconium alloy rods.
Current reconstitution technology may occasionally require removal of an intact fuel rod, e.g., to facilitate removal of a failed rod, and under some circumstances it may be prudent to also replace the intact rod with a solid filler rod, rather than reinsert it into the fuel assembly. The possibility that an intact-rod might be replaced with a filler rod was not explicitly considered in our November 19,1993 submittal. However, in evaluating the reuse of fuel assemblies with solid filler rods, it is the final configuration of the assembly that is important, rather than the integrity of the fuel rods that were replaced. Therefore,we wish to clarify that the term
" reconstituted fuel assembly" can be applied to any North Anna fuel assembly in which fuel rods have been replaced with solid metal filler rods, regardless of whether the replaced fuel rod was failed or intact. Reconstituted assemblies will continue to be explicitly modeled neutronically, and thermal hydraulic calculations will continue to evaluate the exact configuration of any reconstituted assembly, to confirm that the CHF correlation used for DNB predictions remains applicable. Therefore, compliance with NRC requirements on j
the use of reconstituted fuel will not be impacted. Documentation of NRC concurrence on the acceptability of this definition of " reconstituted fuel assembly" is requested, to clarify that the use of such fuel assemblies is within the scope of activity allowed by the North Anna Technical Specifications.
If you have any questions or require additional information on this, please contact us.
Very truly yours, D. A. Christian Vice President-Nuclear Operations Attachment Commitments made in this letter:
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None.
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Regional Administrator
' U. S. Nuclear Regulatory Commission, Region ll Atlanta Federal Center 61 Forsyth St., SW, Suite 23T85 Atlanta, GA 30303 Mr. M. J. Morgan NRC Senior Resident inspector North Anna Power Station i
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Atn.., ment i Discussion of Proposed Program
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INTRODUCTION Virginia Power plans to irradiate a small number of ZlRLO' fuel rods to high burnup.
Irradiation of these fuel rods will provide data on fuel and materials performance that will support industry goals of extending the current fuel burnup limits, and will provide data to j
address Nuclear Regulatory Commission (NRC) questions related to fuel performance behavior at high bumups. The data will also help confirm the applicability of nuclear design and fuel performance models at high burnups.
The fuel rods to be used'i: his program were originally fabricated by Westinghouse as part of a demonstration assembly that was irradiated in North Anna Unit 1 (References 1 through 4). These ZlRLO-clad fuel rods have already been irradiated for three cycles, and currently have cumulative rod average burnups ranging from approximately 49 to 55 GWDMTU. The fuel rods will replace some of the original fuel rods in a once-burned Westinghouse fuel assembly (Assembly 3A4), which will then be irradiated for one additional cycle in North Anna 2. The end of cycle rod average burnups of the ZlRLO fuel rods are expected to range from about 65 to 73 GWDMTU, while the remsinder of the fuel i
in the hast assembly will achieve a burnup of approximately 46 GWDMTU (assembly average bumup). Irradiation of a small number of fuel rods in this manner will generate fuel performance data at high bumups with minimalimpact on core operation.
The use of these fuel rods will be fully evaluated as part of our normal reload design process, and all design crittoria are expected to be satisfied. Based on our preliminary evaluation, no unreviewed safety questions will exist as a result of irradiating this small number of ZlRLO fuel rods to high bumup in the North Anna 2 core. However, as the fuel rods'will operate to bumup levels in excess of the lead rod bumup limit currently identifed for the North Anna units (References 5 and 6), NRC concurrence is requested prior to implementation of the program.
Westinghouse will be performing the fuel rod design analysis for all fuel used in the North Anna 2 Cycle 14 reload design, including the demonstration assembly. Additional conservatisms will be used in the evaluation of the high burnup fuel rods since these rods will exceed the current lead rod burnup limit. The analysis of this assembly will be documented separately, and !s expected to show that all fuel rod design criteria that are applicable for the current lead rod burnup limit of 60 GWDMTU are also satisfied for the l
high burnup fuel rods.
Virginia Power is also documenting a clarification of terminology with respect to reconstituted fuel assemblies. A Technical Specifications change requent (Serial No.93-706, dated Noventer 19,1993) to allow the use of reconstituted fuel assemblies at North Anna Units 1 and 2 described reconstituted assemblies as those in which failed fuel rods have been replaced with solid stainless steel or zirconium alloy rods (Reference 7). The NRC's Safety Evaluation Report associated with this change incorporated similar words
-(Reference 8). Current reconstitution technology may occasionally require removal of an
'ZlRLO is a registered trademark of the Westinghouse Electric Company.
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intact fuel rod, e.g., to facilitate removal of a failed rod. We have determined that under
, some.circt.mstances, it may then be prudent to also replace the intact rod with a solid filler rod, rather than reinsert it into the fuel assembly. The possibility that an intact rod might be replaced with a filler rod was not explicitly considered in our submittal. However, in evaluating the reuse of fuel assemblies with solid filler rods, it is the final configuration of the assembly that is important, rather than the integrity of the fuel rods that were replaced.
As discussed in Reference 7, reconstituted assemblies will continue to be explicitly modeled neutronically. Also, as required by Reference 8, thermal hydraulic calculations will continue to evaluate the exact configuration of any reconstituted assembly, to confirm that the CHF correlation used for DNB predictions remains applicable. Therefore, our application of the term reconstituted fuel assembly" to mean any North Anna fuel assembly in which fuel rods have been replaced with solid metal filler rods, regardless of whether the replaced fuel rod was failed or intact, has no impact on compliance with NRC requirements for the use of reconstituted fuel.
BACKGROUND in March 1981, Virginia Power requested an increase in the North Anna maximum fuel enrichment to 4.1 weight percent U2", to allow an eventual increase in the discharge fuel burnups. The NRC Safety Evaluation Report that allowed implementation of this change limited the fuel to a batch average burnup of 37,000 MWDMTU. In late 1983, we i
requested removal of this batch average bumup limit, citing a Westinghouse topical report supporting higher burnups. The NRC concluded that it was appropriate to increase the limit to 45,000 MWDMTU, but not remove the restriction entirely, as NRC review of the Westinghouse topical report was still in progress. This batch average burnup restriction was unchanged when, in 1990, the NRC approved an increase in the North Anna maximum fuel enrichment to the current limit of 4.3 weight percent U2". In 1992, citing the NRC's approval of the Westinghouse high burnup topical report, we again requested that the NRC remove the batch average burnup restriction that had been imposed on Virginia Power. Upon review of our request, the NRC increased the batch average restriction to 50,000 MWDMTU or above, provided that the maximum rod average burnup of any fuel rod is no greater than 60,000 MWD /MTU. This burnup restriction is documented in References 5 and 6, and currently remains applicable at both North Anna and Surry although it is not explicitly stated in the License Conditions or Technical Specifications for either power station. The proposed irradiation of a small number of fuel rods to extended bumups at Nonh Anna therefore requires NRC approval to exceed this restriction on lead rod burnup.
Fuel rods with Westinghouse's advanced cladding materiat, ZlRLO, were first irradiated in North Anna Unit 1 in 1987 in two demonstration assembi;M (Assemblies AM1 and AM2).
These demonstration fuel assemblies had Zircaloy-4 skeletons and most of the fuel rods were standard fuel rods with Zircaloy-4 cladding, but a limited number of rods in each assembly were made with the advanced cladding material. Assembly AM1 was irradiated for two cycles, and achieved an assembly average burnup of about 37.7 GWDMTU.
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Fuel Assembly AM2 was irradiated for three operating cycles, and achieved an aseembly
. average bumup of about 51.7 GWD/MTU. Several fuel rods were removed from this assembly after its first operating cycle, and replaced with new (unirradiated) fuel rods.
These replacement fuel rods included additional rods with ZlRLO and Zircaloy-4 cladding, as well as rods with an additional zirconium based advanced cladding material. The proposed irradiation program for North Anna Unit 2 uses eight ZlRLO-clad fuel rods from Fuel Assembly AM2 that were irradiated for three cycles.
At the time these assemblies were initially inserted into North Anna Unit 1, the advanced cladding materials had only been irradiated in a test reactor, and exemptions to several i
sections of the Code of Federal Regulations (specifically,10 CFR 50.44,10 CFR 50.46, and Appendix K of 10 CF 50) were required to support the program. Federa! regulation changes have since eliminated the need for these exemptions for ZlRLO, which is now licensed as a fuel cladding material for full reload batches of fuel at a number of domestic
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reactors, including North Anna Units 1 and 2.
I SAFETY SIGNIFICANCE
SUMMARY
The extended burnup of eight ZlRLO fuel rods in once-burned Fuel Assembly 3A4 will be fully addressed as part of the North Anna 2 Cycle 14 Reload Safety Evaluation, using
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Virginia Power's NRC approved reload design methods and Westinghouse % approved fuel j
rod design models and methods. The fuel rods are expected to satisfy all design criteria that are applicable for the current lead rod burnup limit. Ir. addition, the impact on safety analyses will also be determined as part of the cycle specific evaluation. The existing analyses of record are expected to remain applicable. Likewise, there will be no impact on core operation, including setpoints. A preliminary assessment has not identified any unreviewed safety questions as defined in 10 CFR 50.59; a final determination of whether an unreviewed safety question exists will be.nade after the cycle specific reload calculations are complete. NRC approval to exceed the 60,000 MWD /MTU lead fuel rod burnup limit imposed on Virginia Power is requested for these fuel rods.
The separate language clarification on the definition of reconstituted fuel will not affect evaluations performed to support the use of such fuel assemblies. Cycle specific evaluations will continue to be performed to confr.;n that fuel assemblies with solid filler rods meet all design criteria and safety limits, as well as any additional regulatory requirements on the use of reconstituted fuel. These evaluations consider only the final configuration of the fuel assembly, and are therefore independent of the integrity of the cladding of the fuel rods that were replaced. NRC concurrence with the clarification will confirm that use of fully evaluated reconstituted fuel assemblies in which some non-failed rods may have been replaced with solid filler rods is within the existing scope of activity allowed by the current North Anna Technical Specifications.
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' PROPOSED EXTENDED BURNUP PROGRAM 11, Description of Fuel Assembly-Fuel Assembly'3A4 is a once-burned North Anna improved Fuel assembly, which is a
' design incorporating Westinghouse 17x17 VANTAGE SH features (Reference 9). The assembly has 2.ircaloy-4 fuel rod cladding, guide thimbles and mixing vane mid-grids.
Additional features include a removable top nozzle and a debris resistant bottom nozzle, although this assembly does not include the protective grid that has been added to more recent batches of fuel for increased debris resistance. Like other Westinghouse fuel at North Anna, Assembly 3A4 does not include integral poisons, axial blankets, or
- intermediate flow mixing mid-grids.
Fuel Assembly 3A4 was fabricated as part of one of the first reload batches to incorporate the low pressure drop Zircaloy-4 grid that is characteristic of the Westinghouse VANTAGE 5H design. Experience has shown that this foal assembly design may be susceptible to -
self-excited fuel assembly vibration, particulai.y when used in peripheral core locations (where larger gaps permit more assembly vibration to occur). Assembly 3A4 was used in j
an interior core location in its previous operating cycle, and will be used as the center fuel assembly in North Anna 2 Cycle 14. In these positions the assembly would not be expected to experience any significant fuel assembly vibration. However, as a precaution an insert component will be placed in the assembly during Cycle 14 to mechanically stiffen the assembly and reduce the potential for assembly vibration. This insert, which was previously used in fuel assemblies of a similar design in peripheral core locations, consists of Zircaloy-4 rods attached to a baseplate such as that used for discrete burnable absorbers. Like all fuel insert components, the presence of this vibration suppression
- insert will be considered in the core design calculations.
i Assembly 3A4 was previously irradiated in North Anna 1 Cycle g, and currently has an assembly average burnup of approximately 25.8 GWD/MTU.
The assembly was discharged after one cycle of operation because it was determined to contain a felled fuel rod. The failed rod, in assembly location J07 (Figure 1), has since been removed and replaced with a solid stainless steel rod.'
' Eight additional fuel rods in Assembly 3A4 have been removed, and replaced with ZlRLO
' clad fuel rods that were irradiated for three cycles in North Anna Unit 1 in Westinghouse demonstration assembly AM2 (References 1 through 4). The ZlRLO fuel rods are distributed throughit the assembly, with four rods on the periphery and four rods placed in interior locations, as shown in Figure 1. Asse.dly AM2 operated on the periphery of the core in its third operating cycle, resulting in a significant bumup gradient across the i
' The failure mechanism for this rod could not be determined. A review of manufacturing records did not identify any condition or abnormality in processing that might have led to the failure, and it was initially expected that the failure may have boon caused by debns, although very little debns was seen in the fuel when the core was offloaded at the
' end of Cyde 9. Dunng reconstituhon. a large hole was observed at one upper grid elevation, and secondary hydriding crar&s could be soon in the daddin3 n the spans between the hole locatson and the adpecent grids; howrver, no debris i
wear marks evidence of grid to rod fretting, or other primary defect mechanism was readily apparent.
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assembly at end of life. Therefore, the rod average burnup of the ZlRLO rods that were
. removed for use in Assembly 3A4_now ranges from about 49.2 to 54.8 GWD/MTU.
Because these rods were fabricated for use in a demonstration assembly, they were well characterized during fabrication. Data are also available, on some of the rods, from examinations performed after previous operating cycles. The ZlRLO fuel rods were manufactured to the same nominal dimensions as normal production fuel rods, and contain fuel pellets with a nominal density of 95% theoretical, which is the same as production fuel.
The initial enrichment of the fuel in these rods was 4.0 weight percent U23s, slightly lower than the initial enrichment of the fuel in Assembly 3A4 (i.e.,4.2 weight percent U 35). The 2
. fuel rods were prepressurized with helium, to the same pressure used for both Assembly -
' 3A4 and for current reload fuel.
- 2. Mechanical Design Evaluations Westinghouse will perform the mechanical design assessment of Fuel Assemb!y 3A4. The current configuration of the fuel assembly and planned operating conditions in North Anna 2 Cycle 14 wdl be considered. All current licensed fuel design criteria are expected to be satisfied, even when accounting for the end of life burnups of the ZlRLO fuel rods from demonstration assembly AM2.
2.1 Fuel Rod Design for the ZlRLO Rods As for any reload design, Westinghouse will assess the fuel rod design criteria for all rods in Fuel Assembly 3A4, using their approved models and methods. Calculations will be performed to demonstrate that all criteria will be satisfied for the planned operation.
The majority of the fuel rods in Fuel Assembly 3A4 have Zircaloy-4 cladding and have experienced only one cycle of operation. The assembly average bumup of Assembly 3A4 at the end of Cycle 14 is only expected to be about 46 GWD/MTU. At this burnup level, no difficulties are foreseen in showing that all fuel rod design criteria will be satisfied.
The performance of the ZlRLO fuel rods in Assembly 3A4 will also be assessed using NRC-approved models. Calculations will be performed as part of the normal reload design analysis to demonstrate that all fuel rod design criteria that are normally evaluated for r: load fuel will be satisfied for the projected lead rod burnup levels. For the high burnup fuel rods in Assembly 3A4, the most limiting criteria will be those on rod intemal pressure and cladding corrosion. Based on similar calculations already performed for extended bumup of ZlRLO fuel rods at another utility, no difficulties are expected in satisfying all design criteria to the projected end of life bumups.
A developmental corrosion model for ZlRLO will also be used as a conservative I
assessment tool to predict the cladding corrosion 'on the high bumup fuel rods. This developmental ZlRLO corrosion model accounts for variables such as lithium concentration in the primary coolant, oxide layer thicknees, and variations in cladding composition. The j
developmental corrosion-model is based on data from fuel rods in North Anna l
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9 demonstration assemblies AM1 and AM2 as well as lead test assemblies from another utility Use of this model may demonstrate reduced margin to the fuel rod intemal pressure limits, but the _ design criterion is still expected to be satisfied. This is especially true since integral fuel bumable absorber (IFBA) is not used in the North Anna fuel, and fuel-clad gap reopening is not normally predicted. It is therefore expected that the end of life pressure for the ZlRLO fuel rods in Assembly 3A4 will continue to remain below the design pressure limit. In addition, it is not expected that the upper bound steady state corrosion screening criteria will be exceeded, and thus the 10 CFR 50.46 total local oxidation limit of 17% will be satisfied. These additional conservatisms are considered appropriate for assessing the
' fuel rod design criteria to the higher burnups that will be reached by these fuel rods.
It should also be noted that at least one extended burnup program similar to the North Anna program is already in progress at another U.S. utility using Westinghouse fuel. The lead rod bumups in that program will be comparable to those in the ZlRLO rods in Assembly 3A4.
The performance of the high burnup fuel rods will continue to be assessed against the current fuel-related Technical Specifications throughout the cycle. Specifically, the fuel will be required to meet the current reactor coolant activity limits. The impact on the current safety analyses will also be evaluated, as discussed in Section 6 below.
1 2.2 Fuel Assembly Design Because Fuel Assembly 3A4 has only operated for one cycle, irradiation in North Anna 2 Cycle 14 will be well within the operating experience of similar fue! assemblies. No unusual conditions exist that would affect the ability of the assembly to meet all mechanical design requirenants, including areas such as: compatibility with all in-core, fuel handling, and storage interfaces; grid iinpact strength; grid cell force and fretting wear resistance requirements; and fuel assembly growth aiicwances.
Use of ZlRLO fuel rods in a Zircaloy-4 skeleton (guide thimbles and grids) does not present any special concems, because of the similarity in composition and properties of these two materials. Demonstration fuel assembly AM2, from which the ZlRLO rods were taken, similarly contained mostly Zircaloy-4 clad fuel rods in a Zircaloy-4 skeleton, with only a limited number of fuel rods having advanced cladding materials (including ZlRLO).
Assembly AM2 was successfully irradiated for three cycles, and reached an end of life assembly average burnup of about 51.7 GWD/MTU, which exceeds the expected end of Cycle 14 burnup for Assembly 3A4. Fuel assemblies with full complements of ZlRLO fuel rods in Zircaloy skeletons have also been irradiated at other utilities.
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. 3. Thermal Hydraulic Design i
Fuel assembly and coro component pressure drops will not be affected by use of a small number of high bumup fuel rods in a once-bumed assembly. The thermal hydraulic performance of Fuel Assembly 3A4 will therefere be performed in accordance with the Virginia Power's normal reload design methodology (References 10 and 11), using NRC-6
1 approved codes and methods. The fuel assembly will be required to meet the same design
, criteria as other fuel assemblies in the core.
As noted above, Fuel Assembly 3A4 incorporates one solid stainless steel filler rod, which replaced a fuel rod that had failed during the assembly's one operating cycle. This solid stainless steel rod is in an interior location (Figure 1), and is not adjacent to any of the ZlRLO fuel rods that will operate to high bumups. The higher bumup ZlRLO fuel rods will be less reactive than the majority of the fuel in Assembly 3A4, and are thus expected to operate at slightly lower powers than the adjacent fuel rods. Because of their distribution through the assembly, the presence of the ZlRLO rods is not expected to nave a significant detrimental impact on the thermal performance of the fuel assembly, in accordance with Reference 8, Fuel Assembly 3A4 will be specifically evaluated as part of the normal reload design evaluation to verify that the assembly configuration does not introduce a change in radial gradients in the flow and enthalpy distribution that could invalidate the applicability of the CHF correlation used for DNB predictions.
- 4. Neutronic Performance i
i Consistent with Reference 12, a nuclear design evaluation will be performed for North Anna 2 Cycle 14 to demonstrate that the reload core will meet all applicable design criteria.
Appropriate core physics models will be applied to reflect the actual geometry of Assembly
- 3A4.
The presence of the solid stainless steel rod and the eight high bumup ZlRLO fuel rods will affect the reactivity and local peaking due to the redistribution of power within the assembly. Assembly 3A4 will be modeled in a conservative manner on a pin-by-pin basis to determine the effect on local power peaking and corewide reactivity parameters (i.e.,
critical boron concentration and boron coefficient). Past changes made to the model inputs to incorporate neutronically significant changes to the Westinghouse fuel designs, including the modeling of some replacement fuel rods inserted into demonstration assembly AM2 after its first operating cycle, have resulted in fully acceptable predicted-to-measured power distributions and reactivity parameter agreement.
The physics parameters for the high burnup fuel rods will be determined using standard calculational methods and procedures, with the high burnup neutronic effects being incorporated primarily through the microscopic cross sections. Present design calculations account for substantial levels of plutonium and fir,sion products. and the current methods are expected to adequately treat the neutronics change associated with the extended i
bumup of the ZlRLO rods in' Assembly 3A4. The depletion methods used to track the plutonium and fission product isotopics and various normalization procedures are also expected to be equally valid for the high burnup fuel rods.
' As only eight fuel rods in the core will be operated to high burnup levels, the effect on the reload physics design for North Anna 2 Cycle 14 is expected to be relatively small. The fuel assembly containing these rods will be located in the center of the core, and will 7
operate at an assembly average power near the core average power throughout the cycle.
. The high burnup fuel rods will not be in the highest fuel rod power density locations in the core, and will not be limiting with respect to any safety analysis limit. If the effect on the
' design calculations is significant, it will be reflected in a!! phases of the design and safety analysis by either explicit calculations or additional uncertainties, as appropriate, to ensure that the assembly is treated in a conservative manner.
- 5. Impact on Spent Fuel Pool In general, higher bumup fuel is expected to have onlp a minor impact on evaluations for the North Anna spent fuel pool. High burnup fuel could impact both criticality calculations and' calculations cf the decay heat load.
Criticality calculations for the North Anna spent fuel pool currently do not take credit for the decrease in fuel reactivity with increasing burnup. The analyses of record therefore will remain conservatively bounding for the high burnup fuel rods in Ascsmbly 3A4.
With respect to possible impact on the spent fuel pool heat load, the major contributor to the heat load immediately after the core offload is decay heat from short-lived isotopes.
These isotopes tend to reach an equilibrium condition during normal operation, and the concentrations do not change significantly with increasing bumup. Therefore the presence of eight high burnup fuel rods in Assembly 3A4 wi!! not affect the limiting case analysis currently described in the North Anna UFSAR.
A portion of the spent fuel pool heat load is due to long-term decay heat from assemblies stored in the sper,t fuel pool. This long-term decay heat load is caused by longer-lived actinides, which increase with bumup. Therefore, higher fuel bumups will slightly increase this contribution to the pool heat load. However, under the proposed program only eight fuel rods in one assembly will reach high bumups. The ultimate impact on this long-term decay heat contribution to the spent fuel pool heat load is therefore expected to be negligible. In addition, it should be noted that for the limiting case analysis discussed in the UFSAR, the heat contribution from such older fuel is itself only a small portion of the total spent fuel pool heat load. It is therefore concluded that the presence of this small number of high burnup fuel rods in Fuel Assembly 3A4 will not affect the spent fuel pool heat load analysis currently described in the North Anna UFSAR.
- 6. Safety Evaluations No fundamental change in the safety and acci:Jent considerations are anticipated as a result of the extended burnup of these eight fuel rods.
6.1 LOCA Analysis Fuel temperatures and pressures used in the LOCA analysis are calculated by Westinghouse using their fuel performance code. Provided that the fuel red design criteria are satisfied, the limiting conditions for the safety analyses occur near beginning of life, 8
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when fuel temperatures are at a maximum due to fuel densification. As noted above, it is
. anticipated that all fuel rod design criteria, including the criteria on rod internal pressure, will be satisfied for the extended burnup of these fuel rods, in which case the limiting conditions for the input to the safety and LOCA analyses remain unchanged. Therefore, operation of these rods is not expected to impact the existing LOCA analysis for North Anna 2 Cycle 14.
i 6.2 Non-LOCA Safety Analyses The potential impact of the high burnup fuel rods on the North Anna non-LOCA safety analyses will be addressed as part of the North Anna 2 Cycle 14 reload safety analysis.
As Westinghouse will be required to show that all fuel design criteria are satisfied for these fuel rods for the proposed operating conditions, the limiting fuel temperature inputs to the safety analyses will remain unchanged.
The melting temperature of UO, decreases with burnup, which has the potential to affect safety analyses. As part of the normal reload safety analysis, it will therefore be necessary to verify that fuel melt will not occur in the high burnup fuel rods at linear heat generation rates that may be reached during Condition ll transients.
The analyses of record for Condition lli and IV transients will not be affected by this physical property, because the predicted temperatures in most cases remain far below the fuel melting temperature.
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~ One exception is the control rod ejection transient, for whi n v fercentage of the fuel is predicted to melt for the most limiting case. The analysis of the control rod ejection accident is based on conditions in the peak rod in the core, which is most likely to be a i
fresh or once-burned fuel rod. Although the extended burnup fuel rods will be operating at moderately high powers, particularly for a fourth operating cycle, they will not be operating at the peak power position in the core. The high burnup rods are also less i
reactive than the remainder of the fuel in this once-bumed assembly, and so will consistently operate at a lower power than even the assembly average power. In addition, Fuel Assembly 3A4 will be located in the center of the core, which is far away from the
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control rod and core locations that historically have been limiting for this control rod ejection j
transient for North Anna. The base analysis for this accident is therefore not expected to be affected by the presence of these eight high burnup fuel rods. The cycle specific evaluation for North Anna 2 Cycle 14 will explicitly consider the lower UO melting point in 2
the high bumup rods, to confirm that the amount of fuel pellet melting remains acceptable i
for all rod ejection analysis scenarios.
It is therefore expected that the cycle specific reload analyses for North Anna 2 Cycle 14 will confirm that operation of these eight fuel rods to extended bumups will not increase the probability of occurrence or consequences of any postulated accidents.
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i 6.3 RadiologicalImpact For accidents in which the core remains intact (i.e., cladding failure but no fuel melt) the release will involve only volatile fission products. The radionuclides contributing most to these doses are short-lived, and so do not increase with burnup. Therefore no increase in the consequences'of such accidents is expected as the result of operating' eight fuel rods to high burnups in Fuel Assembly 3A4.
For severe accidents where an appreciable amount of fuel melting occurs (e.g., a LOCA),
the impact will be insignificant. Only eight fuel rods in the core (less than 0.02% of the core) will exceed the current lead rod bumup limit of 60,000 MWD /MTU. The bumup of the remainder of the core will remain at levels representative of our standard operation, i.e.,
a batch average discharge burnup of about 45,000 MWD /MTU. The fission product inventory of the North Anna 2 Cycle 14 core as a whole should not differ significantly from that of our standard core, and the analyses of recc;d for assessing the environmental impacts of such severe accidents will remain applicable for high burnup irradiation of a small number of fuel rods in Assembly 3A4.
For the fuel handling accident, which is a single assembly or individual rod accident, the releases are evaluated based on the peak operating assembly. The analysis of record assumes this accident occurs shortly after shutdown, when the short-lived fission products are the major contributors to any calculated doses. These short-lived isotopes tend to reach an equilibrium during operation, and do_ not increase with bumup, so the presence of eight high bumup fuel rods will not affect this analysis. In addition, Fuel Assembly 3A4 will not operate at the highest assembly power in North Anna 2 Cycle 14, and so will have a fission product inventory that is bounded by the analysis of record for this accident.
- 7. Preliminary Safety Assessment The assembly to be irradiated is a reconstituted once-burned assembly of the same mechanical design as fuel used in previous cycles at North Anna. The fuel rods in this assembly will use two different cladding materials, both of which are approved for use at North Anna. The fuel assembly and the ZlRLO rods thrt will operate to high burnup will be required to meet all design criteria for the proposed operating conditions.
Use of this assembly will not affect the set of key analysis parameters defined for the current safety analyses (Reference 12). As discucsed in Section 6 above, the safety analyses of record are expected to remain applicable for the operation of this assembly in N2C14. Cycle specific evaluations will verify that the assumed values for any key analysis parameters are not exceeded.
Irradiation of Fuel Assembly 3A4 is not expected to result in an unreviewed safety question as defined in 10 CFR 50.59:
i The probability of an accident previously evaluated in the North Anna UFSAR will 10-
( '.
not increase, and the possibility of an accident that is different from any already evaluated in the North Anna UFSAR will not be created. Cnly a very small number of fuel rods are to be irradiated to high burnuo, and the fuel assembly containing these rods is fully compatible with the other fuel in the core. The remainder of the core is consistent with the design of normal reload cores for Horth Anna Units 1 and 2.
-Virginia Power's standard reload design methodology will be used to demonstrate that all applicable design criteria and all pertinent licensing basis acceptance criteria will be met. Evaluations will be performed as part of the cycle specific reload safety analysis to damonstrate that existing safety analyses remain applicable for the core containing the small number of high burnup fuel rods. The demonstrated adherence of the fuel and cycle specific core design to applicable standards and acceptance criteria will preclude new challenges to components and systems that could increase the probability of occurrence of any previously evaluated accident, or could create the possibility of a new type of accident. No new failure mechanisms will be created, nor will use of these assemblies cause the core to operate in excess of design basis operating limits.
The consequences of an accident previously evaluated in the North Anna UFSAR
=
are not increased. The reload core design for the cycle in which these fuel rods are irradiated will meet all applicable design criteria and ensure that all pertinent licensing basis acceptance criteria are met. Operation of a limited number of fuel rods to extended bumup in a single fuel assembly will not adversely affect the ability of existing components and systems to mitigate the consequences of any accident, or adversely affect the integrity of the fuel rod cladding as a fission product barrier.
The radiological consequences of accidents previously evaluated in the North Anna UFSAR will remain applicable for the extended burnup operation of a small number of fuel rods.
Neither the probability of occurrence nor the consequences of a malfunction of
=
equipment important to safety previously evaluated in the North Anna UFSAR will increase. The use of a fuel assembly containing a small number of fuel rods that will reach high burnups will not impose new performance requirements on any system or component such that any design criteria will be exceeded, nor will the i
core be operated in excess of pertinent design basis operating limits. No new modes or limiting single failures are created with the irradiation of these fuel rods to high burnup. The existing safety analyses based on normal reload fuel are expected to remain applicable for the core in which the ZlRLO fuel rods are irradiated. The assembly is mechanically comparable to any other reconstituted fuel assembly, and as such no new modes or limiting single failures will be created by its irradiation.
The possibility of a malfunction of equipment important to safety different from any
=
already evaluated in the North Anna UFSAR will not be created. The design for the North Anna Unit 2 cycle in which the ZlRLO-clad fuel rods wili operate to high burnup will be required to meet applicable derign criteria and pertinent licensing 11 r
basis acceptance criteria. The vast majority of the fuel in the core will operate to bumups consistent with normal reload operation, with only a very small number of fuel rods reaching extended burnups, so the possibility of a malfunction of equipment important to safety of a different type than any previously evaluated in the North Anna UFSAR will not be created. No new failure modes will be created for any system, component, or piece of equipment.
No new single failure mechanisms will be introduced, nor will the high burnup fuel rods or the core in general operate in' excess of pertinent design basis operating limits.
The margin of safety as defined in the Bases to any North Anna Technical
=
Specification will not be reduced. Existing safety analyses are expected to remain applicable for the irradiation of this small number of fuel rods to extended burnup.
The normal limits on core operation defined in the North Anna Technical Specifications will remain applicable for the irradiation of these fuel rods to extended -
burnup. The presence of these high burnup fuel rods will be specifically evaluated during the cycle design process using Virginia Power's standard reload design methods. Therefore, the margin of safety as defined in the Bases to the North Anna Unit 1 and North Anna Unit 2 Technical Specifications will not be reduced.
The final determination of whether an unreviewed safety question exists will be made after the cycle specifir reload calculations are complete, and will be documented as part of the normal Reload Safety Evaluation.
RECONSTITUTED FUEL In 1994, a Technical Specifications change was approved for North Anna Units 1 and 2 to allow the substitution of solid stainless steel or zirconium alloy filler rods for a limited number of failed fuel rods in fuel assemblies. The Virginia Power submittal that requested this change (Reference 7) and the NRC letter issuing the amendment (Reference 8) both clearly stated that substitutions are permissible for failed fuel rods. The wording of the revised North Anna Technical Specifications is r;ot as explicit, indicating that limited substitutions of filler rods "for fuel rods" may be made (References 13 and 14).
However, since the North Anna Technical Specifications were amended, changes in reconstitution technology have occurred. It may now sometimes be necessary to remove one or more intact fuel rods from an assembly, e.g., to improve visibility or permit use of certain tools. Under some circumstances, the intact fuel rod may be replaced after the activity is complete. Depending on the nature of the activity, though, a more conservative approach may also be adopted, to replace the intact fuel rod with a solid metal filler rod.
Because of the specific wording in the supporting documentation for the Technical Specifications change, use of such fuel assemblies could be considered to be in noncompliance with the North Anna operating license.
The process of removing intact fuel rods from a fuel assembly and subsequent tracking of the Special Nuclear Material within the rods are govemed by the same processes used for 12
r.
removal and accountability of failed fuel rods.
The technical evaluations performed to determine the acceptability of reusing fuel assemblies with solid filler rods are also independent of the condition of the rods that were replaced.- The precise locations of any solid filler rods in the fuel assembly will continue to be incorporated into the cycle specific nuclear design calculations, to confirm that design criteria are satisfied and that the core is bounded by existing safety analyses. As required by Reference 8, the thermal hydraulic evaluations will also be evaluated to confirm that the final configuration of the fuel assembly does not introduce a change in radial gradients in the flow and enthalpy distribution that could invalidate the applicability of the CHF correlation used for DNR predictions. Therefore, application of the term " reconstituted fuel assembly" to any North Anna fuel assembly in which fuel rods have been replaced with solid metal filler rods, regardless of whether the replaced fuel rod was failed or not, will not impact our compliance with NRC requirements on the use of reconstituted fuel.
Therefore, the NRC is specifically being requested to document their concurrence that the term " reconstituted fuel assembly" can be applied to any North Anna fuel assembly in which fuel rods have been replaced with solid metal filler rods, regardless of whether the replaced fuel rods were failed or intact. This will clarify that the use of such fuel assamblies -
is within the scope of activity allowed by the North Anna Technical Specifications.
13 L
j
i
SUMMARY
Eight ZlRLO fuel rods that have been irradiated for three cycles have been placed in once-bumed Fuel Assembly 3A4, and are scheduled to be irradiated for one additional cycle in North Anna 2 Cycle 14. The proposed irradiation does not require any TeShnical j
Specifications changes. However, because the end of life burnups of these fuel rods will i
exceed the 60,000 MWD /MTU lead fuel rod burnup limit the NRC has imposed on the Virginia Power, NRC concurrence is required for this program to proceed.
The extended ournup of the sight high burnup ZlRLO fuel rods will be fully addressed as part of the North Anna 2 Cycle 14 Reload Safety Evaluation, using the Virginia Power's NRC-approved reload design methods and Westinghouse's approved fuel rod design models and methods. Additional conservatisms will be applied to the fuel rod design analysis since the end oflife burnup of these rods will exceed the current lead rod burnup limit. All fuel rod design criteria that are applicable for the current lead rod bumup limit are
)
expected to be satisfied for these fuel rods.
Operation of this small number of fuel rods to high burnup in the North Anna 2 Cycle 14 core is not anticipated to result in the acceptable safety limits for any incident being
{
exceeded, or in an unreviewed s:sfety question as defined in 10 CFR 50.59. This will be confirmed as part of the cycle specific Reload Safety Evaluation.
We are also requesting NRC concurrence to apply the term " reconstituted fuel assembly" to North Anna assemblies in which limited numbers of non-failed rods have been replaced with solid metal filler rods.
This clarification is required because the supporting.
documentation for the North Anna Units 1 and 2 Techn! cal Specifications change that allowed use of reconstituted fuel discussed only the replacement of failed fuel rods with solid fiHar rods.
I The technical evaluations performed to determine the acceptability of reusing fuel assemblies.with solid filler rods consider only the final configuration of the fuel assembly, accounting for the location of each solid filler rod in the assembly. As such, these analyses are independent of the integrity of the cladding of the fuel rods that were replaced. NRC concurrence with th.4 request will clarify that use of fully evaluated reconstituted fuel assemblies in which some non-failed rods may have been replaced with solid filler rods is i
within the scope of activity allowed by the North Anna Technical Specifications.
i s
14
4 REFERENCES 1.
Letter from W. L. Stewart (Virginia Electric and Power Company) to U.S. Nuclear Regulatory Commission," North Anna Power Station Unit 1, Proposed License Amendment, Fuel Assemblies with Advanced Cladding Materials," Serial No.87-025, February 20,1987.
2.
Letter from Leon B. Engle (U.S. Nuclear Regulatory Commission) to W. L. Stewart (Virginia Electric and Power Company), Issuance of A.mendment No. 94 to North Anna Power Station Unit No.1 Facility Operating License, May 13,1987.
i 3.
Letter from W. R. Cartwright (Virginia Electric and Power Company) to U. S. Nuclear Regulatory Commission," North Anna Power Station Unit 1, Proposed License Amendment and Exemption Request, Fuel Assemblies with Advanced Cladding Materials,* Serial No.88-497, September 30, 1988.
4.
Letter from Leon B. Engle (U.S. Nuclear Regulatory Commission) to W. R. Cartwright (Virginia Electric and Power Company), " North Anna Unit 1 -Issuance of Amendment and Exemption Re: Fuel Rods Clad with Advanced Zirconium-Base Material (TAC No. 69797)", January 3,1989.
5.
Letter from Leon B. Engle and Bart C. Buckley, Sr. (U.S. Nuclear Regulatory Commission) to W. L.
Stewart (Virginia Electric and Power Company), "Surry, Units 1 and 2, and North Anna, Units 1 and 2 - Removal of 45,000 MWD /MTU Batch Average Bumup Restriction (TAC Nos. M87767, M87768, M87812, and M87813," December 14,1993.
6.
Letter from Leon B. Engle and Bart C. Buckley, Sr. (U.S. Nuclear Regulatory Commission) to W. L.
Stewart (Virginia Electric and Pcwer Company),"Surry, Units 1 and 2, and North Anna, Units 1 and 2 - Removal of 45,000 MWD /MTU Batch Average Bumup Restriction (TAC Nos. M87767, M87768, M87812, and M87813 " April 20,1994.
7.
Letter from W. L. Stewart (Virginia Electric and Power Company) to U.S. Nuclear Regulatory Commission, " North Anna Power Station Units 1 and 2, Proposed Technical Specifications Changes, Reconstituted Fuel Assemblies," Serial No.93-706, November 19,1993.
8.
Letter from Leon B. Engle (U.S. Nuclear Regulatory Commission) to J. P. O'Hanlon (Virginia Electric and Power Company)," North Anna Units 1 and 2-Issuance of Amendments Re: Fuel Assembly Reconstitution (TAC Nos. M88364 and M88365)," August 9,1994.
9.
Letter from W. L. Stewart (Virginia Electric and Power Company) to Leon B. Engle (U. S. Nuclear Regulatory Commission), " North Anna Power Station Units 1 and 2 - Proposed Techni21 Specifications Change-North Anna Fuel Assembly Design Change," Serial No.89-795, January 15, 1990.
10.
F. W. Sliz, "Vepco Reactor Core Thermal-Hydraulic Analysis Using the COBRA lilC/MIT Computer Code," VEP-FRD-33-A, October 1983.
11.
R. C. Anderson and N. P. Wolfhope," Qualification of tne WRB-1 Correlation in the Virginia Power COBRA Code," VEP-NE-2-A, June 1987.
12.
Virginia Electric and Power Company Topical Report, " Reload Nuclear Design Methodology," VEP-FRD-42, Rev.1-A, September 1986.
13.
Technical Specifications - North Anna Power Station, Unit No.1, Docket 50-338, through Amendment 216, December 10,1998.
14.
Technical Specifications - North Anna Power Station, Unit No. 2, Docxet 50-333, through Amendment 197, December 10,1998.
13
Figure 1.
Current Configuration of Fuel Assembly 3A4 Showing Previous Locations of ZlRLO Rods in Demonstration Assembly AM2 (As viewed from top)
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