ML20210F612
| ML20210F612 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 07/28/1999 |
| From: | Christian D VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 99-132A, NUDOCS 9908020068 | |
| Download: ML20210F612 (12) | |
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VinciNIA EimcTRIC ANI) POWER CmlPA N Y Ricism>si>, V HGINIA 23261 July 28, 1999 U. S. Nuclear Regulatory Commission Serial No.
99-132A Attention: Document Control Desk 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 SUPPLEMENTAL INFORMATION ON PROPOSED IRRADIATION OF FUEL RODS BEYOND CURRENT LEAD ROD BURNUP LIMIT Virginia Electric and Power Company (Virginia Power) plans to irradiate eight fuel rods to an end of life rod average burnups ranging from about 65 to 73 GWD/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 burnup limits, and will provide data to address Nuclear Regulatory Commission (NRC) questions related to fuel performance behavior at higher burnups. Because the end of life burnups exceed the lead rod burnup limit for North Anna, a submittal was made to the NRC on April 16, 1999 (Serial Number 99-132) requesting approval for the high burnup rods to exceed this limit.
During a meeting with the NRC on June 24,1999, Virginia Power and Westinghouse provided some additional information pertaining to the proposed irradiation program.
This letter documents information that was provided during the June 24 meeting, and supplements our original submittal.
As discussed at the meeting, the cycle specific calculations for North Anna 2 Cycle 14 are currently in progress with the assembly containing the high burnup rods in the center core location. If for any reason the cycle specific calculations do not confirm the acceptability of irradiating the eight fuel rods for a fourth cycle to a higher burnup or if an unreviewed safety question is created, another assembly, not containing high burnup fuel, will be used as the center assembly in North Anna 2 Cycle 14.
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Should you have any further questions or require additional information, please contact us.
Very truly yours, JC
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D. A. Christian Vice President-Nuclear Operations There are no commitments in this letter.
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Attachment i
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Regional Administrator U. S. Nuclear Regulatory Commission, Region ll Atlanta Federal Center 61 Forsyth St., SW, Suite 23T85 Atlanta, GA 30300 l
Mr. M. J. Idorgan NRC Senior Resident inspector North Anna Power Station I
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Attschment 1 Supplementallnformation on i
l Proposed Irradiation Program l
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l Virginia Electric and Power Company North Anna Units 1 and 2 l
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1 Introduction On June 24,1999, Virginie. Power and Westinghouse personnel met with NRC representatives to provide some additional detail regarding the proposed irradiation of eight fuel rods to high burnup in a lead test assembly in North Anna 2 Cycle 14. The discussion focused on several questions that NRC reviewers had regarding the evaluation and examination of the high burnup fuel rods, and regarding the radiological impact of their operation. This letter documents information provided during that meeting to support the NRC's environmental assessment of the proposed program.
Pre-and Post-Irradiation Testing of Fuel Rods Although detailed characterization of these eight fuel rods was not performed immediately prior to insertion into Fuel Assembly 3A4, all eight rods were well-characterized when they were first fabricated for use in the original North Anna advanced materials demonstration program.
Examinations have also been performed on several of the fuel rods after each completed operating cycle, as shown in Table 1. In particular, the oxide thickness was measured on two of the eight ZIRLO fuel rods after the third operating cycle:
AM2 Current Bumup, Oxide Thickness, Rod Location GW/MTU m
F10 53.3 35 K07 52.2 36 The high burnup fuel rods were visually inspected during insertion into Fuel Assembly 3A4. No abnormalities were identified, and no indications of unusual oxidation or crud buildup were observed. The oxide thicknesses measured on the two rods after three cycles are therefore considered representative of all eight high burnup rods, which have current bumaps ranging from 49.2 to 54.8 GWD/MTU.
' The full scope of examinations on the high burnup rods at the end of North Anna 2 Cycle 14 is still being developed. It is currently anticipat that on-site examinations will include fuel rod length, profilometry, and oxide measurementnimilar to those completed at the end of previous operating cycles. Westinghouse has also expressed an interest in shipping at least some of the rods to a hot cell, to obtain data on properties such as fission gas release, clad hydrogen pickup, cladding metallography and fuel microstructure. Virginia Power recognizes the importance of such information in extending the current industry knowledge about fuel performance behavior at high bumups, and will be working with Westinghouse to support the proposed examinations.
Evaluations of High Burnup Fuel Rods Prior to irradiation of the eight high burnup fuel rods in North Anna Fuel Assembly 3A4, cycle specific calculations will verify that all design criteria are satisfied. These cycle specific calculations are currently in progress for North Anna 2 Cycle 14, using Fuci Assembly 3A4 in the center core location. The criteria being used to determine the accep@ility of continued Page1of9
P irradiation of the high bumup fuel rods for North Anna 2 Cycle 14 are the same design criteria j
that are applicable to the other, lower burnup fuel rods in the core.
Because Fuel Assembly 3A4 is a once-burned assembly, considerations that may be more important as the assembly reaches high burnup, such as assembly growth, assembly bow, and relaxation of the grid springs are not a concern for this program. As noted in our original submittal, Assembly 3A4 is a VANTAGE 5H assembly, which may be susceptible to self-excited assembly vibration, particularly when used in peripheral core locations, where larger
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gaps may permit more assembly vibration to occur. This assembly will be used as the center fuel
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assembly in North Anna 2 Cycle 14, a location where significant fuel assembly vibration is not expected. As a precaution, however, a vibration suppression assembly will be placed in the assembly during Cycle 14 to mechanically stiffen the assembly and further reduce the potential for assembly vibration.
h Fuel rod design criteria that become more limiting toward the end of life, and which may therefore be limiting for the high burnup fuel rods, are fuel rod growth, clad fatigue, rod internal pressure, and cladding corrosion. Fuel rod growth will be evaluated as part of the cycle specific fuel rod design evaluation to ensure that sufficient room exists between the top and bottom nozzles of the once-bumed Zircaloy skeleton to accommodate the growth of the thrice-burned ZlRLO fuel rods during an additional cycle of operation. Clad fatigue is typically not a limiting design criterion for fuel rods, and - based on previous cycle specific designs for North Anna and evaluations for other high burnup programs - the cumulative fatigue usage factor is expected to be well below the Westinghouse design criterion at the end of the fourth operating cycle for the high bumup fuel rods. Because the North Anna high bumup fuel rods do not contain ZrB -coated 2
pellets (Integral Fuel Burnable Absorber), which contribute to the gas inside the fuel rod, no difficulty is expected in demonstrating that the end oflife pressure in these high burnup rods continues to satisfy the Westinghouse design criteria on fuel rod internal pressure. These evaluations will be performed using the same NRC-approved models that are normally used for fuel rod design. These models have been used to perform similar evaluations for other high l
burnup lead test assemblies, which have reached lead rod burnups of about 66 GWD/MTU.
l It is similarly expected that the Westinghouse design criterion for clad corrosion will be satisfied for the high bumup rods. This preliminary assessment is based on previous Westinghouse l
evaluations of high burnup fuel and on data available from previous measurements on the high l
bumup fuel rods. With current oxide thicknesses at about 35 m, and projected operating power l
of the high burnup rods of about 0.8 times the core average power, the end of life oxide thicknesses are expected to remain well below oxide thicknesses previously observed for Zircaloy-4 fuel. The cycle specific assessment of the end oflife corrosion thicknesses for the high burnup fuel rods will be performed using the same NRC-approved model normally used for
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fuel rod design of ZIRLO-clad fuel.
As discussed in our original submittal, a developmental corrosion model for ZlRLO will be used as an additional assessment tool. This developmental model has also been used to predict end of life oxidation for other high burnup ZlRLO-clad fuel rods, and was found to accurately predict the irradiation behavior of Ae high burnup fuel, with only a small difference between predicted and measured oxide thicknesses. Because this developmental model is based primarily on data Page 2 of 9
a from fuel irradiated at V. C. Summer and North Anna (including measurements from these high burnup fuel rods after previous irradiation cycles), there is a high degree of confidence that the model will similarly provide accurate end oflife estimates of the oxidation for the high burnup North Anna fuel rods.
The nuclear design evaluation for North Anna 2 Cycle 14 will be performed by Virginia Power using our standard calculational methods and procedures. Current design methods are expected to adequately treat the neutronics changes associated with the extended burnup of the high burnup rods in Fuel Assembly 3A4. The nuclear design model used by Virginia Power (PDQ) uses microscopic cross sections in the calculation of burnup-related isotopic concentration changes. For several key isotopes, the variation of these microscopic cross sections with burnup decreases as the concentration approaches equilibrium. Consequently, at high burnups the change in fuel and fission product concentrations has a much greater impact on fuel reactivity than changes in the microscopic cross sections. Scoping calculations were performed to verify that the PDQ model does not predict any unexpected reactivity behavior at high burnups. The reactivity changes with burnup predicted by PDQ were also compared with results from CELL 2, a transport theory code for generating pin cell cross sections. This comparison also confirmed that the PDQ model performs acceptably at high burnups.
Reactor operation, including core operating limits and setpoints, is not affected by the presence of a small number of fuel rods operating to high burnups. The high burnup fuel rods will operate in an assembly located in the center of the core, but this core location will not experience the highest power density during North Anna 2 Cycle 14. The average assembly power of Assembly 3A4 over the length of Cycle 14 will approximately equal the core average power, but the high burnup rods are less reactive, and will operate at approximately 0.8 times the core average power.
Virginia Power has previous experience with modeling the presence of different cladding materials in a single fuel assembly, including the Westinghouse advanced material demonstration assemblies in which the high bumup rods were originally irradiated, and another lead test assembly program currently in progress in North Anna Unit 1.
As part of the original Westinghouse advanced material program, after the first operating cycle 18 fuel rods were removed from Demonstration Assembly AM2 and replaced with new fuel rods, including some rods with an additional cladding material. Although the enrichment of these replacement fuct rods was specified to match the amount of reactivity in the remaining 246 once-bumed fuel rods, fresh fuel does not deplete at the same rate as bumed fuel, so it was necessary to explicitly model j
these replacement fuel rods over the remaining life of Assembly AM2. Virginia Power will track and model the different burnups and slightly different enrichments of the high bumup fuel rods in 3A4 and the remaining rods in the assembly in much the same manner as the replacement rods in Assembly AM2.
Failed Rod in Assembly 3A4 As noted in our original submittal, Assembly 3A4 was discharged after one cycle of operation i
because it was determined to contain a failed fuel rod. The fuel assembly was ultrasonically inspected to identify the failed fuel rod, which was determined to be well on the interior of the Page 3 of 9 L
4 assembly, with one comer adjacent to a guide thimble. A video inspection did not identify any debris in this assembly.
At the time of reconstitution, the failed rod was examined when it was removed from the assembly, and several hydride blisters were observed. The rod was taken to an inspection station for high magnification video, but during this inspection (which showed a side of the rod not visible during withdrawal from the assembly) a large hole was observed at the Grid 6 elevation (Figure 1). Becc.use of concerna about further damage to the rod, the high magnification video was stopped when this hole was identified, and the rod was transferred to a storage can. The upper portion of the fuel rod was videoed as the rod was placed in storage. The video examinations identified several hydride blisters and associated cracking of the cladding in the fuel spans adjacent to the hole, but there were no marks on the rod that clearly indicated a failure mechanism, such as debris. Because the major defect occurred at a grid location, grid-to-rod fretting must be considered one possible failure mechanism, although no evidence of grid-to-rod
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fretting was mted on the eight additional rods that were removed from the assembly to allow insertion of the high bumup ZIRLO fuel rods. Although a review of manufacturing records revealed no abnormalities during the fabrication of this rod, a manufacturing defect is also possible, such as some sort of contamination of the pellets or cladding that led to primary hydriding.
Assembly 3A4 was reconstituted at the same time the high burnup fuel rods were placed in the assembly, and the failed rod was removed and replaced with a solid stainless steel rod.' Virginia Power has had excellent experience with operation of reconstituted fuel assemblies at both Surry and North Anna, with no additional defects identified after subsequent operating cycles. Many of these assemblies were reconstituted because of debris-induced defects, but c experience also includes reuse of assemblies with isolated failures of unknown causes, similar tt usembly 3A4.
Based on prior experience, the isolated failure of a single fuel rod in Assembly.+ 14 during the first operating cycle does not indicate a increased potential for failure of rods in tus assembly during subsequent operating cycles.
RadiologicalConsequences/ Environmental Assessment The Westinghouse topical report for the VANTAGE + Fuel Assembly Design (WCAP-12610-P-
.A) included a radiological assessment of the effects of operating to lead rod burnups exceeding the end of life burnups of the high burnup fuel rods in Fuel Assembly 3A4. Although the VANTAGE + fuel design has not been licensed for these bumup levels, the information presented in this report supports the conclusion that the proposed high bumup program at North Anna will not affect radiological plant effluents.
Op'eration to high burnups increases the inventory of certain long lived fission products such as Cs ' and Cs'37. Assuming entire reload batches routinely operate to high burnup and no changes 3
in the reactor coolant cleanup, Westinghouse estimated there could be a small increase in the annual release of these isotopes (only about 20%). Even assuming the high burnup fuel rods at North Anna have some increased contribution to the core inventory of long-lived isotopes, with
' It should be noted that the p esence of the solis stainless steel rod in Assembly 3A4 is also being explicitly considered in the cycle specific calculations for North Anna 2 Cycle 14.
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only 8 fuel rods operating to high burnup (0.02% of the fuel rods in the core) there will be no measurable increase in the levels of these isotopes in the coolant, and no effect on normal operating plant releases.
The accidents where the radiological consequences may be impacted by the presence of these high bumup rods fall into three categories: the fuel handling accident; accidents with cladding failure only; and accidents with cladding failure and fuel melt.
The fuel handling accident involves a single fuel assembly. The analysis of this accident for North Anna follows NRC Regulatory Guide 1.25, and is based on a limiting assembly operating at 1.65 times the core average power. The analysis assumes the accident occurs 100 hours0.00116 days <br />0.0278 hours <br />1.653439e-4 weeks <br />3.805e-5 months <br /> after
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the reactor shuts down (even though the North Anna Technical Specifications prohibit fuel j
movement until 150 hours0.00174 days <br />0.0417 hours <br />2.480159e-4 weeks <br />5.7075e-5 months <br /> after shutdown), and assumes the cladding of all rods in the fuel assembly is damaged. The doses from this accident are primarily due to short-lived iodir.e and noble gas isotopes. Because of their short halflives, the quantities of these isotopes present in the fuel-to-clad gap of the fuel rods tend to reach an equilibrium between production and decay during operation, so that the isotopic inventory available for release is primarily a function of operating power and decay time after operation rather than cumulative burnup. For Fuel Assembly 3A4, the assembly average power during North Anna 2 Cycle 14 is approximately the core average power, much lower than the assumed power for analysis of the fuel handling accident. The high burnup rods have a limited amount of reactivity remaining, and will consequently operate at still lower powers of about 0.8 times the core average power. Therefore, the activity releases that would result from damage to the rods in this assembly would be considerably lower than those determined for the North Anna analysis of record for this accident.
For accidents such as the steam generator tube rupture (SGTR) and the main steam ime break (MSLB), no fuel failures are predicted to occur as a consequence of the accident, and the calculated doses are based on failures that exist at the time of the accident. For North Anna, analyses of loss of flow accidents show that the minimura departure from nucleate boiling ratio (DNBR) does not cecrease below the limit, so no cladding failure or release of fission products is expected. For the current North Anna locked rotor accident (LRA) analysis, no fuel rods are predicted to experience DNB. However, the offsite dose calculation for the LRA conservatively assumes failure and gap activity release for 13% of the fuel rods. (The 13% value is based on an analysis of the LRA that has been superseded.) In general, any failures during a LRA would be expected to occur in high power locations because high power fuel rods are more likely to enter a boiling regime during a transient. The high burnup fuel rods in Fuel Assembly 3A4, operating at
'i about 0.8 times the core average power, would therefore not be expected to fa.il for this accident scenario. In addition, releases for the LRA involve only gap activity which, as discussed above for the fuel handling accident, is primarily a function of operating power rather than cumulative burnup. Since current analyses predict no other fuel rod failures during the LRA, even if the eight high burnup rods should fail for some unanticipated reason, there would be no impact on the current dose calculations for this accident.
The third class of accidents encompasses those accidents that predict both cladding failure and some degree of fuel melt. These include the lerr M loss of coolant accident (LBLOCA),
small break loss of coolant accident (SBLOCA) ad the rod gjection a:cident. Dose calculations Page 5 of 9
for these accidents are bounded by the evah-tion of the LBLOCA, which conservatively
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assumes damage to the entire core. The North Anna LBLOCA analysis follows the guidelines of Regulatory Guide 1.4, which requires the dose calculations be based on specific distributions of the core inventory of fission products (not just the material present in the fuel-to-clad gap). As for most accidents, the doses are primarily due to short-lived iodine and noble gas isotopes, and the coce inventory of these isotopes is a function of operating power rather than cumulative burnup. The core operating limits and setpoints will not be affected by the presence of the high bumup rods, so the operation of the North Anna 2 Cycle 14 core incorporating these rods will be comparable to previous North Anna cores in terms of operating powers and core inventory. The presence of eight high burnup fuel rods in Fuel Assembly 3A4 will not result in any change to the calculated releases in accidents involving damage to a significant portion of the fuel in the core.
For the rod ejection accident, limited melting (<10% of the pellet) is predicted to occur at the hot spot in the core. The analysis of this accident is based on conditions in the peak rod in the core, which is typically a fresh or once-burned fuel rod. However, because the melting point of UO2 decreases with burnup, the cycle specific evaluations currently in progress for North Anna 2 Cycle 14 will confirm that the presence of the high burnup rods does not affect the current analysis of the rod ejection transient for North Anna. This is not expected to present any difficulty, because the high burnup fuel rods in Assembly 3 A4 will only be operating at about 0.8 times the core average power. Also, as discussed in a Westinghouse position letter to the NRC (Reference 1) on the Reactivity Insertion Accident (RIA) issue that was raised in the fall of 1994, the predicted effect of an ejected rod is very localized. Fuel Assembly 3A4 is not going to be placed in a rodded location, and will also be located well away from core locations that have historically been limiting for the control rod ejection transient for North Anna. However, if the cycle specific evaluation for North Anna 2 Cycle 14 determines that the current rod ejection analysis is affected by the presence of the high burnup fuel, consistent with the requirements of j
10CFR50.59 either a new analysis will be performed and appropriate NRC approval obtained, or the assembly will be removed from the core loading pattern, as discussed below.
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The current rod ejection analysis for North Anna has demonstrated that the upper limit in fission product release as a result of fuel rods entering departure from nucleate boiling amounts to 10%
i of the core inventory. Because the rods most likely to enter a boiling regime during transient cow
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conditions are those operating at high powers, the high burnup fuel rods in Fuel Assembly 3A4 j
(operating at 0.8 times the core average power) are not expected to be among those that would fail for this accident. In addition, as part of the Reactivity Insertion Accident (RIA) issue that was raised in the fall of 1994, the Nuclear Energy Institute (NEI) issued a position letter to the NRC (Reference 2) that concluded that high burnup fuel failures would have a minimal impact on doses for a rod ejection accident because of conservative assumptions in the dose assessment.
The Westinghouse position letter on RIA (Reference 1) also addressed the impact that an RIA would have on fuel rods of various burnups. Based on the information in these position letters, and the use of Fuel Assembly 3A4 in an unrodded core location, it is concluded that neither Assembly 3A4 nor the eight high burnup fuel rods will contribute to the radiological consequences of a rod ejection accident.
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Alternatives to Use of Assembly 3A4 As noted in our original submittal, the final determination of whether there is an unreviewed safety question will be performed as part of the cycle specific evaluations for North Anna 2 Cycle 14. Based on the current knowledge of the previous operation of the high burnup fuel rods in Assembly 3A4, the examinations performed on the rod to date, and previous vendor experience with similar analyses for other lead test assemblies, we anticipate no difficulty in confirming our preliminary assessment that operation of these rods does not result in an unreviewed safety question. This confirmation will be completed as part of our normal 10CFR50.59 evaluation of the reload cycle.
If the high burnup fuel rods in Fuel Assembly 3A4 do not satisfy the criteria for continued irradiation in Cycle 14, including the requirements of 10CFR50.59, the assembly will not be used. As a contingency, a similar burnup fuel assembly from the same region as 3A4 has been identified as a replacement. This replacement assembly was a symmetric partner of 3A4 in North Anna 1 Cycle 9, so its use in place of Assembly 3A4 would have little impact on the North Anna 2 Cycle 14 core loading pattern, and would require a minimal amount of redesign effort. All fuel rods in the replacement assembly would be in their second operating cycle and would remain below the 60 GWD/MTU lead rod burnup limit currently identified for the North Anna units, so the replacement assembly would not be a lead test assembly.
Summary Based on our original submittal, as supplemented by the above discussion, it is concluded that, subject to successful completion of the cycle specific calculations currently in progress, the proposed irradiation of eight fuel rods to high burnup in Fuel Assembly 3A4 will not affect the probability or consequences of potential reactor accidents, or otherwise affect radiological plant effluents.
Operation of these rods to a higher burnup will provide important data on fuel performance behavior at high burnups while maintaining a high standard of safety performance. If for any reason the cycle specific calculations do not confirm the acceptability ofirradiating the eight fuel rods for a fourth cycle to a higher burnup, or if an unreviewed safety question is created, another assembly, not containing high burnup fuel, will be used to replace Assembly 3A4.
References
- 1. Letter from N. J. Liparulo (Westinghouse) to R. C. Jones (NRC), " Transmittal of the
' Westinghouse Assessment of Topical Report Validity for Reactivity Insertion Accident with High Burnup Fuel,' May 1995," NTD-NRC-95-4438, May 31,1995.
- 2. Letter from A. Marion (NEI) to R. Jones (NRC), transmitting "NEl Response to NRC Staffs Request for Information on Reactivity Insertion Accidents," December 28,1994.
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Table 1 I
l Exams Performed on ZIRLO Fuel Rods in 3A4 after Previous Operating Cycles AM2 Rod Fuel Rod Length Fuel Rod Oxide Fuel Rod Profilometry Location EOC1 EOC2 EOC3 EOC1 EOC2 EOC3 EOC1 EOC2 EOC3 E08 F01 X
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F10 X
X X
X X
X H07 X
X J09 K07 X
X X
X K12 X
X Ll7 X
X Note: High burnup fuel rods in Assembly 3A4 were previously irradiated in Demonstration Assembly AM2. Operating cycles for Assembly AM2 were North Anna 1 Cycles 7,9, and 10.
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Figure 1.
Photographs of Failed Rod from Assembly 3A4 (a) Grid 6 hole e,
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j-(b) Hydride between Grids 6 and 7
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O (c) Hydride between Grids 5 and 6 5
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