ML19273B245
| ML19273B245 | |
| Person / Time | |
|---|---|
| Site: | Zion File:ZionSolutions icon.png |
| Issue date: | 03/07/1979 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19273B235 | List: |
| References | |
| NUDOCS 7904060021 | |
| Download: ML19273B245 (5) | |
Text
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UNITED STATES
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NUCLEAR REGULATORY COMMISSION f,' I f) j WASHINGTON, D. C. 2C555 i Efb' !
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-; / s SAFETY EVALUATION BY THE OFFICE CF NUCLEAR RE CTOR REGULATION SUPPORTING AMENDMENT NO. 44 TO FACILITY OPERATIN^ LICENSE N0. DPR-39 AND AMENDMENT NO. 41 TO FACILITY 0PERAlING LICENSE NO. DPR-48 COMMONWEALTH EDISCN COMPANY ZION STATION, UNITS 1 AND 2 DOCKET NOS. 50-295 AND 50-304 Introduction By letter dated October 27, 1978 as supplemented November 21,1978 and February 13, 1979, Commonwealth Edison Company (tne licensee) r aquested changes to the Technical Specifications appended to Facility Operating Licenses DPR-39 and DPR-48 for Zion Station, Units 1 and 2, respectively.
These changes would extend the FU Ef fective Fuel Power Hour (EFPH)"H rod bow penalty curve and the predicted clad collapse tines to include higher region average fuel burnap. These changes would allow the licensees to reinsert a maximum of four fuel assemblies previously irradiated in Zion Unit 1 for a maximum of two additional fuel cycles (beyond the normal three fuel cycles) in Zion Unit 2 to gain operating experience for an anticipated future extended burnup program.
Background
As discussed in the licensee's proposai, Commonwealth Edison Company is considering an EPRI/ Westinghouse proposal to operate the upcoming Cycle 4 of Zion Unit 2 with four Region 3 fuel assemblies that were discharged from Zion Unit 1 in the fall of 1978. The reinsertion of these four assemblies for an additional (4th) cycle would increase their exposure to about 47,000 MWD /MTU. The EPRI/ Westinghouse program is part of a long range Department of Energy effort to improve uranium fuel utilization by extending the burnup of nuclear fuel assemblies. The licensee's proposal would demonstrate that the Westinghouse fuel asserolies as used at the Zion units are capable of operating satisfactorily to higher burnups.
79040600 M The four assemolies would be examined after the first accitional fuel cycle, and if ceemed appropriate would be reinserted f or a fif tn cycle to increase their exposure to about 55,000 MWD /MTU.
Tne Zion Station Technical Specifications presently contain provisions snich define a roo bow penalty up to a region average burnup of about 38,000 MWD /MTU and predicted clad collapse times of greater than 30,000 EFPH.
In order to allow the reinsertion of four fuel assenblies, the licensee is therefore proposing to extend the above limits for these four assemblies.
The licensee has stated that this extended burnup program at Zion 2 will not adversely affect the safe operation of the Zion plants, increase coolant activity levels or require a reduction in plant capacity, availability or flexibility. Also, although Westinghouse has no experience.with full size fuel assemblies irradiated to these burnups, there have been tests of single fuel rods to extended burnups of 58,000 MWD /'1TU.
Based on this and the methods by which the proposed extended Technical Specification limits were arrived at, the licensee has concluded that the four assemblies to be irradiated up to 55,000 MWD /MTV in Zion 2 will coerate satisfactorily.
The licensee has indicated that there will be a fuel sipping test and a program to Examine the four spent fuel assemblies fecn Zion 1 prior to their insertion in the Zion 2 core and after the first additional cycle of exposure.
Evaluation In our review of tne licensee's proposal we considerec tne mechanical effects on the four fuel assemblies in terms of :ne pctential for increased rod bow and clad collapse effects due to the higher fuel Durnups planned.
In addition, we considered the effects that this increase in burnup of the four assemblies could have on the amounts of radioactivity released from the Zion Station. These areas are furtrer discussed below.
Fuel Mechanir,al Desion Consideraties.
FH Rod Scw Penalty The amount of fuel rod bow of Westinghouse fuel elements in Zion Station has been calculated using a correlation of the form a - b vuurnup.
Using this correlation, the licensee has calculated with a 95/95 confidence level that comolete channel closure will not occur until af ter 47,000 MWD /MTU.
The full DNB penalty for complete closure is aoplied starting at 47,000 MWD /'173, and a proportional penalty for lesser closures is apclied at l aer turnuos.
The Zion rod bowing model, and the resulting DNSR re:uction are consistent with NRC report, " Interim Safety Evaluation Report en Effects of Fuel Rod Bowing on Thermal Margin Calculations for Light Water Reactors", Revision 1, February 16, 1977.
This method treats the rod bow ;;roolem conservatively, assuming a linear relationship between channel closure and DNBR reduct.on.
The reduction in DNBR margin due to the rod bowing can be partially offset by existing thermal margins, as discussed in the abcve referenced report.
The present rod bow penalty curve, which is applicable for up to 38,400 MWD /MTU burnup, includes the generic margin associated with Westinghouse low parasitic fuel. The extended curve of the proposed Technical Specifications also utilizes this margin to minimize the DNBR reduction. The proposed FAH penalty curve is a natural extension of the existing Technical Sp.ecification to burnups resulting in complete channel closure, and is consistent with NRC approved methodology.
Clad Collapse EFPH Limit The proposed change to extend the clad collapse EFP3 limits to higher exposures is based on the analytical techniques described in Westinghouse topical report WCAP-8381 (July 1974).
The licensee's calculations pre-dict that no clad flattening would occur with the revised limits, and that additional burnup margin exists beyond these limits to preclude clad flattening. For all reload fuel regions, the predicted minimum time for clad flattening is >41,000 EFPH.
The extended clad collapse EFPH limits were based on the same modeling and methods used to determine the existing limits. This nethodology has been accepted by the NRC as a basis for establishing conservative fuel residence times for pressurized fuel rods. The minimum time to clad coll 3pse for all reload fuel regions is greater than the expected burnup for the extended burnup program.
Surcary The proposed rod penalty curve and clad collapse EFPH limits were calculated with approved, conservative analytical methods.
The calculational results support use of the pr sed limits at higher region average burnups.
Based on the above considerations, we conclude that the proposed Technical Specification changes for Zion Station are acceptable.
Radiological Considerations Irradiating fuel to extended burnuos will increase the amount of long-lived fission products in the core.
The short-lived fission products will have reachea equilibrium levels at lower burnups and will not be affected.
The potential consequences of the postulated design basis accidents are determined by the short-lived fission products. Therefore, the potential consequences of the postulated design basis accidents given in the Safety Evaluation Report (SER) dated October 1972, for Zion Station Units 1 and 2 will not change due to four fuel assemblies in the core being irradiated to burnups up to 55,000 rm'D/MTU.
We expect that operating Zion 2 with additional irradiation of four spent fuel assemblies from Zion 1 could increase the fraction of failed fuel in the core ~over that previously experienced.
This would increase the concentration of activity in the reactor coolant (RC) and the amount of activity released from the plant as compared to releases during operation of the plant in previous cycles. An increase in the concentra;fon of RC activity would be an indication of fuel failures and an indication of later increases in the anount of activity released frca the plant. However, we do not expect these increases to be significant because, (1) only four assemblies in the core (2.1*.) will be irradiated to the extended burnups, (2) due to restrictions in the plant Technical Specifications, these four assemblies will be located in the core where the operating thermal stresses in the cladding are relatively low and where the thermal limits for cladding should not be approached, (3) there are no design changes in these four asserrblies feca ones previously irradiated at Zion 2, and (4) the license:
will examine the four spent fuel assemblies before their insertion in the Zion 2 core for their fourth cycle and will only irradiate them in a fif th cycle after these assemblies have demonstrated satisfactory fuel performance in the fourth cycle.
If there is a significant increase in RC activity it will be detected by monitoring the activity of samples of the reactor coolant, the radwaste treatment system and the plant effluents.
This is discussed in Sections 9.8,11.1.2.2 and 11.1.2.3 of the Zion Station FSAR and required in Table 4.11-1 and Specification 4.3.6. A of the plant Technical Specifications.
. The plant Technical Specifications contain limits on the maximum concentration of RC radioactivity (Section 3.3.5) and on the amount of radioactivity in the plant effluents (Sections 3.11 and 3.12).
Tnese specifications would not be changed by this action. Therefore, although the licensee may release more radioactivity from Zion 2 during this extended burnup program than during previous cycles, compliance with the plant Technical Specifications will maintain concentrations of radioactivity within the allowed limits. This does not change our conclusion in the Zion Station October 1972 SER that the radwaste treatment system at Zion 2 will be capable of limiting radioactive releases to a few percent of the 10 CFR Part 20 limits. Nor does it change our conclusions that accidental releases (e.g., main steam line failure or stean generator tube failure) will be appropriately limited by the Technical Specifications on coolant aci tivi ty.
Summary On the basis of the above, we conclude that it is acceptable from a radiological standpoint to operate Zion 2 with four fuel assemblies previously irradiated for 3 cycles in Zion 1 withcut additional restrictions in the Technical Specifications beyond those proposed by the licensee for the four fuel assemblies.
Conclusion We have concluded, based on the considerations discussed above, that:
(1) because this action does not involve a significant increase ir the probability or consequences of accidents previously considered and does not involve a signiricant decrease in a safety margin, it does not involve a significant hazards consideration, (2) there is reasonable assurance that the health and safety of the public will not be en1 angered by operation in the proposed manner, and (3) such activities will be conducted in compliance with the Commission's regulations and the issuance of this amencment will not be inimical to the common defense and security or to the health and safety of the public.
Date: March 7,1979