ML20101U584
| ML20101U584 | |
| Person / Time | |
|---|---|
| Site: | Palo Verde  |
| Issue date: | 07/17/1992 |
| From: | Boger B Office of Nuclear Reactor Regulation |
| To: | ARIZONA PUBLIC SERVICE CO. (FORMERLY ARIZONA NUCLEAR |
| Shared Package | |
| ML20101U586 | List: |
| References | |
| NUDOCS 9207230101 | |
| Download: ML20101U584 (7) | |
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UNITED STATES OF AMERICA EUCLEAR RE,GVLATORY COMMISSION In the matter of
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ARIZONA PUBLIC SERVICE COMPANY,
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=(Palo Verde Nuclear Generating
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Station, Vait No. 3)
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EXEMP1108-I.
Arizona Public Service Company '(APS),1111. (the licensee) is the
. holder of Facility.0perating License No. NPF-74, which authorizes operation of
- Palo Verde Nuclear Generating Station (PVNGS), Unit 3.
The facility consists of'a. pressurized water reactor (PWR) at the licensees' site
-located in Maricopa County, Arizona.1This license provides, among other
' things,. that the licensee-is subject to all rules, regulations, and Orders of t
- the Commission nowJor hereafter-in effect.
II.
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Section 50.46 of Title 10 of the Code of Federal Regulations (10 CFR 50.46) contains acceptance criteria for emergency core cooling systems (ECCS) for _ light water. nuclear-power reactors fuelcd with uranium oxide pellets within cylindrical zircaloy-cladding.
Further,-10 CFR-50.46 states'that ECCS cooling performance following postulated loss-of-coolant accidents must be calculated in accordance with an acceptable evaluation model.
Appendix K to 9207230101 920717 PDR ADOCK 05000530 P
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- 10.CFR Part 50 contains the' required and acceptable features for ECCS
= evaluation models.
Finally,10 CFR 50.44 contains requirements for the~
Econtrol of hydrogen gas -that may be generated 'after a postulated loss-of-coolant accident in light water power reactors fueled with uranium oxide pellets within cylindrical zircaloy cladding.
111.
By letter dated December 20, 1991, APS submitted an amendment request l
for PVNr,S Unit 3 to allow the substitution of up to a total of 80 fuel rods.
clad with advanced zirconium-based alloys, other than the conventional Zircaloy-4, in two fuel assemblies. These assemblies would be used for evaluation of in-reactor performance during fuel cycles 4, 5, and 6.
By letter dated December 20, 1991, APS also submitted an exemption
-request-to 10 CFR 50.46, 10 CFR Part 50, Appendix K, and 10 CFR 50.44. These-regulations refer to the use of zircaloy, but do not clearly specify what is considered zircaloy. Therefore, the use of advanced zirconium-oased alloys, rather_than conveational Zircaloy-4, may not be within the regulatory basis.
Pursuant to 10 r.FR 50.12(a), "The Commission may, upon application by any interested person or upon its own initiative, grant exemptions from the requirements of the regulations of this part, which are - (1) Authorized by law, will not present an undue risk to the public health and safety, and are consistent with the conson defense and security. -(2) The Commission will not consider granting an exemption unless special circumstances are present, o
Special circu.astances_ are present whenever
...(ii) Application of the regulation in the particular circumstances would not serve the
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3 underlying purpose of the rule or is not necessary to achieve the underlying-purpose of the rule...".
'The Code of Federal Regulations at 10 CFR 50.46 states:
"Each boiling.
and pressurized-light-water nuclear power reactor fueled with uranium oxide pellets within cylindrical Zircaloy cladding must be provided with an emergency core cooling system (ECCS) that must be designed such that its
- calculated cooling performance following postulated loss-of-coolant accidents conforms to the criteria set forth in paragraph (b) of this section. ECCS cooling performance must be calculated in accordance with an acceptable
-evaluation model and must be calculated for a number of postulated loss-of-coolant ' accidents of different sizes, locations, ar.d other properties sufficient to provide assurance that the most severe postulated loss-of-
. coolant accidents are calculated." The Code of Federal Regulations at 10 CFR 50.46 then goes on to give specifications for peak cladding temperature, maximum cladding oxidation, maximum hydrogen-generation, coolable geometry, andilong term cooling.
Since.10 CFR 50.46 specifically refers to fuel with
- Zircaloy cladding, the use of fuel clad with advanced zirconium-based alloys would,-in--effect, place the licensee outside the applicability of this section
-of the Code.
The underlying purpose of.the rule is to ensure that facilities have adequate acceptance criteria for ECCS.
The fuel rods clad with the advanced zirconium-based alloys 1will be identical in design and dimension to the fuel rods clad-with conventional Zircaloy-4. The advanced cladding materials used in the' demonstration fuel assemblies were chosen based on the improved corrosion resistance exhibited in ex-reactor autoclave corrosion tests in both L
high-temperature water and steam environments.
Fuel rods clad with similar l
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-types of: advanced zirconium-based alloys have been successfully irradiated in
--high-temperature PWRs in Europe.
The= mechanical properties of the clad made from the advanced zirconium-based alloys meet all the mechanical requirements of the conventional
.Zircaloy-4 procurement specifications, lhus, the cladding and structural i
integrity of the fuel rods and fuel assemblies that have the advanced zirconium-based alloys will be maintained, i
Therefore, due to these similarities between advanced zirconium-based alloys and Zircaloy-4, the advanced alloys are expected to result in clad and fuel performance similar to Zircaloy-4, such that the 10 CFR 50.46 LOCA Lacceptance criteria will be satisfied for the advanced zirconium-based cladding. Thus, the underlying purpose of the rule has been met.
Strict interpretation of the regulation would render the criteria of 10-CFR 50.46 inapplicable to the advanced zirconium-based alloys,. even though analysis-shows that applying the Zircaloy criteria to the advanced zirconium-based alloys yields acceptable results.
A strict application of the regulation in this instance is not necessary to achieve the underlying' purpose of the rule. Therefore, special circumstances exist'to grant an exemption from 10 CFR 50.46(a)(1)(i) that would allow the licensee to apply the acceptance criteria of 10 CFR 50.46 to a reactor with 80 fuel rods clad with advanced zirconium-based alloys The Code of Federal Regulations at 10 CFR 50.44 provides requirements for. control of hydrogen gas generated in part by Zircticy clad fuel after a
.p_ostulated loss-of-coolant-accident (LOCA). The intent of this rule is to
. ensure that an adequate'means is provided for the control of hydrogen gas that may be generated following a LOCA.
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' The hydrogen produced in a post-LOCA scenario comes from cladding oxidation from a metal-water reaction. Most of the high temperature oxidation occurs in the 8-phase since the diffusion coefficient for oxygen in B-phase of zirconium is significantly greater than that in a-phase zirconium.
The B-phase oxidation resistance of the alloys is expected-to be as good 4
as-or better than that of.Zircaloy-4.
It is expected that the alloying element levels adjusted.to improve the corrosion resistance of the a-phase of these alloys with respect to the a-phase of Zircaloy-4 will result in an improvement of the corrosion resistance of the B-phase of these alloys cs well.
It is therefore concluded that the 6-phase oxidation rate of the alloys will be comparable to or lower than that of Zircaloy-4 and that the Baker-Just correlation will overpredict the B-phase oxidation of the alloys. A strict interpretation of the rule in this instance would result in the criteria of 10 CFR 50.44 being inapplicable-to advanced zirconium-based alloys.
Since application of the regulation is not necessary to achieve the underlying purpose of the rule, special circumstances exist to grant an exemption from 10 CFR 50.44.-to a reactor containing 80 fuel rods clad with advanced-zirconium-based alloys.
Paragraph I.A.3'of' Appendix K to 10 CFR Part 50 states that the rates of energy release, hydrogen generation, and cladding oxidation from the metal-water reaction shall be calculated using the Baker-Just equation.
- However, since-the Baker-Just equation presumes the use of Zircaloy clad fuel, strict application of the rule would not permit use of the equation.
The intent of this part of the Appendix, however, is to apply an equation that conserva-tively bounds all post-LOCA scenarios. -Due to the similarities in the
' composition of the advanced zirconium-based alloys and Zircaloy, the
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application-of the Baker-Just equation in the analysis of advanced zirconium-
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-i based _ clad fuel will conservatively bound all post-LOCA scenarios.. Since the use of. Baker-Just equation presupposes Zircaloy cladding and post-LOCA scenarios are conservatively bounded, the underlying purpose of the rule will be met.1Thus, special circumstances exist to grant an exemption from Paragraph I.A 5'of Appendix K to 10 CFR Part 50 that would allow the licensee to apply the Baker-Just equation.to edvar.ced zirconium-based alloys.
IV.
Accordingly, the Commission has determined, pursuant to 10 CFR 50.12(a)(1),-that an exemption as described in action !!! above is authorized iby law, will not preser.t an undue risk to the public health and safety, and is -
consistent with the_ common defense and security. The Commission has determined, pursuant to 10 CFR 50.12(a)(2)(ii) that special circumstances exist, as noted in Section 111 above. Therefore,-the Commission hereby grants Arizona-Public Service Company, gi 11, an exemption from 10 CFR 50.46,10 CFR Part-50, Appendix X,-and_10_CFR 50.44.
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. Pursuant to 10 CFR 51.32, the Commission has determined that the granting of this exemption will not have a significant impact on the quality of the_ human environment (57 FR 24511).
This exemption is effective upon issuance.
FOR THE NUCLEAR REGULATORY COMMISSION t
Bruce A. Boger, Director Division of Reactor Projects III/IV/V Office of Nuclear Reactor Regulation Dated at Rockville, Mar"yland this 17th-day of JULY
, 1992.
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