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**UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, D.C. 20555-0001**

August 3, 1998

**Addressees**:

All holders of operating licenses for nuclear power reactors, except those licensees who have permanently ceasedoperations and have certified that fuel has been permanently removed from the reactor vessel.

**Purpose**:

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to inform addressees of recentWestinghouse experience with one of its reactor fuel designs which has exhibited higher than expected rates of oxidation ofzircalloy cladding at high burnups . It is expected that recipients will review the information for applicability to theirfacilities and consider action as appropriate, to avoid similar problems. The material and discussion contained in thisinformation notice are not NRC requirements; therefore, no specific action or written response is required.

**Description of Circumstances**:

On October 28, 1997, Westinghouse notified NRC that modification of its fuel cladding corrosion model in its fuel roddesign code, PAD, to reflect new data on Zircaloy-4 oxidation at high burnup may create compliance issues for its IntegralFuel Burnable Absorber (IFBA) fuel with Zircaloy-4 cladding. The modified code may predict higher fuel temperaturesand internal pressures at high burnup conditions. This, in turn, may lead to code results that do not meet the Westinghousecriterion prohibiting gap reopening and that do not meet the loss-of-coolant accident (LOCA) criterion in 10 CFR50.46(b)(2).

The Westinghouse criterion prohibiting gap reopening was approved by the NRC staff for steady-state operation wheninternal pressure in the rod exceeds reactor coolant system pressure. The staff approved this criterion in lieu of a criterionrequiring that the internal pressure of the fuel rod not exceed reactor coolant system pressure. Both criteria have the samepurpose, which is to not allow separation between the fuel pellet and the cladding late in life; this limits temperaturedifference between fuel and clad and therefore minimizes maximum fuel temperature.

The acceptance criterion in 10 CFR 50.46(b)(2) requires that the calculated maximum total oxidation of the cladding notexceed 0.17 times the total thickness of the cladding before oxidation. Total oxidation includes both pre-accident oxidationand oxidation occurring during a LOCA. If this total oxidation limit were to be exceeded during an accident, the claddingcould become embrittled. The cladding could then fracture and fragment during the reflood period and lose structuralintegrity. This in turn could compromise the structural soundness and coolable geometry of the core and ultimately theability to keep the core cooled.

Historically, the focus of compliance with 10 CFR 50.46 has been on 10 CFR 50.46(b)(1), "Peak Cladding Temperature,"which usually is most limiting at the beginning of fuel life. Because the oxidation rate is known to be dependent ontemperature, total oxidation was also deemed most severe at the beginning of life (BOL). The contribution of preaccidentoxidation to the calculated total oxidation had not previously been thought to be significant, but as measured claddingoxidation thickness in the later stages of assembly service life increased faster than had been predicted, it became so.

On November 6, 1997, the NRC staff, Westinghouse, and the Westinghouse Owners Group (WOG) met in a publicmeeting to discuss this matter. At that meeting, the WOG stated that it would provide a list of affected plants, the projecteddates when each might become vulnerable to potential non-compliance, and details of its plans to address the issue. TheWOG also stated that each affected plant would take appropriate individual actions in terms of reporting pursuant to10 CFR 50.46(a)(3)ii before the plant reached its projected date of vulnerability.

Westinghouse stated that it planned to perform more detailed assessments for individual plants and to make timelyrecommendations to each licensee for compensatory actions with regard to the compliance issue. In the longer term,Westinghouse will correct its model in PAD to better account for recent higher burnup oxidation data and will begin usingthe revised model by August 1998.

The NRC staff found that this approach was adequate to address in the near term the specific problems reported byWestinghouse and that plants with Westinghouse IFBA fuel could continue to operate in compliance with 10 CFR 50.46.The staff noted that the burnup related phenomena, which could result in noncompliance with the oxidation requirements of10 CFR 50.46, may not be limited to Westinghouse IFBA fuel but might affect any Zircaloy fuel used in high burnupapplications. The staff also notes that the oxidation-related phenomena discussed in this information notice may affectlicensees' compliance with the reporting requirements of 10 CFR 50.46(a)(3), as well as the performance criteria of 10CFR 50.46(b).

**Discussion**:

Westinghouse employs the NRC-approved PAD computer code to evaluate fuel performance.  In 1996, Westinghousefound that two cladding-related models in PAD were nonconservative in analyses of fuels at high burnup. It has recentlybeen shown that the effects of these non-conservatisms in the models could lead to nonconservative calculation of post-LOCA cladding oxidation. These analyses are used to show compliance with 10 CFR 50.46 (b), criterion (2).

The first deficient model deals with fuel rod gap pressure. For the last several years, Westinghouse plants have usedhigh-duty fuel rods, with IFBA and Zircaloy-4 cladding in their core designs. The IFBA rods have a boron coating on theUO2 pellet surface. Westinghouse discovered that for higher burn up IFBA fuel, the rod internal pressure buildup attributedto helium released from IFBA was higher than the buildup previously modeled by the PAD code. Westinghouse revised thePAD model to account for increased helium release from IFBA rods and the increased rod pressure buildup resulting fromthis helium release.

The second deficient model is the Zircaloy-4 cladding oxidation calculation in the PAD code. Westinghouse corrected thecorrosion model for Zircaloy-4 cladding material to address the accelerated levels of corrosion actually being measured forhigh burnup fuel rods. The measured corrosion levels were higher than had been calculated using the previous oxidationmodel. Using the corrected corrosion model, Westinghouse interpreted the PAD results to indicate that the degradedthermal conductivity of the cladding due to the higher oxidation levels produced an increase in fuel cladding temperaturesand consequent higher clad creep rates. These higher creep rates could, in turn, lead to gap reopening, which would becontrary to a Westinghouse design criterion. In addition, Westinghouse concluded that with potential gap re-opening,degraded thermal conductivity of the fuel pellets due to high burnup further elevated the local fuel temperature.

The accompanying higher stored energy level and the high pre-LOCA oxidation level could, as early as the second half ofthe second duty cycle, make this higher burnup fuel more limiting with respect to the LOCA criterion of 10 CFR50.46(b)(2) than the analysis of record for BOL fuel. Westinghouse further indicated that the gap reopening is a concernnot only for IFBA rods, but also for gadolinia rods which contain gadolinia powder mixed homogeneously with UO2pellets. The gadolinia degrades the thermal conductivity of the fuel pellets, resulting in a higher operating temperature ofthe fuel.

Westinghouse stated that exceeding the criterion prohibiting gap reopening did not directly lead to clad failures. However,fuel rods with gap reopening could be more vulnerable to swelling and rupture during LOCAs and could challenge the 17percent oxidation limit. Therefore, high burnup or high duty-fuel rods with a tendency toward gap reopening would bemore vulnerable under LOCA conditions. Licensees and fuel vendors with other types of Zircaloy clad fuels may wish toconsider the relevance of this information to the oxidation models in use for their specific fuels in light of this newexperience, which suggests that oxidation levels at high burnup may be more severe than previously expected.

This information notice requires no specific action or written response. If you have any questions about the information inthis notice, please contact one of the technical contacts listed below or the appropriate Office of Nuclear ReactorRegulation (NRR) project manager.

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