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UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION & OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS WASHINGTON, D.C. 20555 August 31, 1994 NRC INFORMATION NOTICE 94-64: REACTIVITY INSERTION TRANSIENT AND ACCIDENT LIMITS FOR HIGH BURNUP FUEL Addressees All holders of operating licenses or construction permits for nuclear power reactors and all fuel fabrication licensees. Purpose The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to recent information on high burnup fuel performance that could affect previously approved fuel burnup limits and enthalpy limits of highly exposed fuel. It is expected that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to minimize adverse effect on design and operation of reload cores and to properly evaluate their 10 CFR Part 21 reporting responsibilities if continuing evaluation indicates limits should be reduced. However, suggestions contained in this information notice are not NRC requirements; therefore, no specific action or written response is required. Description of Circumstances Recent experimental data have suggested that high burnup fuel may be more prone to failure during design-basis transients and reactivity insertion accidents (RIAs) than previously thought. Recent data on the relationship between fuel failure enthalpy and burnup for pressurized water reactor fuel rods tested in foreign experimental facilities indicate lower failure initiation enthalpy thresholds (measured in differential cal/gm) than was considered in the evaluation of currently approved fuel burnup limits. The NRC is closely following work to confirm the validity of the preliminary results by verifying the conditions of the experimental tests. The NRC is also reviewing data reduction, data interpretation, and the implications of the observed failure results at low enthalpy rise levels on the safety evaluation of LWR design-basis transients and accidents. The NRC is scheduled to complete its review of the high burnup fuel data by the summer of 1995. The results of this study may invalidate the basis for the previous staff approvals of fuel design topical reports and fuel failure and fuel burnup limits. 9408250234.IN 94-64 August 31, 1994 Page 2 of 3 Discussion Recent information suggests that fuel rod damage during reactivity transients can increase with high burnup. Various investigators have reported a "rim" effect, consisting of a high burnup peripheral region of the fuel pellet, in which the solid fuel matrix has a small grain size and is surrounded by fission gas bubbles (Ref. 1). This porous microstructure could enable fuel to disperse in powder form during transients that heat and expand the fission gas bubbles. A recent study for the Electric Power Research Institute (EPRI) was conducted on fuel irradiated up to burnups of 60 GWD/MTU (Ref. 2). The objective was to evaluate the degradation of mechanical properties in cladding and assembly structural components caused by oxidation, hydriding, and radiation embrittlement. While strength increases, ductility was found to be very low and dependent on hydrogen content and fluence, both of which increase with burnup. This appears to contradict a previous belief that metal annealing at operating temperatures would prevent radiation damage from accumulating after some initial period. Recent integral tests with high burnup fuel under reactivity transient conditions have indicated that the bounding peak fuel enthalpy that may cause fuel cladding failure decreases significantly with increasing fuel exposure. Of the fuel rods tested, the one with the highest burnup (approximately 65 GWD/MTU) failed at an enthalpy level of only 30 cal/gm with a reactivity insertion corresponding to less than 15 cal/gm; some fuel dispersal may be associated with this failure. An earlier integral test at the intermediate burnup level of 32 GWD/MTU showed cladding failure at about 85 cal/gm (Ref. 3). If the scatter of data points for high burnup failure is similar to that observed in lower burnup failure data, the recent cited data are not conclusive regarding the enthalpy failure limit for a specific fuel rod. However, the transient data, the declining cladding ductility with increasing burnup, and the fuel pellet "rim" effect, when considered together, suggest that the trend of these results may be real. LWR fuel suppliers and reactor operators (licensees) may wish to consider this preliminary information in planning for fuel cycle designs, particularly those fuel design changes and longer core operating cycles which may approach or exceed approved fuel burnup limits. A higher burnup fuel may be required to meet more restrictive enthalpy limits for fuel failure, fragmentation, and dispersal than those permitted by the acceptance criteria of the standard review plan (Ref. 4). The NRC review of pending and future requests to extend burnup beyond the limits previously approved for existing fuel designs (up to approximately 60 GWD/MTU, fuel rod average) will more carefully consider the adequacy of the reactivity insertion experimental data included or referenced by such requests, until the implications of these high burnup experimental data on licensing safety evaluations are resolved. .IN 94-64 August 31, 1994 Page 3 of 3 On October 26, 1994, the NRC will present additional technical information on this subject at the NRC 22nd Water Reactor Safety Information Meeting. 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 NRC project manager. /s/'d by Robert F. Burnett/s/'d by AEChaffee/for Robert F. Burnett, DirectorBrian K. Grimes, Director Division of Fuel Cycle SafetyDivision of Operating Reactor Support and SafeguardsOffice of Nuclear Reactor Regulation Office of Nuclear Material Safety and Safeguards Technical contacts: L. E. Phillips, NRR (301) 504-3232 E. D. Kendrick, NRR (301) 504-2891 J. Roth, NMSS (301) 415-7257 Attachments: 1. References 2. List of Recently Issued NRC Information Notices 3. List of Recently Issued NMSS Information Notices .Attachment 1 IN 94-64 August 31, 1994 Page 1 of 1 REFERENCES 1.M. E. Cunningham, et al., "Development and Characteristics of the Rim Region in High-Burnup UO2 Fuel Pellets," Journal of Nuclear Materials, Vol. 188, page 19, June 1992. 2.G. P. Smith, et al., "Hot Cell Examination of Extended Burnup Fuel from Calvert Cliffs-1,' EPRI TR-103302, November 1993. 3.P. E. MacDonald, et al., "Assessment of Light-Water-Reactor Fuel Damage During a Reactivity-Initiated Accident," Nuclear Safety, Vol. 21 No. 5, September-October 1980. 4."NRC Standard Review Plan," NUREG-0800, Rev. 2, July 1981.
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