ML20321A093

Appendix 8B - Fuel Cladding Creep

ML20321A093
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Issue date:	04/30/2020
From:	Office of Nuclear Material Safety and Safeguards
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References
NUREG-2215
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APPENDIX 8B FUEL CLADDING CREEP Creep is the dominant mechanism for cladding deformation under normal conditions of storage.

The relatively high temperatures, differential pressures, and corresponding hoop stress on the cladding will result in permanent creep deformation of the cladding over time. Several laboratory programs have demonstrated that spent nuclear fuel (SNF) has significant creep capacity even after 15 years of dry storage. NUREG/CR-6831, Examination of Spent Fuel Rods After 15 Years in Dry Storage, issued September 2003, reported that irradiated Surry-2 pressurized-water reactor (PWR) fuel rods (35.7 gigawatt days per metric ton of uranium (GWd/MTU)) that were stored for 15 years at an initial temperature of 350 degrees Celsius (°C)

(662 degrees Fahrenheit (°F)) (with temperatures reaching as high as 415 °C (779 °F) for up to 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />) experienced thermal creep, which was estimated to be less than 0.1 percent.

Post-storage creep tests were conducted to assess the residual creep capacity of the Surry-2 fuel rods. One rod segment experienced a creep strain of 0.92 percent without rupture at 380 °C (716 °F) and 220 megapascals (MPa) in 1,820 hours0.00949 days <br />0.228 hours <br />0.00136 weeks <br />3.1201e-4 months <br /> (75.8 days). A different rod segment was tested at 400 °C (752 °F) and 190 MPa for 1,873 hours0.0101 days <br />0.243 hours <br />0.00144 weeks <br />3.321765e-4 months <br /> (78 days), followed by 693 hours0.00802 days <br />0.193 hours <br />0.00115 weeks <br />2.636865e-4 months <br /> (28.9 days) at 400 °C and 250 MPa, and experienced a creep strain of more than 5 percent without failure (Tsai et al. 2006). Profilometry measurements on that fuel rod indicated that the creep deformation was uniform around the circumference of the cladding with no signs of localized bulging, which can be a precursor for rupture. A report of the literature (Beyer 2001) also indicates that some SNF cladding can accommodate creep strains of 2.87.5 percent at temperatures between 390 and 420 °C and hoop stresses between 225 and 390 MPa. Other significant contributions to the understanding of the effects of creep on SNF cladding can be found in several references (Einziger et al. 1982; Rashid et al. 2000; Hendricks 2001; Rashid and Dunham 2001; Machiels 2002). In general, these data and analyses support the conclusions that (1) deformation caused by creep will proceed slowly over time and will decrease the rod pressure, (2) the decreasing cladding temperature also decreases the hoop stress, and this too will slow the creep rate so that during later stages of dry storage, further creep deformation will become exceedingly small, and (3) in the unlikely event that a breach of the cladding from creep occurs, it is believed that this will not result in gross rupture.

Based on these conclusions, the staff has reasonable assurance that creep under normal conditions of storage will not cause gross rupture of the cladding and that the geometric configuration of the SNF will be preserved, provided that the maximum cladding temperature does not exceed 400 °C (752 °F).

References Beyer, C.E., Letter from C. E. Beyer, Pacific Northwest National Laboratory, to K. Gruss, NRC.

Subject:

Transmittal of Update of CSFM Methodology for Determining Temperature Limits for Spent Fuel Dry Storage in Inert Gas. November 27, 2001.

Einziger, R.E., S. D. Atkin, D. E. Stellbrecht, and V. Pasupathi, High Temperature Postirradiation Materials Performance of Spent Pressurized Water Reactor Fuel Rods Under Dry Storage Conditions. Nuclear Technology, Vol. 57, p. 65. 1982.

Hendricks, L., Letter from L. Hendricks, NEI, to M. W. Hodges, NRC.

Subject:

Transmittal of Responses to the NRC Request for Additional Information on storage of high burnup fuel, August 16, 2001.

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Machiels, A., Regulatory Applications Lessons Learned -- Industry Perspective. NEI Dry Storage Information Forum. Naples, FL. May 15-16, 2002.

NUREG/CR-6831, Examination of Spent Fuel Rods After 15 Years in Dry Storage, ANL-03/17, Argonne National Laboratory, September 2003.

Rashid, Y.R., D. J. Sunderland, and R. O. Montgomery, Creep as the Limiting Mechanism for Spent Fuel Dry Storage - Progress Report. EPRI TR-1001207. 2000.

Rashid, Y.R., R. S. Dunham, Creep Modeling and Analysis Methodology for Spent Fuel in Dry Storage. EPRI TR-1003135. 2001.

Tsai, H. and M.C. Billone, Thermal Creep of Irradiated Zircaloy Cladding, Journal of ASTM International, Vol. 3, No. 1, pp. 1-16, 2006.

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