This report summarizes the results of EPRI's multi-year research effort to assess cladding performance under normal and hypothetical accident conditions of spent nuclear fuel transportation.
The structural performance of high-burnup spent fuel cladding during dry storage and transportation has been the subject of research and evaluation at EPRI for several years. The major issues addressed in this research program have included the following:
- Characterization and development of predictive models for damage mechanisms perceived to be potentially active during dry storage
- Modeling and analysis of deformation processes during long-term dry storage
- Development of cladding failure models and failure criteria, considering cladding material and physical conditions during dry storage and transportation
- Failure analysis, considering end-of-dry-storage conditions, of spent fuel systems subjected to normal and accident conditions of transport, prescribed in Part 71 of Title 10 of the Code of Federal Regulations (10 CFR 71)
While issues related to dry storage have largely been resolved, transportation issues have not, at least for spent fuel with discharge burnups greater than 45 GWd/MTU. A research program was launched in late 2002 following two NRC-industry meetings held on September 6, 2002 and October 23, 2002. The aim of the research program was to assess the performance of high-burnup spent fuel cladding under normal and accident conditions of transportation, as prescribed by 10 CFR 71, considering the physical characteristics and mechanical properties of cladding at the end of dry storage. Since its inception in the fall of 2002, the research program has resulted in the publication of eight EPRI reports.
To present a synthesis of the information contained in eight previously published EPRI reports, which collectively form a part of a technical basis intended to facilitate resolution of regulatory issues associated with the transportation of spent nuclear fuel characterized by discharge burnups greater than 45 GWd/MTU.
The research team reviewed the large body of modeling and analysis work carried out during the past several years and then organized it into three technical areas. The first technical area examines dry storage effects on cladding geometry (due to creep) and hydride morphology in the cladding alloy (due to hydride re-orientation). The second technical area deals with the development of failure models and failure criteria for application to transportation accident analysis. The third technical area applies the above-described developments to the evaluation of fuel rod failures in transportation casks subjected to loading conditions specified in 10 CFR 71.
The results of the analytical studies conducted during the past several years indicate that damage to high-burnup spent fuel under prescribed regulatory conditions of dry storage and transportation will not impair its operational management. Dry storage effects on cladding physical and material conditions, including creep-related deformations, have the potential to impact fuel rod performance during transportation. Specifically, cladding resistance to failure under the dynamic loading of transportation accidents depends on fuel-cladding gap size and radial hydride formation, both of which could—to a limited extent—evolve during long-term dry storage. The results of the hypothetical accident analysis indicate that cladding failure would be bi-modal—taking the form of 1) a state of failure initiation at the cladding inside diameter (ID) remaining as part-wall damage, with less than a 2% probability of occurrence and 2) a through-wall failure with a probability of 1E-5. The response analysis under normal conditions of transport shows a large margin against fuel rod failures. The grids and guide tubes, which form the structural elements of the fuel assembly, are predicted to remain structurally competent. As a result, the geometric form of the spent fuel assemblies will not be substantially altered.
Failure to resolve, in a timely manner, regulatory issues associated with interim dry storage and transportation of high-burnup spent fuel would result in severe economic penalties and in operational limitations to nuclear plant operators. The Nuclear Regulatory Commission (NRC) Spent Fuel Storage and Transportation (SFST) Division issued Revision 3 of Interim Staff Guidance 11, Cladding Considerations for Transportation and Storage of Spent Nuclear Fuel
, in November 2003. Revision 3 contains generic acceptance criteria for the dry storage of spent fuel, but does not specify any such criteria for the transportation of high-burnup (>45 GWd/MTU) spent fuel. Proposed approaches for resolving technical issues associated with fuel assembly integrity under hypothetical transportation accident conditions have been discussed with SFST. The results are documented in eight EPRI reports: 1009694, June 2004; 1009693, December 2004; 1009929, June 2005; 1011816, September 2005; 1011817, December 2005; 1013447, October 2006; 1013448, December 2006, and 1015049 (June 2007). This report provides a synthesis of the work reported in these eight reports. For completeness, past work related to the dry storage phase in the life cycle of managing spent fuel is summarized, given that transportation may occur after an extended period of dry storage. Two additional reports assessing criticality risks during transportation of spent fuel (1013449, December 2006) and addressing effects of fuel relocation on nuclear reactivity (1015050, June 2007) provide further information intended to generically resolve the high-burnup spent fuel transportation issue.