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Review of Used Nuclear Fuel Storage and Transportation Technical Gap Analyses

The U.S. Department of Energy Office of Nuclear Energy (DOE-NE), Office of Fuel Cycle Technology, has established the Used Fuel Disposition Campaign (UFDC) to conduct the research and development activities related to storage, transportation, and disposal of used nuclear fuel and high-level radioactive waste. The mission of the UFDC is to identify alternatives and conduct scientific research and technology development to enable storage, transportation, and disposal of used nuclear fuel (UNF) and wastes generated by existing and future nuclear fuel cycles. The Storage and Transportation activities within the UFDC are being developed to address issues regarding the extended storage of UNF and its subsequent transportation. The near-term objectives of the storage and transportation task are to use a science-based, engineering-driven approach to develop the technical bases to support the continued safe and secure storage of UNF for extended periods, subsequent retrieval, and transportation.

While both wet and dry storage have been shown to be safe options for storing UNF, the focus of the program is on dry storage of commercial UNF at reactor or centralized locations. Because limited information is available on the properties of high burnup fuel (exceeding 45 gigawatt- days per metric ton of uranium [GWd/MTU]), and because much of the fuel currently discharged from today’s reactors exceeds this burnup threshold, a particular emphasis of this program is on high burnup fuels.

The first step in establishing the technical bases for storage and transportation was to determine the technical data gaps that need to be addressed. The Gap Analysis to Support Extended Storage of Used Nuclear Fuel (UFDC 2012a, referred to as the UFDC Gap Analysis) was prepared to document the methodology for determining the data gaps and to assign an initial priority (Low, Medium, High) of importance for additional research and development to close the data gaps. The analysis considered only normal conditions of extended dry storage of commercial light water reactor (LWR) uranium dioxide fuel. An update to the UFDC Gap Analysis report is planned to include data gaps associated with transportation as well as some design-basis phenomena (e.g., design-basis seismic events) and accident conditions (e.g., cask tipover). UFDC also performed a more quantitative prioritization of the research to close the high and medium priority gaps in the Used Nuclear Fuel Storage and Transportation Data Gap Prioritization report (UFDC 2012b, referred to as the UFDC Gap Prioritization).

In order to verify that the UFDC identified all of the technical gaps and properly prioritized them, this report was commissioned to compare the UFDC Gap Analysis and UFDC Gap Prioritization reports to those recently published by others, including the U.S. Nuclear Waste Technical Review Board (NWTRB), the U.S. Nuclear Regulatory Commission (NRC), the Electric Power Research Institute (EPRI), and the International Atomic Energy Agency (IAEA). The documents reviewed are:

 

Evaluation of the Technical Basis for Extended Dry Storage and Transportation of Used Nuclear Fuel (cited as NWTRB 2010)

Identification and Prioritization of the Technical Information Needs Affecting Potential Regulation of Extended Storage and Transportation of Spent Nuclear Fuel, Draft for comment (cited as NRC 2012a)

International Perspectives on Technical Data Gaps Associated with Extended Storage and Transportation of Used Nuclear Fuel, Draft (cited as EPRI 2012)

Extended Storage Collaboration Program (ESCP) Progress Report and Review of Gap Analyses (cited as EPRI 2011)

Long Term Storage of Spent Nuclear Fuel - Survey and Recommendations (cited as IAEA 2002).

The EPRI 2012 report provides the priorities of additional research of Extended Storage Collaboration Program (ESCP) committee members from six countries in addition to the United States: Germany, Hungary, Japan, South Korea, Spain, and the United Kingdom. Priorities given for the six countries are opinions of the EPRI/ESCP International Subcommittee participants and may not represent the official position of the organization or country. Each organization and country has a different focus when evaluating the research needed for closing technical gaps. These differences stem mostly from differences in the storage systems used (e.g., casks, vaults), future waste management needs and strategies, and organizational perspectives (e.g., industry, regulator). Both the NRC report (NRC 2012a) and the international report from EPRI/ESCP (EPRI 2012) are draft reports subject to change.

There are a collective total of 94 technical data gaps identified by the various reports to support extended storage and transportation of UNF. This report focuses on the gaps identified as Medium or High in any of the gap analyses and provides the UFDC’s gap description, any alternate gap descriptions or different emphasis by another organization, the rankings by the various organizations, evaluation of the consistency of priority assignment and the bases for any inconsistencies, and UFDC-recommended action based on the comparison. Gaps that are ranked Low by all organizations and countries are not evaluated in this report.

Of the 94 gaps identified in the various gap analyses, there are 14 cross-cutting gaps and 80 structure, system, and component- (SSC-) specific gaps. For the cross-cutting gaps, the UFDC identifies eight and others identify six. Thirteen of the 14 cross-cutting gaps were identified as Medium or High by at least one of the gap analyses. The UFDC assigns a high priority to all the cross-cutting gaps it identified. For most of these, there is general agreement of their high priority. The six gaps identified by others are either covered by other UFDC gaps or are not applicable to UNF storage and transportation in the United States. Therefore, it is concluded that no changes to the UFDC cross-cutting gap analysis are necessary.

For the 80 SSC-specific gaps, the UFDC identifies 52 and others identify 28. The gaps identified by others either do not meet the UFDC’s definition of a gap for extended storage and subsequent transportation, are grouped differently by the UFDC, or are given less than low priority by the UFDC. For example: “Cladding - Oxide Thickness” is a property of UNF, not a degradation mechanism, “Cladding - Propagation of Existing Flaws” is covered by the UFDC under the individual degradation mechanisms, and “Canister - Irradiation Damage” is considered by the UFDC to be insignificant.

Of the 80 SSC-specific gaps, 48 were identified as Medium or High by at least one of the gap analyses. For 25 of these 48 Medium and High priority gaps, there is either consistency in evaluation and priority assignment across the gap analyses or the UFDC assigns a higher priority. Gaps with consistent high priority evaluation receiving five or more high ratings include:

Cross-cutting gaps

Thermal Profiles Examine Fuel After Storage Monitoring

SSC-specific gaps

Cladding – Delayed Hydride Cracking Cladding – Hydride Reorientation and Embrittlement Casks/Canisters – Atmospheric Corrosion (especially SCC at the welds)

In some instances, the UFDC gives a higher priority for additional research and development to gaps where experts disagree on the mechanisms (e.g., delayed hydride cracking and clad oxidation). Other differences in priorities are mostly because of differences in the various countries’ or organizations’ storage and transportation programs and ultimate waste disposal strategies. For example, the UFDC places a higher priority on many of the cladding gaps in an effort to maintain retrievability at the fuel assembly level.