Nuclear Executive Update   
An EPRI Progress Report, January 2011
TECHNICAL HIGHLIGHTS
EPRI Examines Criticality Risks During Transportation of Used Nuclear Fuel

A new report proposes a pathway for addressing several regulatory issues associated with used nuclear fuel transportation.

Industry efforts to license transportation packages for used nuclear fuel characterized as “high-capacity” or containing “high-burnup” (>45 GWd/MTU) fuel have not been successful to date considering existing used fuel inventories. EPRI has developed a multi-faceted framework to potentially resolve several used fuel transportation issues. The framework considers criticality risks during transportation, used fuel and cask-design features, and defense-in-depth in the context of current and potential regulations that would reflect a risk-informed, technically state-of-the-art approach.

To date, much of the debate has identified cladding integrity under transportation accident conditions as key to satisfying the regulatory requirements, specifically those requiring that the transportation package contents remain subcritical under hypothetical accident conditions. For fuel burnup less than or equal to 45 GWd/MTU, the Nuclear Regulatory Commission (NRC) has concluded that sufficient evidence exists to provide reasonable assurance that used fuel will remain structurally intact under hypothetical accident conditions. For burnup greater than 45 GWd/MTU, the NRC has concluded that such reasonable assurance did not exist. Therefore, changes in the packaging and contents under hypothetical accident conditions that could cause the nuclear reactivity to increase would have to be addressed.

For representative transportation package designs, the EPRI research showed that the conditions required for criticality during hypothetical transportation accidents had an extraordinarily low probability of occurrence. With regard to the nuclear reactivity of a transportation package with re-configured fuel and water in the fuel cavity, which has been of particular regulatory concern, the analysis showed that realistic and achievable configurations of nuclear fuel materials following an impact accident could not result in a critical configuration.

The research highlights the importance of incorporating risk analysis into decisions on used fuel transportation. Misallocation of regulatory requirements can potentially lead to higher overall risks, specifically by increasing the number of shipments by overly restricting payloads. Because non-radiological risks – such as collision injuries and deaths and property damage – dominate transportation risks, payloads should be maximized to minimize their occurrence.

For more information, contact Albert Machiels at 650.855.2054 or amachiel@epri.com.