Spent Fuel Performance Assessment and Research (SPAR III)

Knowledge Base
The accumulated reporting of the IAEA’s BEFAST and SPAR CRPs now exceeds 30 years. In the context of 100+ years storage it is important that this knowledge base is continued to be updated and on on-going spent fuel and system performance reported.
Spent Fuel Performance
For zirconium and stainless steel based LWR clad spent fuels there is now >50 years wet storage and >30 years dry storage experience. Performance in storage remains excellent with no generic failure mechanism identified or experienced. In the case of stainless clad AGR spent fuels there is >30 years of wet storage experience. Storage performance is good provided the fuel is stored in the presence of corrosion inhibitor. For MAGNOX spent fuel, magnesium alloy clad there is >50 years’ experience in handling and storage. The tendency is only to store for relative short periods of time in wet storage, but the fuel will remain intact for longer periods provided optimum storage chemistry is maintained.
Spent Fuel Degradation
A considerable number of studies on hydrogen behaviour in zirconium based clad material have been undertaken during this phase of the SPAR project. Whilst studies using empty clad tubes have improved the understanding of hydrogen behaviour in zirconium based clad material, the properties associated with the presence of fuel may be different. A gap in this area remains in establishing experiments under actual storage conditions.
Impact of Interim Storage on Associated Spent Fuel Management Activities
Improving the knowledge base on spent fuel behaviour for retrieval and transport situations has been investigated. The amount of fuel material released upon impact appears to be independent of fuel burnup, test method, is localized to the area of rupture and has been found to be <2 pellets worth of material for all fuel types tested. The studies have enabled establishing reference conditions for safety assessments.
Materials Performance in Storage Systems
Knowledge of the long-term performance of materials used in spent fuel storage systems is required for continued operations and for storage system relicensing. A number of studies on the behaviour of materials have continued or been initiated during this phase of the SPAR project. In some areas, such as the influence of stress corrosion cracking (SCC) on dry storage canisters in marine environment, studies are still ongoing. In other areas, for example metal seals, the experience to date and on-going long-term studies have not established limiting factors for their continued use.
Issues Related to Extended Storage of Spent Fuel
In the very long-term, helium generation from a-decay and the accumulation of helium may become an important mechanism in fuel matrix evolution. Although there remains unknowns in relation to gas retention by the fuel matrix, bulk pellet swelling or release mechanisms to the free volume, even where all the gas generated in the first 300 year of UO2 spent fuel storage is released, there is a negligible effect on cladding stress during storage. More attention has to be devoted to spent MOX fuel behaviour.
Way Forward
Continued spent fuel storage and future transitions from one phase of the back-end of the nuclear fuel cycle to the next require that the operational experience and research results to be reported and disseminated to the Member States for input to continued operations safety assessments and the licensing of new facilities. In this respect field experience needs to be collected and reported; especially from the deployment of new technology.