SPAR III is a continuation of the SPAR II CRP dealing with research of spent fuel and storage materials behaviour over long term spent fuel storage (100 years and potentially more).Programs carried out so far (BEFAST, SPAR I and II) have identified some potential deterioration mechanisms of spent fuel elements that required detailed investigation. Investigations of these mechanisms indicate unlikely impact of most of them on spent fuel integrity over storage periods. There are still impacts of some deterioration mechanisms that have to be investigated over long time periods (i.e. hydrogen effects on fuel elements) and countries that participated in SPAR II continue this research. In addition, behaviour and impact on these mechanisms under transportation and some extreme conditions need to be investigated. Surveillance and monitoring results of spent fuel in storage are important inputs in assessing spent fuel performance in storage. The long term objective of spent fuel integrity is to keep all future spent fuel management options open.
Development of a technical knowledge base on long term storage of power reactor spent fuel through evaluation of operating experience and research by participating member states.
Collection and exchange of relevant experience of spent fuel storage and the impact on associated spent fuel management activities in the participating countries
Creating synergy among research projects of the participating member states
Developing capability to assess the impact of potential deterioration mechanisms on fuel and spent fuel storage components
Facilitate transfer of knowledge by documenting technical basis for spent fuel storage
Fuel and materials performance evaluation under wet and dry storage, and assessment of impact of interim storage on associated spent fuel management activities (like handling and transport)
Surveillance and monitoring programmes of spent fuel storage facilities as one of the means to evaluate spent fuel performance during storage
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.
Spent fuel management has always been one of the most important stages in the nuclear fuel cycle, and stands among the most vital problems common to all Member States with nuclear reactors. In order to demonstrate safety over these long time periods a good understanding of the processes that might cause deterioration of the system are needed. The CRP wholly supports providing this understanding through reporting on-going behaviour of both spent fuel and the storage system. The major players in the CRP have now been storing spent fuel for decades and sharing this accumulated knowledge is essential both for existing operators and to new comers. In addition the CRP reports the latest R&D findings and thinking related to spent fuel storage and storage system performance.