Subtask 1.1 – Creep and hydride reorientation of fuel rods under simulated dry storage conditions:
Whereas many creep and hydride reorientation tests of unirradiated cladding have been performed, hardly any data are available on the thermal creep properties of irradiated fuel rods with fuel pellets inside. In high burnup fuel rods, fuel-cladding bonding could restrict cladding creep out. In addition to the effects on creep behaviour, bonding might also affect hydride reorientation behaviour in the cladding, leading to local stress concentrations favouring local hydride reorientation and creating potential spots vulnerable to crack initiation and propagation under long-term dry storage conditions. Possible effects due to fuel-cladding bonding in high burnup fuel rods will be investigated. Creep properties of rod segments with fuel inside will be compared to defueled cladding properties. Potential hydride reorientation will be assessed and mechanical properties of the cladding before and after creep testing will be determined.
Subtask 1.2 – Hydride reorientation:
During back end handling and dry storage, fuel cladding temperatures will be high enough to dissolve hydride precipitates back into solid solution. When temperature drops later on, hydrogen will be precipitated again. If the cladding is under high enough hoop stress, the precipitated hydrides will be oriented in radial direction, which impacts ductile-to-brittle transition behaviour of the cladding material of concern. The conditions and mechanism for hydride reorientation in irradiated cladding material will be determined, in order to predict both the hydride reorientation and ductile-to-brittle transition behaviour of the material, based on the understanding of these parameters.
Subtask 1.3 – Spent fuel rods in transport and handling operations and in accident scenarios:
Independent from the back end concept, fuel assemblies are handled, loaded into transport casks and unloaded or stored in dry-storage casks when removed from the on-site spent fuel pool. A very large number of transports have been performed successfully worldwide. Only for special transportation conditions or accident situations is there a substantial need to verify spent fuel behaviour and suitability for further storage. This subtask will concentrate on three areas of concern. It aims at generating valuable experimental data on the mechanical response of irradiated fuel rods under transport accident conditions. The data will support analytical models for regulatory accident evaluation. In addition, they will also be useful for seismic and vibratory evaluations. In order to support cask containment analysis and the definition of source terms for accident scenarios, the particulates which might be released from high burnup fuel rods due to impact events will be characterised. Finally, the strength of weak or slightly damaged fuel rods under transportation and handling operations will be investigated. The aim is to verify that weak or slightly damaged rods will not degrade or jeopardise cask safety functions during transportation and storage.
Subtask 1.4 – Failed fuel:
In most countries, no standard procedures have yet been established to take care of failed fuel for interim storage and final disposal. For safe long-term stabilisation of failed fuel, the radiological confinement needs to be restored and the geometry and environment needs to be controlled and stable. There are different concepts available to encapsulate damaged and failed fuel rods, either by canning in-pool or by conditioning and encapsulation at a hot cell. In this context, drying of failed fuel is essential to avoid gas generation by radiolysis of residual water and moisture. The presence of oxygen and hydrogen gas could have undesirable consequences, such as oxidation of the fuel, hydriding of the cladding, corrosion and pressure build-up. Whereas standards have been established for drying of intact spent fuel in dry storage casks, for failed fuel these standard drying procedures may not be sufficient to guarantee the required moisture level for encapsulation. Therefore, test methods to measure moisture content need to be developed and validated to prove that criteria on moisture content can be met. Furthermore, available drying procedures need to be evaluated for failed fuel and possibly optimised. Within this subtask, experimental data on the issue of safe encapsulation and storage of failed fuel rods will be generated, using established characterisation methods and assessment of residual water
