Implementation of dry storage models in Falcon: cladding creep, annealing of cladding irradiation damage, and fuel helium-swelling

Dry storage is an important phase of the nuclear fuel cycle as it allows a reduction in the decay heat so that the used fuel can be safely transported to and stored in a final repository. However, the dry storage period lasts several decades and the integrity of the fuel rods must be guaranteed during this time. The fuel performance code Falcon allows simulating nuclear rods in reactor operating conditions, but the models are not taking into account the dry storage conditions which have a greater time scale and different thermal and mechanical environments. The introduction of specific models in Falcon is necessary for the reliable simulation of fuel rods in dry storage. Therefore, new cladding creep, cladding annealing and fuel helium-swelling models have been implemented in the code and the validation is performed for each model. Base irradiation simulations have been performed for a Swiss PWR representative rod and a BWR rod. Dry storage conditions from a benchmark have been applied. Gap closure is identified as the main contributor to stress increase in the cladding. In the BWR case, no gap closure is calculated and little difference is observed in the cladding mechanical response for different models. In the PWR case, gap closure is calculated and it is observed that the annealing model and cladding temperature history have a limited effect on the cladding hoop stress. On the other hand, the fuel helium-swelling model has a pronounced impact on the calculated stresses. In realistic conditions and with the use of the dry storage models, the calculated hoop stress is below the critical values of 90 and 70 MPa for PWR and BWR, respectively. Also, the calculated Hoop strain is below 1 % in realistic temperature conditions.