Mechanical interaction between fuel and cladding during normal transient operating conditions

In order to avoid pin failure, it is necessary to be sure that the operating conditions will not induce in the cladding some damaging form of deformation. Stresses and strains taking place in the cladding have different origins. 1. The cladding deformation due to fission gas pressure occurs by irradiation creep and this is not a damaging strain. 2. The thermal stresses induced in the cladding at the beginning of life by the thermal gradient relax rapidly by thermal and irradiation creep. The resulting damage is negligible. 3. Stresses induced by the swelling gradient in the thickness of the cladding probably relax by irradiation creep. But we cannot be sure that the swelling laws deduced from axial profile still apply in the cladding thickness. In particular the influence of stress on swelling is not yet well known. 4. Analysis of fuel-cladding mechanical interaction (FCHI) shows that during steady-state operation it is very unlikely a damaging form of strain due to FCHI to take place in the cladding. It is only during non-steady state operation at power increase that such a strain can occur. Therefore two types of operating conditions may be feared the increase to full power after continued operation at reduced power and the load follow power cycling