LOAD FOLLOW OPERATION IN ELECTRICITE DE FRANCE POWER PLANTS AND FUEL BEHAVIOUR EVALUATION.

Power changes, totally or partly made by control rod movements, lead to important variations in the axial profile and therefore in the local fuel power. Due to differential thermal expansion, the uranium dioxide strains the cladding and the resulting stresses increase the risk of PCI failure. Some studies on fuel rod behaviour under load follow duty have been performed to evaluate such a risk. These studies consist of the calculation of the stress evolution, according to the power and the velocity of the load variation, and also the calculation of the associated stress relaxation and therefore the cladding deformation. The results are compared with the ramp test data and, based on these tests, the PCI failure criterion is defined as the ratio of the maximum stress in the cladding to the zircaloy yield point in the temperature and irradiation conditions at which interaction occurs. The results of the calculation for the more constrained conditions under load follow duty show that the risk of fuel failure is almost zero. In addition, the contribution to the fatigue behaviour of the cladding is less than 10%, all the other design criteria being satisfied, of course.