Thermal and mechanical modeling of pellet clad interaction with the COCCINEL/TITUS fuel rod calculation program

For some years FRAMATOME and CEA have undertaken an important R and D program on pellet cladding interaction (P.C.I.) during power ramps. In parallel, a modelisation of the mechanical behaviour of the fuel has been done with the computer system COCCINEL-TITUS which is presented in this paper. In a first part the state of the fuel rad before the ramp is defined by COCCINEL taking into account the rad power history. In particular the diameter of the pellet and the gap between pellet and cladding are used as inputs for the three dimensional finite element code TITUS. In, a second part, the mechanical analysis of the rod is performed with TITUS. Three types of modelisation are presented. The first two are bidimensional whether axisymmetric or of translation. They allow to predict separately the effect of the 'hour-glassing' and of the cracks. The third one is a three dimensional. The main results are that the stress concentration due at the 'hour-glassing' is of about 15% and that the power for which the elastic limit is reached is reduced by 10% if cracks are taken into account. Different initial gaps are considered ta quantify the influence of the uncertainty on this parameter. In the axisymmetric case two boundary conditions have been chosen corresponding ta perfect sliding or no-sliding between pellet and cladding. In the radial case whether cracks are considered or not. A simplified combination of these two results consists of adding to the stress given by the axisymmetric solution with perfect axial contact between pellet and cladding, the variation of stress, for the considered power, between the radial solution with and without crack. Then a complete three dimensional analysis permits to obtain the precise local stress. This complete calculation, combining the 'hour-glassing' and the crack effects, shows a very good agreement with the simplified estimation done before. Nevertheless, the phenomena cannot be superposed linearly and the stress intensification due to the hour-glassing is modified by the presence of the cracks. Finally the validity of the computer system COCCINEL-TITUS is demonstrated by comparison with the results of the ramp test on the PRISCA 04 bis rod