Modeling of pellet cladding interaction during power ramps in pressurized water reactors
Résumé
Pellet-cladding interactions (PCI) which can occur during power transients (Class 2 incidental operating conditions) in nuclear reactors may lead to Zircaloy-4 cladding failure by stress corrosion cracking. Indeed under transient power conditions, a rapid increase of the linear heat generation rate (i.e. thermal power released per unit length) induces dilatations of fuel pellets and a strong mechanical pellet-cladding interaction. Observations of cracks on inner surfaces of failed claddings show a preferential localisation in continuity of radial fuel cracks, with a higher number of occurences at the vicinity of pellet-pellet interfaces. Acceptable maximum power levels and rate of power increases are often defined according to the notion of threshold hoop stress. The aim of this work is thus to propose a numerical simulation of representative power ramps to provide a good estimation of local strain and stresses induced by fuel pellet extension. This paper presents several simulations of power ramps by the Finite Element method. Attention was paid to the description of irradiation creep and creep under high stress of the Zy-4 cladding with two specific anisotropic viscoplastic models including irradiation damage effects. Moreover the viscoplastic behavior of the pellet was considered. In the first part, 2D calculations focus on the description of the stress concentration that occurs in the inner surface due to the presence of fuel pellet cracks. Results show that the local hoop stress is four to six times higher than the mean hoop stress. Effects of frictions conditions are especially analyzed. It appears that a high friction coefficient (due to the formation of solid compounds such U-Cs-Zr) drastically increases stress and strain levels in the cladding inner surface. In the second part, a 3D modeling is also proposed to complete 2D investigations and to study geometrical aspects of PCI, such as the evolution of pellet cladding gap and fuel rod radial deformation during the ramp, especially the main and secondary ridges of the cladding. Calculations taking into account a realistic deformation of the pellet provide a better agreement with experimental data with respect to classical axysymmetric 2D calculations. These simulations provide a good description of the gap closure kinetics and allow to evaluate respective effects of pellet creep and gas swelling in the appearance of the secondary ridge of the cladding.
Domaines
Sciences de l'ingénieur [physics]
Origine : Fichiers produits par l'(les) auteur(s)