Modelling of the fuel mechanical behavior: from creep laws to internal variable models Original Research Article

Creep laws such as that of Bohaboy are commonly used to simulate the fuel pellet response to the demands placed upon it during its use. These laws are sufficient for describing the base operating conditions (where only creep appears), but they require improvement for describing power ramp conditions (where hardening and relaxation appear). The aim of the present paper is to develop a framework in which it will be possible to build models that are more representative of the fuel pellet in pile conditions. The approach presented here is based on the thermodynamics of irreversible processes and continuum mechanics. It is postulated that the material is made of a mixture of porous and ‘sound’ material. The evolution of porosity is deduced from experimental results in order to be consistent with the second law of thermodynamics. This implies the assumption of a threshold value for the existence of densification and swelling.