For nuclear pressurized water reactors (PWR), safety analysis are carried out on operating modes involving quick and large amplitude changes in thermal power produced. In this frame, assessments have to conservatively account for initiating fuel melting localized in the pellet central region, where temperatures and stresses are the highest. In the present paper we propose a thermal-mechanical coupled model to describe the behavior of fuel pellet at melting onset, developed by means of homogenization methods. The model has been implemented in the finite element code Cast3m and a simple test case is presented to verify and validate the modeling principle. The dependency of the thermal mechanical behavior of the system on the melted fraction is discussed.