Computational evaluation for the mechanical behavior of U10Mo fuel mini plates subject to thermal cycling

Mechanical behavior of the monolithic mini-plates during a post-fabrication furnace annealing was investigated. Monolithic fuel is a proposed fuel form to accomplish higher uranium densities in the reactor core and thermal cycling is a standard performance evaluation procedure for these fuel elements. To evaluate the mechanical performance of the plate under a thermal loading, a thermo-mechanical finite element simulation was performed. All three stages of the thermal cycling process were considered: (1) heating of a newly fabricated plate to 500 °C, (2) holding at a constant temperature of 500 °C for 60 min, and finally (3) cooling the plate to room temperature. Fabrication induced residual stress fields were implemented as the initial state for the thermal cycle model. It was shown that the fuel foil remains in the elastic regime during the entire process, while the cladding material exhibits additional plasticity. In particular, simulations have revealed the existence of a critical temperature at which the net stress fields on the fuel foils change directions. This stress reversal occurs between 400 and 450 °C which matches the experimental blister temperature of irradiated plates. It was shown that the fuel foil would be in fully tensile state above this transition temperature, facilitating the initiation of blisters. Long transverse edges and the regions around the corners of the fuel foil were identified as possible blister locations. The results have implied that a higher post-fabrication compressive stress field of the foil yields higher threshold temperatures; however, each thermal cycle would progressively relieve compressive stresses of the foil. Comparison with experiments has shown agreement, thus substantiated the capability of the model.