Uranium-molybdenum (U-Mo) alloy, which is applied in the heat-pipe cooled reactor (referred as heat-pipe reactor), has the advantages of high thermal conductivity, high density of uranium and excellent irradiation performance. At the same time, U-Mo alloy has significantly thermal expansion and irradiation swelling whilst the high temperature will aggravate the irradiation swelling of U-Mo alloy and reduce the performance of the material. Hence research on swelling under high temperature of U-Mo alloy is essential in the design of this fuel.

The study aims to comprehensively evaluate the effect of fuel swelling under high temperature of U-Mo alloy on reactor core structure.

Firstly, based on the irradiation data of U-Mo alloy under high temperature, a new type of swelling model considering the effect of high temperature was established. Secondly, a three-dimensional (3D) thermal-mechanical coupling analysis model of U-Mo alloy fuel was set up using finite element analysis (FEA) software COMSOL Multiphysics (referred to as COMSOL). Thirdly, a thermal-mechanical coupling analysis was carried out with concern of the thermal expansion effect to verify the validity of the 3D FEA model. Finally, this swelling model was used to study the fuel swelling effect of reactor core by considering the irradiation swelling of U-Mo alloy at high temperature, and the stress and deformation analysis under different burnup were carried out to evaluate the effect of fuel swelling on the core structural stability.

Under steady-state operating conditions, the core fuel of 1 kWe Kilopower heat-pipe reactor has a large deformation at the end of life (EOL) due to thermal expansion and irradiation swelling, and the maximum deformation reaching 5.28 mm. The maximum stress caused by deformation is 57.4 MPa, which is concentrated on the wall where the heat pipe is connected to the core fuel. Thermal expansion is the main factor that causes stress and deformation of fuel. As the burnup continues to deepen, the irradiation swelling of U-Mo alloy at high temperature leads to greater deformation and greater stress of the fuel. The maximum deformation of the fuel is 6.63 mm when the burnup is 0.4%, which increases by 1.69 mm compared with the calculation results considering only thermal expansion. The maximum core fuel stress reaches 85.1 MPa, which is close to the yield limit of U-Mo alloy. And the stability of fuel structure may be threatened.

The results of this study indicate that the swelling effect of U-Mo alloy at high temperature leads to more severe deformation and greater stress on the fuel. The influence of thermal expansion and irradiation swelling on the structural stability of the core at high temperature and high burnup needs to be considered in reactor fuel design. In addition, it is necessary to accelerate the irradiation test of U-Mo alloy at high temperature to optimize the irradiation behavior model.