Behavior of irradiated ATR/MOX fuel under reactivity initiated accident conditions

Pulse irradiation experiments with irradiated ATR/MOX fuel rods of 20 MWd/k gHM were conducted at the NSRR in Japan Atomic Energy Research Institute to study the transient behavior of MOX fuel rod under reactivity initiated ac cident conditions. Four pulse irradiation experiments were performed with p eak fuel enthalpy ranging from 335 J/g to 586 J/g, resulted in no failure o f fuel rods. Relatively large radial deformation of the fuel rods due to pe llet-cladding mechanical interaction occurred in the experiments with peak fuel enthalpy above 500 J/g. Significant fission gas release up to 20% was measured by rod puncture measurement. The generation of fine radial cracks in pellet periphery micro-cracks and boundary separation over the entire re gion of pellet were observed. These microstructure changes might contribute to the swelling of fuel pellets during the pulse irradiation. This could c ause the large radial deformation of fuel rod and high fission gas release when the pulse irradiation conducted at relatively high peak fuel enthalpy. In addition, fine grain structures around the plutonium spot and cauliflow er structure in cavity of the plutonium spot were observed in the outer reg ion of the fuel pellet.