Fission gas release and swelling in uranium–plutonium mixed nitride fuels

Two uranium–plutonium mixed nitride, (U,Pu)N, fuel pins with different He-gap width were irradiated at a linear heating rate 75 kW/m to 4.3% FIMA in the experimental fast reactor JOYO, and nondestructive and destructive post irradiation examinations were carried out. Fission gas release rates were about 3.3% and 5.2%, and swelling rates were about 1.8% and 1.6%/% FIMA. From the radial distributions of Xe concentration measured by EPMA, it was determined that approximately 80% and 15% of fission gases were retained in the intragranular region and in the fission gas bubbles, respectively. Deformation of the fuel cladding differed between the two tested fuel pins. A uniform diameter increase was observed in the small gap fuel pin, while ovalities, which seemed to be caused by relocation of the fuel fragments, were found in the large gap one.