Behavior of (U,Pu)O₂ Sphere-Pac Fuel, with Plutonium in the Large Spheres Only, in Low Burnup Tests Under Pressurized Water Reactor Conditions

Two irradiation tests were conducted in the pressurized water reactor loop of the High Flux Reactor at Petten to compare the low burnup (0.4% fima), high power (35 to 60 kW·m⁻¹) performance of MO₂ sphere-pac fuel with the plutonium incorporated only in the large (1000-µm) diameter spheres and that of homogeneously enriched UO₂ sphere-pac and pellet fuel The fuel columns in the Zircaloy-4 clad pins were fabricated with a smear density of 87 to 88% of the theoretical UO₂ density. Postirradiation examinations showed that fission gas release, axial migration of cesium, and relocation of small (10- to 40-µm) and medium (∼100-µm) diameter spheres were considerably larger in the MO₂ sphere-pac pins than in the UO₂ sphere-pac pins. Contrary to observations in the pellet pins, no hard mechanical fuel-cladding interaction has been observed in the sphere-pac pins. Although these tests showed the technical feasibility of the inhomogeneous MO₂ sphere-pac fuel pin, its further development seems appropriate only in connection with the use of recycle light water reactor fuel.