The analysis of the insertion of UO2 microspheres in the gap between pellet and clad as a new design solution to increase the burnup of the nuclear fuel Original Research Article

The insertion of UO2 microspheres (eventually graphite coated) in the gap between pellet-clad is observed to decrease substantially the clad hoop plastic strain concomitantly with the elimination of the rim effect at high burnup if low enrichment is used for the microspheres. Taking into account the special features of the specialized finite element code ELFINʹ90 for the behavior of fuel elements, it was possible to introduce this new type of material viewed as a granular media. The results of the new code version ELFINʹMS applications to a PHWR fuel for a power ramp irradiation history show that the hoop plastic strain is reduced by about three times in comparison to standard fuel, and that the ridge phenomenon disappears. To establish critical plastic strain limit for irradiated clad failure onset, quantitative evaluations of iodine chemisorbtion on graphite and at the surface of the irradiated zircaloy, are presented. The indications on technology procedure are also discussed. Therefore, the insertion of 2–3 layers of UO2 microspheres of 100 μm diameter, graphite coated to retain corrosive fission products for clad and with the diameter greater than the design gap, can be considered a design solution to increase the burnup of nuclear fuel.