Dissolution and Storage Experiment with 4.75-wt%-²³⁵U-Enriched UO₂ Rods

The Nuclear Safety Department of the French Commissariat à l’Energie Atomique has achieved an important experimental program relating to the criticality of 4.75-wt%-²³⁵U-enriched UO₂ rods in water with various configurations. Two steps of this program are set out. The first one consisted of doing the parametric study of regular lattices of rods in a sodium nitrate solution (study connected with the problems raised by the dissolution of irradiated elements). The results obtained show the poisoning effect due to the nitrate ion at various concentrations. The second step concerns the study of the effect due to the interpositioning of hydrogenous materials between four 18 × 18 assemblies at 13.5-mm square pitch (study connected with the problems raised by the accidental sprinkling of a mist into a fuel storage). The results obtained allow one to determine the optimum theoretical moderation conditions for the storage of new fuel elements. In all cases the theoretical results are in good agreement with experimental results.