Trefoil bundles of NPD 7-element size fuel irradiated to 9100 MWd/tonne U

NPD prototype elements (1 in. OD, 19 in. long) were assembled into trefoil bundles and irradiated in the X-5 pressurized-water loop of NRX. The first tests were for only a few weeks but showed that elements made by sheathing UO2 pellets in Zircaloy-2 behaved well under irradiation; later similar elements were irradiated for 18000 hours to a burn-up of 9100 MWd/tonne U at ∫kdθ = 40 W/cm. The dimensional stability of all the elements was good. Only those subjected to long irradiation showed progressive diametral increases, and these were attributed to relocation of the UO2 during interim inspections. Length measurements demonstrated that pellet end-dishing is effective in controlling axial expansion, but that for a given depth of dishing the amount of expansion depends on the shoulder width. The extent of grain growth in the UO2 was compatible with previously reported results when the duration of irradiation, density of the fuel, and variations in growth characteristics of the different batches of UO2 are considered. The elements taken to high irradiation released up to 135 ml of fission-product gases, which is 2% of the amount formed. The transverse tensile strength of ring samples from the Zircaloy-2 sheaths increased from 75000 to 95000 lb/in2 at room temperature, but the ductility dropped. The completely brittle fracture of some rings was due to ZrH2 precipitation. The failure of one element was caused by increased stress due to a higher heat rating, combined with low ductility of the Zircaloy-2 resulting from radiation damage and with precipitation of ZrH2 because of a lower coolant temperature. The fission-product release from the split was not excessive, and the element was easily withdrawn from the loop after operating at full power for four days from the time of the failure. (author)