Lattice location of fission products in UO2 single crystals

Abstract

Heavy fission product ions (Te, Xe, Cs) were implanted in UO2 single crystals at 50 keV to high fluences between 5 × 1015 and 1 × 1017 ions/cm2. The implantations were performed at 77 K (Te, Xe) and 293 K (all three elements). The crystals were subsequently analyzed in situ using the Rutherford backscattering (RBS) channeling technique. All implants exhibited a large (apparent) substitutional fraction SF = 50–70% when implanted at 293 K. In contrast, SF = 0 for the implantations at 77 K. The annealing behaviour was also studied. For the crystals implanted at 293 K the release of impurity atoms (Te, Cs) sets in at temperatures above 750 K and is clearly correlated with radiation damage recovery. Xe-release starts above ~1300 K. The release of Te implanted at 77 K begins already at 400 K and is nearly completed at 600 K. In this temperature range, no changes in the damage distribution were noticed. These results give the first indication of coherent precipitation of solid rare gases and volatiles in UO2.