Lattice Location of Fission Products in UO sub 2 Single Crystals

Heavy fission product ions (Te, Xe, Cs) were implanted in UO sub 2 single crystals at 50 keV to high fluences between 5 x 10 exp 15 -1 x 10 exp 17 ions/cm exp 2 . The implantations were performed at 77K (Te, Xe) and 293K (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 293K. In contrast, SF = 0 for the implantations at 77K. The annealing behaviour was also studied. For the crystals implanted at 293K, the release of impurity atoms (Te, Cs) sets in at temperatures > 750K and is clearly correlated with radiation damage recovery. Xenon-release starts above approx 1300K. The release of Te implanted at 77K begins already at 400K and is nearly completed at 600K. 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 UO sub 2 .