Abstract

Phosphates are known to exhibit superior resistance to radiation damage, as evidenced by natural phosphate minerals that are minimally damaged despite containing alpha-emitting actinides for hundreds of millions of years. This study looks to investigate radiation damage in xenotime (YPO4) and zircon (ZrSiO4). Radiation damage occurs by two processes: the recoil of the heavy nucleus (70-100 keV) and the alpha particle itself (4.5-5.5 MeV), which loses energy primarily by ionizations but causes several hundred atomic displacements near the end of its path. Here we present results of Pu doping into xenotime, with insight into solid solubility and response to radiation damage compared to previous results for zircon. We also separate damage due to alpha particles from damage due to the heavy nucleus recoil and compare to Pu-doped studies.

For solid solubility and accelerated ageing studies, 239 and 238Pu-doped xenotime samples were prepared at the Institute for Transuranium Elements, Karlsruhe. At 4 mol% 238Pu, xenotime is single phase, and at 8 mol% 238Pu a secondary PuP2O7 phase is present. Preliminary calculations using lattice parameters indicate Pu solid solubility of 7.4 mol% (9.4 wt%), which is compared to Pu solid solubility in zircon. Radiation damage over time was monitored by X-ray diffraction in the 4 mol% sample. Lattice parameters of damaged xenotime over time are compared to initial xenotime lattice parameters to observe swelling of the crystalline unit cell.

To separate the damage due to the α-particle from heavy recoil damage, xenotime and zircon were synthesized with natural boron (containing 19.9 wt% 10B) and irradiated for 1, 10, and 28 days to achieve the reaction 10B n—> 7Li α. The dose for each irradiation was calculated by 7Li MAS -NMR spin-counting, and amorphous regions quantified by 31P and 29Si MAS -NMR. X-ray diffraction shows maximum unit cell volume swelling of 0.46% at a dose of 6.72×1018 α-events/g for xenotime, and 1.7% at a dose of 20.43×1018 α-events/g for zircon. Preferential swelling along the a- or c-axis is discussed and compared to Pu-doped samples.