Bubble formation and lattice parameter changes resulting from He irradiation of defect-fluorite Gd2Zr2O7

Pyrochlores have long been considered as potential candidates for advanced ceramic waste-forms for the immobilization of radioactive waste nuclides. This work provides evidence that Gd2Zr2O7, often considered the most radiation tolerant pyrochlore, could be susceptible to radiation damage in the form of bubble nucleation at the highest He doses expected over geological time. Ion irradiations were utilized to experimentally simulate the radiation damage and He accumulation produced by alpha-decay. Samples were pre-damaged using 7 MeV Au3+ to induce the pyrochlore to defect-fluorite phase transformation, which would occur due to alpha-recoil damage within several hundred years of storage in a Gd2Zr2O7 waste form. These samples were then implanted to various He concentrations in order to study the long-term effects of He accumulation. Helium bubbles 1-3 nm in diameter were observed in TEM at a concentration of 4.6 at.% He. Some bubbles remained isolated, while others formed chains 10-30 nm in length parallel to the surface. GIXRD measurements showed lattice swelling after irradiating pristine Gd2Zr2O7 with 7 MeV Au3+ to a fluence of 2.2 x 10(15) Au/cm(2). An increase in lattice swelling was also measured after 2.2 x 10(15) Au/cm(2) + 2 x 10(15) He/cm(2) and 2.2 x 10(15) Au/cm(2) + 2 x 10(16) He/cm(2). A decrease in lattice swelling was measured after irradiation with 2.2 x 1015 Au/cm2 + 2 x 1017 He/cm2, the fluence where bubbles and bubble chains were observed in TEM. Bubble chains are thought to form in order to reduce lattice strain normal to the surface, which is produced by the Au and He irradiation damage. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.