XAS study of Am2Zr2O7 pyrochlore - Evolution under alpha self-irradiation

Management of long-lived nuclear wastes is, after safety, the main issue of nuclear industry, both in terms of scientific challenge as well as public acceptance. Among the different options that have been envisioned and explored for minor actinides over the past thirty years, two alternatives currently remain: long term disposal in a safe repository or “burning” of nuclear wastes in a so-called transmutation process. Materials selected for such applications have to meet the following criteria: high incorporation of actinides, good structural and chemical stability, low thermal dilatation, and resistance to radiation.
Among the various ceramics envisaged, zirconia based pyrochlore oxides of composition An2Zr2O7 (An=actinide) appear to be ideal candidates. Results obtained with lanthanide pyrochlores submitted to ion-beam irradiation show that Zr-based pyrochlores, unlike Ti-based, remain crystalline with a transition from the pyrochlore to a defect fluorite structure [1]. The ordered pyrochlore belongs to the Fd-3m space group and is a superstructure of the ideal defect fluorite structure (Fm-3m) with exactly twice the lattice constant. In the pyrochlore structure, the coordination shell of Am cations (8 (2+6) oxygen atoms) is distinct from that of Zr cations (6 oxygen atoms), whereas the second coordination shell (cation-cation) is the same for both cations (6xAm+6xZr at the same distance). The structural phase transition from pyrochlore to defective fluorite involves the randomization of the oxygen atoms into the anionic and vacant positions of the pyrochlore structure. In the ideal fluorine defect structure, the Am and Zr cations share the same position and thus have the same local environment: a first coordination shell consisting of 7 oxygen atoms at the same distance and the same second coordination shell as observed in the pyrochlore structure.
The objective of this study was to investigate the radiation tolerance of Zr-pyrochlore during a more realistic experiment using alpha emitting 241Am as structural component.