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 nuclear wastes “burning” in a so-called transmutation process. Materials selected for such applications have to meet the following criteria: high incorporation amount 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 a very good candidate. Results obtained with lanthanide pyrochlores submitted to ion-beam irradiation show that Zr based pyrochlores, unlike Ti based, remain crystalline with a transition from pyrochlore structure to a defect fluorite structure [1]. However, this strong radiation tolerance still has to be confirmed on alpha emitter materials (e.g. americium). In that prospect, 241Am2Zr2O7 sample was synthesized [2] and its evolution under the effect of alpha self-irradiation as a function of time was followed by XRD. Figure 1 shows the transition from the pyrochlore phase to a defect-fluorite phase after ~200 days as the superstructure peaks (marked ) slowly vanish and completely disappear after about 200 days [3].