Ion-beam irradiation and 244Cm-doping investigations of the radiation response of actinide-bearing crystalline waste forms

Candidate materials for actinide immobilization are subject to alpha-decay event doses that accumulate to values of more than 10(20) alpha-decays per gram (tens displacements per atom, dpa) over the extended periods of geologic disposal. To evaluate the radiation-response of actinide-bearing materials, two experimental techniques have been used to accelerate the damage accumulation process: ion-beam irradiations and Cm-244-doping experiments. Based on modern characterization techniques, such as high-resolution transmission electron microscopy, and experimental results that involve ion-beam irradiation and chemical doping with highly active actinides, crystalline ceramics for the immobilization of actinides can be divided into three groups on the basis of their critical doses, D-c, i.e., the dose required for amorphization at 300 K: (i) low resistance to radiation damage accumulation (D-c approximate to 0.2 dpa) - murataite, Ti-perovskite, Fe-garnet; (ii) resistant (0.4 < D-c < 0.6 dpa) - Al-garnet, Ti-Zr-pyrochlore, Al-perovskite; and (iii) highly resistant (D-c > 0.8 dpa) - Zr-, Zr-Ti-, and Sn-pyrochlores. Phases with low critical temperatures (T-c below 600 K) will not become amorphous in a deep geologic repository, as long as the temperature remains between 300 and 550 K, but rather, they will remain crystalline. Only Zr-rich pyrochlore is fully resistant to radiation damage and will remain crystalline over the entire period of its disposal.