As the energetic beta and alpha particles traverse through a crystalline medium, atoms are displaced from their stable lattice positions, forming Frenkel pairs (ion pairs, vacancy and interstitials). For example, the autora­diolysis in metallic 239Pu (T1/2 = 2.41 x 104 y) causes significant structural changes. When the 239Pu atom decays, an a-particle with an kinetic energy of 5 MeV and the recoil nucleus of 235U atom with an energy of 86 keV are created. In metallic Pu, the a-particle deposits its kinetic energy in approxi­mately 10 pm, while for the heavier recoil atom it is only 12 nm. For each disintegration event, approximately 2600 ion pairs are formed, and over the course of 20 years, every atom in a piece of Pu metal changes its position. This property becomes critical when trying to predict the long-term behav­ior of Pu materials (Keogh, 2005). However, while metallic structures can undergo a significant degree of self-annealing (heating to a melting point and cooling over a long period of time), molecular complexes can suffer irreversible structural damage. A typical example would be autoradiolysis causing amorphization of crystalline compounds of relatively short-lived americium 241Am (T1/2 = 432.7 y). As confirmed by the single crystal X-ray diffraction analysis, the intensity of the diffraction decreases with time. Visible damage of crystal lattice can be observed by opacity of originally clear crystal and increased solubility. Similar structural disordering in mate­rials was observed after irradiation with high-energy electron beam. Electrons, being much smaller than alphas or heavy ions, penetrate deeper through the bulk of the irradiated crystal, and cause amorphization of the crystalline lattice. Uranophane and other uranium solid matrices (Douglas et al., 2002; Utsunomia et al., 2003), important for the development of radia­tion resistant waste forms have been studied using accelerated beams. Thermochemical investigation of stability of microporous and mesoporous materials such as frameworks in silica zeolites or selected metal-phosphates (Petrovic et al., 1993; Hu et al., 1995) is another example of material studies
for high-level nuclear waste immobilization (Navrotsky et al., 2009; Weber et al., 2009).

A material very resistant to radiation damage is glass, because it is a noncrystalline “solid” liquid. Though at an atomic level the same phenom­ena occur in glass, it is not appropriate to speak of dislocations in glass: the random structure of the glass allows it to accommodate foreign species throughout the sample (Sales, Boatner, 1984; Oelkers, Montel, 2008). Glasses have been intensely studied as the matrix for high active waste consisting of fission products and actinides (Ewing, Wang, 2002).