Radiation effects and the leach rates of vitrified radioactive waste

Abstract

There is much concern that long-lived radioactive elements incorporated in glasses for the disposal of highly active nuclear waste might eventually return to the environment. The most important mechanism begins with leaching of the glass by groundwater and a leach rate, usually based on laboratory tests of simulated vitrified radioactive waste, has been used as the basis of analyses of the radiological safety of a waste repository^1–3. The possibility that radiation damage to the glass from the products of nuclear decay could change the leach rate has been studied both by the incorporation of active elements in the glass and by irradiation from external sources. The most important contribution to radiation damage comes from the recoil nuclei during α decay⁴. Typically the recoil nucleus has a kinetic energy of ∼100 keV and displaces >1,000 atoms in the glass. Experiments which simulate this damage by incorporating short-lived α-emitting isotopes such as ²³⁸Pu into the glass have not shown significant increases in leach rate at doses equivalent to >10¹⁸ α decays g⁻¹ (refs 5–9). Figure 1 shows that for wastes considered in the UK this corresponds to a time after vitrification of at least 10³–10⁴ yr. Recent work with ion beams inducing the radiation damage^10–12 has suggested that large increases (up to a factor of just over 50) in leach rate will occur after a critical dose of radiation from the α-decay processes. In addition, experiments in which the leaching solution is irradiated with γ rays^13–15 have shown that radiolysis effects in the water can increase the leach rates of glasses. We present here new data and calculations which relate these studies to the conditions expected in a real waste repository. Our analysis is described in more detail elsewhere^16,17.