Nuclear waste form glasses: The evaluation of very long-term behaviour

ne of the unique and scientifically most difficult aspects of the design an d evaluation of nuclear waste form materials is the extrapolation of short- term laboratory data (hours to years) to the long time periods (10(3) to 10 (5) years) required by the performance objectives set in regulations. Prese nt performance assessments routinely extrapolate the performance of a mater ial beyond 10,000 years. In some of the performance assessments (such as at the proposed high-level waste repository at Yucca Mountain in the United S tates), long-term materials performance is an essential element of establis hing compliance with regulatory release or exposure limits. Thus, it is ess ential to establish the extent to which such long-term extrapolations are u seful or valid. Although there are many materials issues in a repository (e .g., corrosion of actinide-bearing crystalline ceramics, spent nuclear fuel , and the metallic waste package materials), in this paper I present approa ches that have been developed to describe the long-term behaviour of nuclea r waste borosilicate glasses.