The effect of waste age on the design of a geologic repository

Spent fuel from civilian power reactors has been accumulating since 1969, and will be as old as 29 years out-of-reactor in 1998, when the first US repository for commercial radioactive waste is scheduled to begin operation. In an oldest-waste-first scenario, the first heat-producing waste committed to a repository will be 29 years old at the time of emplacement. Age at emplacement will decrease as the inventory of old waste is depleted, and will vary throughout the operating life of the repository. The age of the waste influences the deposition of decay energy in the waste packages themselves and in the geologic medium in which the waste packages are emplaced for disposal. In the short term, there is a local temperature pulse that is the principal consideration in the design of waste canisters-which, of course, are tailored to the individual waste forms to be emplaced. In the longer term, and on a more regional scale, the thermomechanical response of the host rock to areal energy deposition determines the design of the disposal arrays within the repository. This report is concerned with the latter problem. It is shown that the effect of waste age variability on the thermomechanical stability of the host geology can be minimized by the proper choice of waste canister spacings