Criteria derived for geologic disposal concepts

As part of the Advanced Fuel Cycle Initiative (AFCI) programme conducted by the Department of Energy in the United States (US), analyses are being performed to quantify the benefits of a geologic repository that may accrue as a result of processing spent nuclear fuel and recycling several of the recovered chemical elements. Previous work identified and quantified the benefits to utilising the available space in a repository like the one proposed for Yucca Mountain in the US, and demonstrated that large gains in utilisation were possible by reducing the decay heat of the emplaced materials. The results identified a group of chemical elements, including certain actinides and fission products, that needed to be removed from the spent fuel and either recycled or placed in interim storage, with only processing waste being sent to the repository. For example, as a result of the lower decay heat, removal of 99.9% of the plutonium, americium and curium, along with removal of 99.9% of the caesium and strontium, would allow the waste from 225 times more spent fuel being placed in the repository, as compared with the direct disposal of spent fuel. The actinides would be recycled, while the fission products would be placed in interim storage for 200-300 years to allow sufficient time for decay before ultimate disposal. The recent focus of the analysis effort has been to examine the impact of greater utilisation of the repository space on the estimated peak dose rate for releases from a repository such as Yucca Mountain. The results show that, while many of the same elements responsible for the decay heat are also contributors to the dose rate, there are other elements that are not significant contributors to the decay heat but that are very important in determining the dose rate. In addition, changing the form of the emplaced materials from directly disposed spent fuel to a high-level waste form alters the relative importance of some of the chemical elements in affecting the estimated peak dose rate. The potential reduction in dose rate by removing individual elements, or groups of elements, is quantified. The results are used to provide additional specifications for separations at the processing plant as well as to potentially impose limits on increased space utilisation in the repository. (authors)