Repository Benefits of Partitioning and Transmutation
R.A. Wigeland and T.H. Bauer
Nuclear Engineering Division
Argonne National Laboratory, Argonne, Illinois
Abstract
Geologic repositories such as the one proposed for Yucca Mountain are designed to both safely store hazardous nuclear materials for very long periods of time and minimize the release rate of these materials to the environment. To succeed in this mission, these repositories need to satisfy a number of design and operational constraints which usually take the form of temperature limits, restrictions on inventory, etc. In general, the ability of a repository to satisfy the constraints depends on the nature and characteristics of the emplaced materials. This paper reports the results of several studies into the benefits that can be obtained by partitioning spent nuclear fuel, and recycling certain chemical elements in reactors to transmute them into less hazardous materials.
The first part of the discussion focuses on identifying the chemical elements (isotopes) that are responsible for the applicable limits being approached. In particular, in the case of a repository at YuccaMountain, the temperature limits imposed on the engineered systems and on the mountain result in drift loading limits for the direct disposal of spent nuclear fuel. Analyses have shown that removal of plutonium and americium is essential as the first step towards better utilization of the repository drifts, with the general removal of all transuranic (TRU) elements also being considered. With 99.9% removal of plutonium and americium, the repository benefit, as measured by the increase in drift loading, can be a factor of 5-6 over the direct disposal case, using anticipated repository operating conditions of disposing of spent fuel 25 years after reactor discharge, and ventilating the repository for 75 years after waste placement.
The second part of the discussion considers the options for further increasing the drift loading benefit, either by additional processing of the spent fuel to remove cesium and strontium, or by altering the repository operating conditions to make the contributions from these elements less important. The results show that it is possible to increase the drift loading by up to a factor of 100 with 99.9% removal of TRU and cesium and strontium, for the same repository operating conditions(assuming that suitable waste forms are available to take advantage of such potential). It is also shown that substantial drift loading benefits can also be achieved by delaying the placement time for the process waste and by extending the ventilation period, for example, by delaying placement for 75 years and ventilating for 125 years, a drift loading benefit in excess of a factor of 50 can be achieved without removing cesium and strontium from the waste.
The last part of the discussion examines possible recycling scenarios for the recovered plutonium, americium, and other elements. The need for ongoing processing of the spent fuel was identified, along with requiring repeated recycling of the recovered plutonium and americium. It was determined that this is possible with either thermal or fast neutron reactors. Results indicate that substantial drift loading benefits can only be achieved by keeping the contents of TRU in the waste (or any other emplaced materials) as low as possible.