Analytical Models for Migration of Radionuclides in Geologic Sorbing Media

In safety evaluation of nuclear waste repository systems, the analysis on migration of radionuclides in geologic media is important. In previous analytical models called Band- Release, Step-Release and Impulse-Release for decay chains, nuclide-specific release rate and time-dependence of release rate can not be taken into consideration. In this paper, two new release models are proposed. In Preferential-Release model, both of nuclide-specific release rate and time-dependence of leach are taken into account. Exponential-Release model is a special case of Preferential-Release model, in which no preferentiality in leach is assumed. The obtained analytical solutions for these new models are applicable even to four or more member decay chains and are as simple as those for the previous models.

As an example of the application of the new models, the discharge rates of the four nuclides in the decay chain of ²³⁸Pu→²³⁴U→ ²³⁰Th→²³⁶Ra→ into a surface water body are calculated as a function of time after the start of leach and path length from the repository. By using the result, heavy hazard due to ²³⁸Pu at short distance and ²²⁶Ra at long distance are also discussed briefly.