Congruent Release of Long-Lived Radionuclides from Multiple Canister Arrays

Abstract

Results are presented of an analytical study of mass release of a long-lived radionuclide from multiple waste canisters placed in a water-saturated repository in a two-dimensional array configuration. The radionuclide is assumed to be released congruently with the dissolution of the waste matrix. The concentration and release rate of the radionuclide from the downstream side of the repository region are numerically calculated to observe the effects of canister multiplicity and the leach time of the waste form. Peak values of the concentration and the release rate have been analytically formulated.

For numerical illustration, the case of a Japanese repository concept is considered, where canisters containing vitrified wastes are placed in a water-saturated granitic rock. For the illustration, the nuclide ¹³⁵Cs is chosen, which is characterized by a long half-life and high mobility in the assumed geologic media.

The peak exit concentration becomes independent of the number of waste canisters in the flow direction if the number is sufficiently great. This peak value is a theoretical upper bound of the exit concentration, regardless of the number of canisters or the waste matrix leach time. The model is suitable for assisting in the design of a repository since the effects of the canister array configuration are reflected by the peak exit concentration and the peak release rate.