Near-field transport of radioactive chains

Much attention has been given to predicting the near-field mass transfer of a single radioactive species from a waste solid into surrounding porous material, but only limited consideration has been given to predicting the coupled mass transfer of species in a radioactive decay chain. Here we present an analysis that assumes that the members of a decay chain dissolve congruently with a solubility-limited matrix. Here ''dissolution'' refers to the net process of dissolving in ground water. A solid species that may temporarily dissolve in interacting with water to form a precipitate is not considered to have dissolved. We give the general, non-recursive analytical solutions for the transport of a radioactive decay chain in a finite porous medium when nuclides are released congruently with the matrix. To illustrate we consider the decay chain 234U → 230Th → 226Ra from spent fuel. These solutions are useful and potentially important in performance assessment of nuclear waste repositories. 6 refs., 4 figs., 1 tab