Implications of sorption kinetics to radionuclide migration in fractured rock

Abstract

A new model framework for transport of radionuclides in fractured rock, which includes sorption kinetics and surface diffusion, is developed and verified experimentally. Interpretations of two types of complementary batch tests indicate that sorption kinetics in the rock matrix is governed by migration phenomena along intragranular microfissures. About 40% of the matrix pore volume consists of such pores not active in the main matrix diffusion process along the intergranular fissures. The adsorption rate coefficient obtained for the surface sorption kinetics on small granite particles is 2 or 3 orders of magnitude larger than that obtained for intact rock. The studied rock material was diorite collected in a typical Swedish crystalline bedrock at the Äspö Hard Rock Laboratory. An exact solution is derived in a form that expresses the relative error in the temporal variance of a pulse travelling in a fracture resulting if sorption kinetics is omitted. The relative error of the peak value of the pulse increases with distance and can be several hundred percent in cases typical of performance assessment analyses of a nuclear waste repository.