Assessment of water-filled borehole as a preferred pathway penetrating radioactive waste repository

This paper discusses the effect of human intrusion into the repository for radioactive wastes. It has been numerously suggested that an inadvertent direct drilling at the repository could provide a pathway for the release of radionuclides immobilized in waste forms even with its low occurrence probability. The diffusive mass transport in the water-filled borehole is modeled accounting for diffusion out of the waste forms and into the rock. The fully implicit integrated finite difference method is used to obtain the solution and is compared with the numerical inversion of Laplace transform method. One interesting finding is that the diffused radionuclides from the waste forms do not diffuse farther in the borehole due to low concentration gradient in early time and rock matrix diffusion so long as the borehole is not pumped continuously. The results show that the releases of radionuclides through the borehole pathway are insignificant even though the concentrations of radionuclides released into the borehole water exceed the MPC. It is shown that the model developed here could also be applied to the drilling at a non-cylindrically shaped repository and to the off-center drilling of the cylindrical repository unless the drilling occurred near the repository boundary