Numerical simulation of radon transport from subsurface to buildings

Abstract

The finite element code FRACTure was conceived for the stimulation of forced fluid flow in fractured rock. For the treatment of radon transport through the subsurface only minor changes were necessary in this code (extension by a radioactive decay term). The calculations performed so far simulate steady state pressure and radon concentration fields in the ground surrounding a cylindrical building. Comparisons of numerical6and analytic calculations for a simple geometry show excellent agreement. Successive simulations demonstrate the significance of individual transport mechanisms. All models assume constant underpressure in the building and the validity of Darcy's law for mass transport in the underground as well as Fick's law for molecular dispersion. The results show that the radon transport by advection and by diffusion strongly depends on the gas permeability of the underground. The source region of indoor radon extends over a limited volume of a few meters only. In soils with low permeability the diffusive flux is dominating even at high pressure differences between the building interior and the sub-surface. In these cases the radiation risk due to radon entry is small. On the other hand a high soil gas permeability leads to a strong increase in radon entry into the building. For these advective dominated regimes even small pressure changes produce large changes in the indoor radon content. © 1994.