NUMERICAL-SIMULATION OF RADON TRANSPORT FROM SUBSURFACE TO BUILDINGS

The finite element code FRACTure was conceived for the simulation of f orced fluid flow in fractured rock. For the treatment of radon transpo rt through the subsurface only minor changes were necessary in this co de (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 numer ical and analytic calculations for a simple geometry show excellent ag reement. Successive simulations demonstrate the significance of indivi dual transport mechanisms. All models assume constant underpressure in the building and the validity of Darcy's law for mass transport in th e underground as well as Fick's law for molecular dispersion. The resu lts show that the radon transport by advection and by diffusion strong ly depends on the gas permeability of the underground. The source regi on 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 inc rease in radon entry into the building. For these advective dominated regimes even small pressure changes produce large changes in the indoo r radon content.