Is Diffusion or Forced Flow the Main Mechanism of Radon Infiltration into Underground Openings? A Theoretical Study

Abstract

Abstract The diffusion length in porous materials is usually determined from core samples and not for an underground opening as a whole. Underground structures usually have widely differing porosity and crack systems through which equally widely differing diffusive and convective flows deliver gases into underground openings. This paper discusses whether diffusion, rather than forced flow, can be the major cause of radon infiltration into underground openings. To calculate the diffusion through cracks across a slab, an approach developed by Landman was used in which a single rectangular crack of porosity (cr gives a diffusion flow. The two-dimensional calculation is repeated for a case where there are two cracks per unit length, three cracks, four cracks, etc. while keeping the crack porosity (cr constant. The corresponding forced flow for identical cracks is also calculated for a typical pressure difference across the slab. When the diffusion and forced flow are plotted as a function of the number of cracks per unit length, a crossover occurs where the diffusion flow starts to dominate. In another case where the forced flow is kept constant, an even more dramatic increase in the diffusion flow is observed as the number of cracks per unit length increases.