Theoretical considerations regarding the migration of 222Rn and 220Rn from uranium- and thorium-bearing underground environments.

Theoretical calculations are presented for partially enclosed uranium- and thorium-bearing subterranean environments, such as tunnels and underground uranium mines. The variables of practical interest considered here are the 222Rn and 220Rn concentrations in the wall, and the flux densities of the same radioactive gases in the wall and at the wall/air interface of these underground sites. Calculations have been conducted based on a plane, semi-infinite geometry model (commonly used to predict radiation levels in mines) and a cylindrical (i.e., tunnel) geometry model. The 220Rn flux density, J(220Rn), calculated according to the plane and cylindrical geometries agree with each other within 5% for wall media of porosity equal to or greater than about 2%, even for tunnels of small radii. However, for 222Rn the cylindrical geometry gives values for the 222Rn flux density, J(222Rn), substantially higher (by a factor of 1.4 to approximately 3) than those predicted by the plane semi-infinite geometry. A practical difficulty arises in the experimental verification of the models in underground environments. The results are relevant for predicting radioactivity levels (222Rn, 220Rn, and their progeny) in underground environments such as uranium mines. Health Phys.