Simulation of the concentrations and distributions of indoor radon and thoron.

The technique of computational fluid dynamics (CFD) was used to study the concentrations and distributions of indoor radon (222Rn) and thoron (220Rn) as well as their progeny in three dimensions. According to the simulation results, in a naturally ventilated room, the activity distribution of 222Rn is homogeneous except for the places near air diffuser (supply and exhaust) locations. The concentration of 220Rn exponentially decreases with the distance from the source wall which is considered independently. However, as the ventilation rate increased, the concentrations of both 222Rn and 220Rn decreased and their activity distributions become complicated due to the effect of turbulent flow. It suggests that the impact factors of monitoring conditions (sampling site, airflow characteristics, etc.) should be taken into account in obtaining representative concentrations of 222Rn/220Rn for dose assessment. Both the simulation results of activities and their distributions agreed well with the experimental results in a laboratory room. It suggests that the CFD models may be applicable for the estimation of indoor 222Rn and 220Rn as well as their progeny.