Numerical investigations of the deposition of unattached 218Po and 212Pb from natural convection enclosure flow

Abstract

We report numerical predictions of the deposition to enclosure surfaces of unattached 218Po and 212Pb, short-lived decay products of 222Rn and 220Rn, respectively. The simulations are conducted for square and rectangular two-dimensional enclosures under laminar natural convection flow with Grashof numbers in the range 7 × 10 7 to 8 × 10 10. The predictions are based upon a finite-difference natural-convection fluid-mechanics model that has been extended to simulate the behavior of indoor radon decay products. In the absence of airborne particles, the deposition velocity averaged over the enclosure surface was found to be in the range (2–4) × 10 −4 m s −1 for 218Po and (1–3) × 10 −4 m s −1 for 212Pb. In each simulation, the deposition rate varied by more than an order of magnitude around the surface of the enclosure with the largest rates occurring near corners. Attachment of decay products to airborne particles increased the deposition velocity; for example, attachment of 218Po at a rate of 50 h −1 increased the predicted average deposition velocity by 30–70% over values in the absence of attachment. The simulation results have significance for assessing the health risk associated with indoor exposure to 222Rn and 220Rn decay products and for investigating the more general problem of the interaction of air pollutants with indoor surfaces.