Migration of gaseous radionuclides through soil overlying a uranium ore deposit: a modeling study

This study presents several one-dimensional mathematical models derived to simulate the distribution of radionuclides in soil overlying uranium ore deposits. The migration of gaseous products away from the ore is simulated by mechanisms of molecular diffusion and advective transport. A homogeneous overburden containing no uranium or radium is assumed throughout. The study is intended to be a first step in analyzing the feasibility of detecting uranium ore by remote geochemical measurements. A steady-state diffusion model predicts the detectability of radon-222 and some of its daughter products several tens of meters from a uranium deposit. In reality this range would be shortened by the masking effect of radon generated in the overlying soil itself. Krypton-85 is found to be only marginally detectable even in the immediate vicinity of the ore. Xenon-133 would be detectable at half the range of 222Rn. The effect of soil gas motion as an additional transport mechanism is evaluated using both steady-state and transient models. A constant soil gas velocity of 1 x 10-4 cm/sec causes gaseous radioisotope activities to change by several orders of magnitude. Detectability of uranium could be significantly increased if long-term soil gas motion is confirmed. A transient numerical model is used to simulate barometric influences on the migration of 222Rn away from an ore deposit. It was found that near-surface effects are +-15% which would not appreciably affect the detectability of uranium by geochemical means