Assessment of the environmental impact of a tritium gas release

A fraction of the tritium gas which is released to atmosphere can be rapidly oxidized at the surface of soil, and then resuspended to atmosphere as HTO. The resuspension rate has been determined hereafter by mathematical modeling and experimental simulation. In the model, the soil-plant system is assumed to consist of N soil layers and one plant compartment. The tritiated water content of the soil and the plants is described by a system of simultaneous, first-order, linear, differential equations with discontinuous, variable coefficients. The coefficients are derived from the phenomenological equations of transfer processes at plant and soil surfaces and through the soil profile. The parameters needed as entry to the model are water vapor concentrations at soil-atmosphere interface, soil moistures and water fluxes at different depths in the soil. The resuspension rate, defined as the fraction of the deposited HTO which is resuspended per unit time, is computed from the evolution of HTO content in the totality of soil profile. In parallel to the development of the model, an experimental simulation was done to determine the resuspension rate and its evolution during a period of 10 days. This was carried out by covering a small area of undisturbed soil by a field chamber, exposing the enclosed soil to tritium gas, then flushing the chamber at a high flow rate and collecting periodically the resuspended tritiated water. Both model and experimentation show that the evolution of the resuspension rate can be represented approximatively by a single exponential function for plant-covered soil, and by a sum of exponentials for a bare soil. The model and experimental values of the initial resuspension rates were similar and comprised between 1 and 5%. This study is an introductory work to a large-scale release experiment which had been carried out very recently, and will permit to make other estimations of deposition and resuspension rates. 15 refs, 6 figs, 1 tab