Modelling the effect of air exchange on 222Rn and its progeny concentration in a tunnel atmosphere

The effect of air exchange on the concentration of 222Rn and its progeny in the atmosphere of the Roselend tunnel, in the French Alps, is estimated using a box modelling scheme. In this scheme, the atmosphere is divided into a small number of well mixed zones, separated by flow restricted interfaces, characterized by their exchange rate. A four-box model, representing the three sections of the tunnel present until 2001 and an adjacent inner room, accounts for the spatial variations of the background 222Rn concentration, and for the time structure of transient bursts observed regularly in this tunnel since 1995. A delay of the order of one day, observed during some transient bursts in the inner room with respect to the end of the tunnel, is accounted for if the bursts are assumed to be mainly generated in the end section of the tunnel, and stored temporarily in the inner room via air exchange. The measured radon concentration is reproduced by this model for an air exchange rate of 1.6x10(-6) s-1 between the room and the tunnel, in a context of a global ventilation rate of 10(-5) s-1 in the tunnel. Gradual onset and decay phases, varying from burst to burst, are also suggested. The equilibrium factor of 222Rn with its progeny, measured in 2002 with values varying from 0.60+/-0.05 to 0.78+/-0.06, is interpreted with a five-box model representing the five sections of the tunnel present after 2001. This model indicates that the equilibrium factor does not provide additional constraints on the air exchange rates, but the value of the deposition rate of the unattached short-lived radon progeny can be inferred, with results varying from 0.2 to 6 h-1 in the various sections. This study illustrates the benefits of a simple modelling tool to evaluate the effect of natural ventilation on 222Rn and its progeny concentration in underground cavities, which is important for radioprotection and for a reliable characterization of signatures of hydrogeological or geodynamical processes. Conversely, this study shows that 222Rn and progeny measurements provide a non-invasive method for characterizing natural ventilation conditions in delicate underground cavities, such as painted caves.