Ventilation, air confinement and high radon levels in an underground gallery studied from profiles measurements

An horizontal closed-end tunnel, 128 m long and 2 m in diameter, located within the eastern margin of the Belledonne crystalline basement, French Alps, near the west shore of the Roselend artificial lake, 600 m NE of the dam, has been instrumented since 1995 for radon emanation and deformation measurements. Radon bursts are repeatedly associated with transient deformation events induced by variations in lake levels. This high radon anomalies in the air of the tunnel result from its particular geometry, its excellent confinement, the water and radium 226 contents of rocks, and the crossing of several faults. We calculated the equilibrium factor F, directly proportional to air ventilation, from the ratios of radon-222 gas activity measured with an AlphaGUARDTM, and the Potential Alpha Energy Concentration of its short-lived daughters measured with a TracerlabTM, simultaneously in five locations along the tunnel. The calculated equilibrium factors of 0.60 to 0.78 show that confinement is very good all along the tunnel. Fast Fourier Transform of the radon 222 signals measured during six months simultaneously with six BarasolTM distributed along the tunnel shows also the poor ventilation and the weak influence of atmospheric pressure and air temperature. This work has been done with the collaboration of the CEA and the BRGM. (author)