Methods for measuring diffusion coefficients of radon in building materials

Two methods for determining the 222Rn diffusion coefficient in building materials are presented.Experimentally, the measurement of radon release rates under well-controlled conditions, using aflush and adsorption technique, underlies both methods. However, the theoretical principle of thetwo methods differs. The first method uses samples with a cubical or rectangular geometry andhas, as a prerequisite that a 100% radon tight surface covering method is available. The cover issuccessively applied on each side of the sample and after each application the radon release ratesof the partly covered sample is measured. The diffusion coefficient is than determined from theratios of the radon release rates in partly covered situations and the rate corresponding to theuntreated specimen. To assess the practical application of such a method the sealing capacity forvarious coverings have been investigated but none showed to be very effective in reducing radonexhalation. This indicates that radon-tight sealing of surfaces is far from trivial and no clearconclusion was reached if the method is practicable. The second method can be applied tospecimens that are first reshaped into hollow cylinders. Inside this hollow cylinder, a radon sourceis enclosed by a (radon tight) sealing of the two circular surfaces with an aluminum disk and arubber disk. This type of sealing was tested to be almost radon tight on an aluminum dummy ofsimilar dimensions. The radon diffusion coefficient is deduced from a radon release measurementon the building material specimen with enclosed radon source and using a steady-state solution tothe radon transport equation. It is concluded that although the second method requires someadditional information about the porosity and moisture content of the sample it is still practicable.