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METHODS FOR MEASURING DIFFUSION COEFFICIENTS OF RADON IN BUILDING MATERIALS

I.Cozmuta, E.R.van der Graaf
Nuclear Geophysics Division, Kernfysisch Versneller Instituut, University of Groningen,
Zernikelaan 25, 9747 AA Groningen, The Netherlands

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

Key words: Radon, diffusion coefficient, building materials, concrete, release rate, cubical
geometry, cylindrical geometry, covering, controlled conditions, steady-state.