Radon Dose Determination for Cave Guides in Czech Republic

Abstract

According to recommended approach there are six (from total of twelve) open‐to‐public caves in Czech Republic, reaching near to an effective lung‐dose of 6mSv/year. A conservative approach for estimating the potential effective lung‐dose in caves (or underground) is based on two season's measurements, using solid state alpha track detector (Kodak in plastic diffusion chamber). The obtained dataset is converted into an annual effective dose, in agreement with the ICRP65 recommendation, using the “cave factor” 1.5. The value of “cave factor” which depends on the spectrum of aerosol particles, or on the proportional representation of the unattached/attached ratio (6.5 : 93.5 for residential places, 13.6 : 86.4 for caves due to lower concentration of free aerosols) and on the equilibrium factor. Thus conversion factor is 1.5 times higher in comparison with ICRP 65. Is this correct? Because a more precisely determined dose value would have a significant impact on radon remedies, or on restricting the time workers stay underground, a series of measurement was initiated in 2003 with the aim to specify input data, computation and errors in effective dose assessment in each one of the evaluated caves separately. The enhancement of personal dosimetry for underground work places includes a study of the given questions, from the following points of view in each cave: continual radon measurement; regular measurements of radon and its daughters to estimate the equilibrium factor and the presence of free 218Po; regular indoor air flow measurements to study the location of the radon supply and its transfer among individual areas of the cave; natural radioactive element content evaluation in subsoil and in water inside/outside, a study of the radon sources in the cave; determination of the free fraction from continual unattached and attached fraction measurement (grid and filter); thoron measurement. Air flow measurements provide very interesting information about the origin of “radon pockets” with very high radon concentration, and enable study of the location of the radon supply and its transfer among individual areas of the cave. Most of the results show the equilibrium factor around F = 0.2−0.7 and the unattached fraction around 2%–30%. One of the most important question remains: how accurately was the unattached fraction measured? Part of this project was to verify the influence of etched track detector position in the cave.