Nuclear Engineering Section


Exhalation Container

The exhalation rate of building materials with relatively high radium concentrations was assessed using a specially designed steel clad 1m³ air-tight container ([10, 1994], [11,1994]); the volume of the sample material, which could also be a structural module such as a test wall or slab, should not exceed 10% of the total volume of the container. The following important environmental parameters, which affect an exhalation experiment, are monitored in the container through analogue-to-digital converters interfaced to the bus of a PC : pressure changes, using a ±50 mbar differential pressure transducer, temperature and relative humidity. An aerosol generating system is also used to produce particles.

Three methods are employed for determining the radon concentration in the container environment. Two of these methods are based on grab sampling of a small portion (15-20%) of the container gas through filters. The filters are then analysed using:

A third method, was applied in order to conduct in-situ radon progeny measurements inside the container, using an Am-241 doped 2x2" NaI detector, which records the area under the 609 keV photopeak of Bi-214.

For the calibration of the container a Pylon 2000A - NIST cross-certified source of a nominal Ra-226 activity of 102.8 kBq was used. According to the systematic results already obtained in a clean nitrogen environment, within the range of radon concentrations 0.1-30 kBqm¯³, temperature 19-21 °C and relative humidity 30-40%, the collection efficiency, when sampling 160 L of gas through 37 mm glass microfiber submicron filters for 5 minutes, was determined as 10±1.5%. Furthermore, preliminary results show that the collection efficiency is increasing with the aerosol concentration inside the container, up to a maximum of 4 times.

Exhalation Rate Results

A least squares fitting to the radon growth curve of the examined materials was used to calculate their radon exhalation rate; for this purpose each one of the specimens was enclosed in the container for a period of about 20 days. The estimated effective decay constants did not significantly differ from the decay constant of Rn-222; the total uncertainty associated with these calculations was about 25%. According to the results obtained the cement specimens with Ra-226 concentrations in the range 100-140 Bq/kg ([9, 1994], [11, 1994]) present exhalation rates between 15-20 µBqkg¯¹s¯¹. Brick specimens pulverised to less than 90 µm with Ra-226 concentrations in the range 28-48 Bq/kg present exhalation rates between 3-10 µBqkg¯¹s¯¹.

Furthermore, two typical greek structural modules, a clay brick wall and a concrete slab, of an exhaling area of 1m², constructed from raw materials with the highest Ra-226 concentrations detected, were also tested. The exhalation rate of the wall, with calculated value of Ra-226 content equal to 40 Bq/kg, was 2mBqm¯²s¯¹ while that of the slab with Ra-226 content equal to 26 Bq/kg, was 3 mBqm¯²s¯¹. Assuming a typical Greek room (4x4x3.5m) constructed using materials with the above exhalation rates, and an air-exchange rate of 0.5/h the maximum radon concentration is assessed equal to 34 Bq/m³, which is much lower than the 150 Bq/m³ recommended action level.

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