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Report No.
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Study on the measurement method of diffusion coefficient for radon in the soil (VII)

Iida, Takao*

To evaluate radon exhalation from the ground surface of a soil-covered landfill for waste rock from a uranium mine, the followings were carried out: (1) continuous measurement of radon concentration in soil air, radon exhalation from ground surface and soil moisture content, (2) experimental evaluation of the effect of bentonite-mixed cover soil to radon exhalation, and (3) numerical simulations on these items. According to the measurement results, the radon exhalation rate varied in the rage from 0.004 to 0.351 Bq m-2s-1 with an average value of 0.010 m-2s-1. These values are considerably smaller than those obtained in the measurements in previous years. This can be attributed to wetting of the ground surface due to precipitation much greater than that in the previous years. The radon exhalation from the cover soil surface, on the other hand, was measured to be as small as 0.0015 to 0.0030 Bq m-2s-1 for the variation range and 0.0020 Bq m-2s-1for the average. The radon concentration in soil air was 200 to 400 kBq m-3 and varied in accordance with the exhalation. Therefore, it can be concluded that the soil cover effectively and sufficiently reduce the radon exhalation. The numerical simulations showed results pertinent to the observational results except for some short-term variations. By summarizing the results obtained in the series of studies (I to VII), it was concluded that main parts of radon transfer in the surface soil were understood and the reduction of radon exhalation by cover soil was effective. The remaining issues are development of soil radon monitor which is applicable to nearly saturated soils, measurement of physical properties of cover soils, and study on maintenance of soil cover soundness.

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