No abstract

The effectiveness of bentonite-mixed soil as a barrier to radon exhalation from soil is investigated experimentally at a closed uranium mine in Japan. Continuous observations of radon flux from a clay-covered soil surface and a bare soil surface were conducted for a period of 80 days. The mean radon flux from the clay-covered plot was one fifth of that from the bare plot, demonstrating that the clay-mixed soil is effective as a boundary suppressing radon exhalation from the contaminated soil. Numerical analyses using a one-dimensional radon transport model for an unsaturated soil for the dry weather conditions confirmed the effectiveness of the clay-mixed soil cover under a range of weather conditions.

Because of radioactive fallout resulting from the Fukushima Daiichi Nuclear Power Plant (NPP) accident, water discharge from many outdoor swimming pools in Fukushima was suspended out of concern that radiocesium in the pool water would flow into farmlands. The Japan Atomic Energy Agency has reviewed the existing flocculation method for decontaminating pool water and established a practical decontamination method by demonstrating the process at eight pools in Fukushima. In this method, zeolite powder and a flocculant are used for capturing radiocesium present in pool water. The supernatant is discharged if the radiocesium concentration is less than the targeted level. The radioactive residue is collected and stored in a temporary storage space. Radioactivity concentration in water is measured with a NaI(Tl) or Ge detector installed near the pool. The demonstration results showed that the pool water in which the radiocesium concentration was more than a few hundred Bq L was readily purified by the method, and the radiocesium concentration was reduced to less than 100 Bq L. The ambient dose rates around the temporary storage space were slightly elevated; however, the total increase was up to 30% of the background dose rates when the residue was shielded with sandbags.

Approximately two months after the Fukushima nuclear accident, the Japan Atomic Energy Agency (JAEA) led off a series of demonstration tests to develop effective but easily applicable decontamination methods for various school facilities in Fukushima. This effort included (1) dose reduction measures in schoolyards, (2) purification of swimming pool water, and (3) removal of surface contamination from playground equipment. Through these demonstration tests, they established practical methods suitable for each situation: (1) In schoolyards, dose rates were drastically reduced by removing topsoil, which was then placed in 1-m-deep trenches at a corner of the schoolyard. (2) For the purification of pool water, the flocculation coagulation treatment was found to be effective for collecting radiocesium dissolved in the water. (3) Demonstration tests for playground equipment, such as horizontal bars and a sandbox wood frame indicated that the decontamination effectiveness considerably varied depending on the material, paint or coating condition of each equipment piece. These findings were summarized in reports, some of which were compiled in local/national guidelines or handbooks for decontaminating the living environment in Fukushima.

The transfer of 226Ra from soil to plant has been studied for wild trees and grasses to identify the site-and the plant-s~ecific variability in relation to the transfer of Ca. Twelve species of plants and soils were sampled from three different forests and one flood plain in Okayama, Japan. The concentrations of 226Ra in plant varied about 60 times by species and sites, while those in soil were not different significantly among sites. The transfer factors defmed as~e concentration ratio of 226Ra between plant and soil were linearly correlated with that of Ca, implying that~he abI1~ty of Ca uptake b~plant r~f1ect the ability of 226Ra uptake. The Ca concentrations in plant showed spe~le~-.speclfic value and vaned 22 tImes. The range of transfer factor (plant/soil) for Ca increased by the vanablhty of the Ca concentration in soil among sites. These results suggest that the wide variability of 226Ra concentration in plant can be ascribed to both the differences in plant and soil factors of Ca concentration. K1!YWORDS