Monitoring network of atmospheric Radon-222 concentration in East Asia and backward trajectory analysis of Radon-222 concentration trend at a small solitary island on Pacific Ocean

Okura, Takehisa; Yamazawa, Hiromi*; Moriizumi, Jun*; Hirao, Shigekazu*; Guo, Q.*; Tojima, Yasunori*; Iida, Takao*

Proceedings of 3rd Asian and Oceanic Congress on Radiation Protection (AOCRP-3) (CD-ROM), 3 Pages, 2010/05

Monitoring network of Rn concentration in air which was measured in Beijing, Nagoya, Hegura-jima, Hachijo-jima and Hateruma-jima, as a tracer for long-range transport in East Asia was established. At inland sites, Beijing and Nagoya, high concentrations Rn were measured, at marine sites, Hachijo-jima and Hateruma-jima, Rn concentrations level was very low. Seasonal variations of the Rn concentration show that Rn concentration was the lowest in the summer and the highest in the winter. Diurnal variations were measured at inland sites. At marine sites several-day-cycle variations were measured. It was pointed out by this study that the several-day-cycle variations at Hachijo-jima were dependent on synoptic-scale atmospheric disturbance. Backward trajectory analysis of the relationship between atmospheric Rn concentrations at Hachijo-jima and transport pathway of air mass indicates that atmospheric Rn at Hachijo-jima has much to do with transport pathway of air.

Simulation of the atmospheric dispersion of Kr from a reprocessing plant over a coastal area; Performance examination of an emergency environmental radiation dose evaluation code system "SIERRA-II"

Takeyasu, Masanori; Iida, Takao*; Watanabe, Hitoshi; Takeishi, Minoru; Yamamoto, Asao*

Journal of Radioanalytical and Nuclear Chemistry, 275(1), p.43 - 54, 2008/01

https://doi.org/10.1007/s10967-006-7000-3

The performance of the computer code system "SIERRA-II" for calculating the environmental radiation dose due to an accident was assessed using the environmental monitoring data around the coastally located Tokai Reprocessing Plant (TRP) when Kr was discharged during its operation. The agreement within a factor of 5 between the calculated and observed air concentrations of Kr in areas of a few km from the TRP was achieved in 57% of the comparison data, and the agreement within a factor of 2 was obtained in 29%, disregarding the internal boundary layer. According to the vertical profile of atmospheric temperatures, the internal boundary layer was modeled in the SIERRA-II and the agreement became better than when the internal boundary layer was not considered.

Ratio of aerosol and gases of radioactive chlorine and particle size distribution of aerosol formed by high-energy proton irradiation

Yokoyama, Sumi; Sato, Kaoru; Manabe, Kentaro; Noguchi, Hiroshi; Kaneko, Hirohisa; Oki, Yuichi*; Iida, Takao*; Tanaka, Susumu*

Radiation Protection Dosimetry, 127(1-4), p.392 - 397, 2007/11

https://doi.org/10.1093/rpd/ncm475

To estimate internal doses due to the inhalation of radionuclides produced by the nuclear spallation of the air nuclei in high-energy proton accelerator facilities, the physicochemical properties of radionuclides are very important. However, there is not enough information of the properties for airborne radioactive chlorine formed from argon gas in the air of a high-energy proton irradiation field. Thus we have measured the ratio of aerosol and gases of radioactive chlorine which are formed by irradiating argon gas-added air with a 48 MeV proton beam. In addition, the particle size distribution and chemical form of non-radioactive aerosol were examined. It was found that Cl-38 and Cl-39 exist as aerosol, acidic and non-acidic gases. The percentages of Cl-38 and Cl-39 aerosols are more than 70%. In total radioactive chlorine gas, about 30% and 70% are acidic and non-acidic gases, respectively. The concentration of non-radioactive aerosol had a peak at an aerodynamic diameter of 20-30 nm in the early irradiation period. The particle size of non-radioactive aerosol shifted to larger with time. After 10 min, the aerosols grew very slowly and many of the non-radioactive aerosols were distributed widely under 200 nm. This suggests that the diameter of the radioactive chlorine aerosol formed by the attachment of the radioactive chlorine to the non-radioactive aerosol around a beam line of the accelerator is much smaller than the default value adopted in ICRP Publication 66.

Suppression of radon exhalation from soil by covering with clay-mixed soil

Ota, Masakazu*; Iida, Takao*; Yamazawa, Hiromi*; Nagara, Shuichi; Ishimori, Yuu; Sato, Kazuhiko; Tokizawa, Takayuki

Journal of Nuclear Science and Technology, 44(5), p.791 - 800, 2007/05

https://doi.org/10.3327/jnst.44.791

A study on the effectiveness of clay-mixed soil as a barrier against radon exhalation from soil surface was carried out at a closed uranium mine in Japan. Continuous observations of radon flux at a clay-covered soil surface and a bare soil surface, radon concentration in soil air and soil moisture content were carried out for 80 days. The mean of radon flux of the clay-covered plot was one fifth of that of the bare plot. This result indicates that clay-mixed soil worked effectively for reduction of radon exhalation from the soil surface. A numerical analysis with a one-dimensional radon transport model for unsaturated soil was carried out to evaluate the effectiveness of the barrier under dry weather conditions. Calculation results suggested that the barrier works effectively even under the dry condition.

Diurnal and seasonal variations in Rn concentration profile in soil

Iskandar, D.*; Yamazawa, Hiromi*; Yamasoto, Kotaro; Ota, Masakazu*; Koarashi, Jun; Moriizumi, Jun*; Bunawas*; Iida, Takao*

Hoken Butsuri, 42(1), p.98 - 104, 2007/03

https://doi.org/10.5453/jhps.42.98

The Rn concentrations in soil and Rn flux were measured at 6 depths in soil to analyze the seasonal- day-to-day and diurnal variations. The variations of Rn concentrations in soil were larger in shallow depths. It seems that the Rn concentration in soil during night-time was slightly lower than day-time especially at depth of 15 cm. The precipitation may have affected the concentration insoil differently depending on its amounts. The large decrease in air pressure during one day increased the concentration in soil at shallow soil layers. The effect of soil water content on Rn concentration in soil was also observed.

Analysis of Rn-222 concentration in the upper atmosphere during rainfall using the wet scavenging model for its decay products

Takeyasu, Masanori; Iida, Takao*; Takeishi, Minoru

Proceedings of 2nd Asian and Oceanic Congress Radiological Protection (AOCRP-2) (CD-ROM), p.754 - 757, 2006/10

The atmospheric Rn-222 concentration in rain clouds was estimated using the wet scavenging model of Rn-222 decay products with the observed data of the environmental ray dose rate. In this method, the pattern of the increase of the dose rate calculated by the model was adjusted to the observed one, in order to estimate the atmospheric Rn-222 concentration which was one of parameter of the model. The Rn-222 concentration was estimated for the rainfall event on Dec. 27, 2003, when a large increase of the dose rate was observed in Tokai-mura. The concentration was estimated to be 24 Bq m, which was higher than the measured one at a height of a few kilometers around central Japan by aircraft. From a back trajectory analysis, the origin of Rn-222 atoms for the events on Dec. 27 was back-tracked to the northeastern part of China, and it was guessed that the large amount of Rn-222 was transported by the Eurasian continental air mass.

Measurement of radiolytic yield of nitric acid in air

Kanda, Yukio*; Oki, Yuichi*; Yokoyama, Sumi; Sato, Kaoru; Noguchi, Hiroshi; Tanaka, Susumu*; Iida, Takao*

Radiation Physics and Chemistry, 74(5), p.338 - 340, 2005/12

https://doi.org/10.1016/j.radphyschem.2005.03.006

The operation of high-energy accelerators leads to the production of radiolytic noxious gases, such as ozone and nitrogen compounds in the air of the beam-line tunnels. Among nitrogen compounds, nitric acid is a principal radiolytic compound produced in large quantities. In the high-intensity proton accelerator, such as J-PARC, a very strong radiation environment leads to an abundant production of nitric acid. A quantitative assessment of the damage due to corrosion with nitric acid is essential for machine maintenance and information concerning the formation of nitric acid is a requisite for that. In this work, the G-value for nitric acid production was measured by irradiating atmospheric air with 48MeV-protons. The G-value was obtained to be 1.460.12. The G-value for nitric acid production obtained in this experiment was larger than the estimated value, but very close to the value of 1.5 given by irradiating the air with Co-60-rays.

Study on the measurement method of diffusion coefficient for radon in the soil (VII)

Iida, Takao*

JNC TJ6400 2004-002, 49 Pages, 2005/01

JNC-TJ6400-2004-002.pdf:1.78MB

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.

Study on the measurement method of diffusion coefficient for radon in the soil (IV)

Iida, Takao*

JNC TJ6400 2004-001, 66 Pages, 2004/03

JNC-TJ6400-2004-001.pdf:3.16MB

The aim of this study is to clarify the radon behavior in the soil at the waste rock yards of uranium mine. We have developed an apparatus for measuring radon diffusion coefficient in soil under the control of soil temparature. The radon diffusion coefficients changed suddenly around the soil temperature of 0 degrees centigrade. The radon diffusion coefficients in dry soil have little temparature dependency, and were comparable coefficients obtained by empirical formula of Rogers & Nielson. To study the restraint effect of radon exhalation by covering with bentonite on soil, we carried out the measurements and the calculations by using one-dimensional transport model of atmosphere and soil. The decrease of radon exhalation rate was 2.9 Bq m-2 s-1 when soil covered with the bentonite of 5 cm thick. The radon consentrations in snow cover and the radon flux from the snow surface were measured at the waste rock yard in Ningyo-toge Environmental Engineering Center. The simulation of the radon transport in snow also carried out using measured snow parameters. The radon concentrations in snow cover ranged from 500 to 3,000 Bq m-3, and the radon flux from the snow surface was about 9 mBq m-2 s-1. The small radon flux results mainly from the wet condition of soil surface caused by melting of the snow.

Measurements of concentrations and its ratio of radon decay products in rainwater by gamma-ray spectrometry with a low background germanium detector

Takeyasu, Masanori; Iida, Takao*; Tsujimoto, Tadashi*; Yamasaki, Keizo*

International Congress Series, 1276, p.289 - 290, 2004/00

The each concentration of Pb and Bi in rainwater was measured by a low background Ge detector. The concentration ofPb was a inverse relation with rainfall rate in some rainfall events, and the concentration ratio ofBi toPb was a weak inverse relation with rainfall rate in all rainfall events. These relations were the same as these obtained by the removal model of radon decay products from stratiform cloud.