Numerical reproduction of the seasonal variation in dissolved uranium in Lake Biwa.

The seasonal variation in dissolved uranium (DU) concentrations in the epilimnion of Lake Biwa, the largest lake in Japan, was successfully reproduced by geochemical calculations. The DU in Lake Biwa's epilimnion was calculated using an ion-exchange-equilibrium model. The model used the water-DU simple mixing based on monthly observations of water sources (river water, rainwater, evaporation, and groundwater) and thermocline depths from previous studies (Mochizuki et al. (2016)), the adsorbed uranium (AU) on the soil surface based on work by Saito et al. (2021), and the chemical equilibrium of major chemical species such as H, C, and Ca. The total uranium (TU = AU + DU), which determines the peak DU level in summer, and cation-exchange capacity of the soil surface (CECZp), which determines the DU decrease in winter, were optimized by reproducing the DU measurements. The DU value calculated by PHREEQC with the ion-exchange-equilibrium model can be expressed as the sum of three dissolved chemical species (DUeq), which constitute on average 97% of the DU calculated by PHREEQC. DUeq values were decomposed and the contribution of each factor was ranked by comparing their seasonal variabilities. The complexation equilibrium had the greatest variability and was proportional to the reciprocal of the cube of hydrogen ions. This indicates that the seasonal variation in DU is caused mainly by variation in pH. The peak in the DU observations lagged behind the peak in the pH observations by approximately 1 month. However, DUeq did not reproduce this peak delay. Therefore, we assumed that when DU fails to reach the ion-exchange equilibrium concentration and the peak delay occurs, it follows a first-order reaction, and the rate coefficient is determined by the reproducibility of the DU measurements. The DUlim, as the adsorption/desorption rate-limited DU, improved the reproducibility of the DU and the delay after the pH peak. This implies that the delay of the DU peak after the pH peak represents the lag from equilibrium to a first-order rate reaction caused by the soil uranium adsorption/desorption rate.

ANALYSIS OF 210PB DEPOSITION DISTRIBUTION CHARACTERISTICS IN WINTER AT ROKKASHO IN AOMORI BASED ON HIGH RESOLUTION ATMOSPHERIC TRANSPORT/DEPOSITION MODEL CALCULATION.

To evaluate 210Pb deposited on the surface of sea around Japan in detail, the performance of atmospheric transport/deposition model for 222Rn and its progenies was examined. To test the model's reproducibility of 210Pb deposition in winter, it was applied at Rokkasho in Aomori, where the model had been reported to significantly underestimate when the horizontal grid interval of 9 km was used. It was shown that the precipitation on the Pacific Ocean side and Mutsu Bay and hence the 210Pb deposition at Rokkasho were significantly improved with the grid interval of 3 km although the deposition was still underestimated by the observation. This underestimation was considered to be caused by model's neglect of horizontal drift of snow and rain. It was also pointed out that the deposited 210Pb was mainly contributed by the lower atmosphere up to 2-3 km.

Analysis of volatile nuclides' behavior in the atmosphere released due to the FDNPP accident.

The volatile nuclides, 131I, 132I, and 132Te, which were released in large quantities during the early stage of the Fukushima Dai-ichi Nuclear Power Plant accident, are the most dominant ones in determining the impact on the public and the environment. Since the behavior of these nuclides in the environment and their effects on human health differ depending on their physicochemical forms, it is necessary to understand more about the release fraction and the deposition process of each form. In this study, a behavior model was developed to calculate physical and chemical processes of organic, inorganic, and particulate 131I and 132I, and gaseous and particulate 132Te. This behavior model was used to estimate the physicochemical composition of iodine at the time of release, the 132Te/132I activity ratio at the time of release, and the dry deposition velocity of 132Te. The estimation was performed for five plumes transported during the period from March 14 to 21, 2011, and the estimated values were obtained by comparing with the values measured in previous studies. As a result of the estimation, the organic iodine fraction at the time of release was estimated to be 0.1 or less for most plumes, while a significantly higher value of 0.3 was estimated for the plume released in the night of March 14. This result implied that the possibility of formation and release of organic iodine in the liquid phase in Unit 3, which was considered to be the source of this plume. As for the 132Te/132I ratio at the time of release, it was estimated to be about 0.3-0.4 for most plumes, while it was about 1 in the plume released in the early morning of March 15, which is consistent with the release process in Unit 2 on the night of 14 and early morning of the 15. In addition, a remarkably high dry deposition velocity of 132Te was estimated in this plume, which was consistent with the existence of radiocaesium-bearing microparticles (CsMP) with a particle size of a few micrometers that had been found to be specific to this plume.

LONG-RANGE ATMOSPHERIC TRANSPORT OF RADON IN EAST ASIA AND DEPOSITION OF ITS PROGENIES IN JAPAN.

A three-dimensional long-range atmospheric transport model for 222Rn and its progenies was applied to simulate 210Pb deposition in Japan and its adjacent waters. Monthly precipitation and 210Pb deposition simulated by the model for winter months were favorably compared with observational data although they were exceptionally underestimated in Aomori. The large monthly deposition along the Japan Sea coast lines during winter was successfully simulated by the model to be nearly 200 Bq m-2, which was contrasted by about 10-fold smaller deposition in other regions. It was also pointed out that the heavy deposition areas formed a narrow band structure along the coastline with width of several tens of kilometers and 210Pb deposition amount positively correlated with the height of mountains located leeward of the heavy deposition areas.

Numerical reproduction of dissolved U concentrations in a PO4-treated column study of Hanford 300 area sediment using a simple ion exchange and immobile domain model.

We succeeded at numerical reproduction of dissolved U concentrations from column experiments with PO4-treated Hanford 300 Area sediment using a simple ion exchange and immobile domain model. The time-series curves of dissolved U concentrations under various Darcy flow rate conditions were reproduced by the numerical model in the present study through optimization of the following parameters: the mass of U in mobile domain (on surface soil connected to the stream) to fit the starting U concentration at the column exit, and the rest of the total U was left as precipitation in immobile domain (isolated in deep soil); the mixing ratio between immobile and mobile domains, to fit the final recovering curve of concentration; and the cation exchange capacity (CECZp) and equilibrium constant (kZp) of the exchange reaction of UO22+ and H+ on simulated soil surface (Zp), to fit the transient equilibrium concentration, forming the bed of the bathtub curve. Numerical setting of no U in immobile domain or no mixing between immobile and mobile domains caused all U flushed out of the column exit, and setting of no CEC on Zp, formed no transient equilibrium concentration. The ion exchange immobile domain model is so common that it has become a standard process in the general-purpose geochemical program Phreeqc. Optimization of this model led to the development of the model presented here, which was capable of explaining the fluctuations in dissolved U concentration well and reproducing column experiments under various conditions.

Measurement and modeling of oxidation rate of hydrogen isotopic gases by soil.

Measurements of oxidation rate of hydrogen isotopic gases by soil were made to model HT oxidation rate by soil. Soil was sampled at a cultivated farmland and laboratory measurements of the oxidation rate of H(2) and D(2) gases simulating HT gas were carried out under controlled conditions of soil. The oxidation rate increased with increase of H(2) or D(2) concentration in air and nearly saturated at high concentration. The oxidation rate was low under extremely dry and wet soil conditions and was the highest at soil water content of 8-14 w/w%. The oxidation rate increased exponentially with increasing soil temperature and was the highest at 46 degrees C. Michaelis constant K(m) increased exponentially with increasing soil temperature. Oxidation rate of H(2) was generally higher than that of D(2), while K(m) of H(2) was generally lower than that of D(2). From these results, oxidation rate of HT was modeled as a product of the functions that represent dependency on each soil factor.

Evaluation of aerosol sizing characteristic of an impactor using imaging plate technique.

The activity-size distribution of radon decay products are normally determined using two approaches: direct and indirect. The present study utilises the direct approach to evaluate sizing information of a low pressure cascade impactor using imaging plate (IP) technique for radon decay products. The experiment verified the use of the collection media as suggested by the manufacturer of impactor and proposed a few improvements toward sizing characteristics of impactor. The obtained relative activity-size distribution of radon decay products presents a sharp unimodal log-normal distribution of the particle characterised by activity median aerodynamic diameter (AMAD) of 268 nm and geometric standard deviation (sigma(g)) of 1.66. The obtained data with all the suggested improvements were evaluated by the data obtained from a scanning mobility particle sizer (SMPS, Model 3934, TSI Inc), as reference data. The verification lead to a derivative area ratio of 0.803 between the reference and experimental data.

The study of activity median aerodynamic diameter using imaging plate technique for assessment of effective dose from radon and its decay products.

The indoor workplace environment was evaluated for exposure to radon and its decay products at two Universities in two metropolises of Japan. The mean radon concentrations in Nagoya University (NU) and Hokkaido University (HU) were 16.7 Bq m(-3) and 18.0 Bq m(-3), respectively. Activity median aerodynamic diameter (AMAD) of particles at NU ranged from 172 nm approximately 205 nm and at HU from 186 nm approximately 300 nm. Estimated effective doses for five usual sites of NU by ICRP approach was 0.15 mSvy(-1) approximately 0.76 mSvy(-1) and by UNSCEAR approach was 0.05 mSvy(-1) approximately 0.24 mSvy(-1). The effective dose for the same of HU by ICRP 66 approach was 0.16 mSvy(-1) approximately 0.79 mSvy(-1) and by UNSCEAR 2000 approach was 0.05 mSv y(-1) to 0.26 mSv y(-1). The two approaches differs on an average by a factor 2.3- approximately 4.7. The AMAD of obtained particle size distribution at NU has an average total deposition of 23% in human respiratory tract and 20% at HU.

Depth profiles of radiocarbon and carbon isotopic compositions of organic matter and CO2 in a forest soil.

Depth profiles of the specific activities of (14)C and carbon isotopic compositions (Delta(14)C, delta(13)C) in soil organic matter and soil CO(2) in a Japanese larch forest were determined. For investigating the transport of CO(2) in soil, specific activities of (14)C, Delta(14)C and delta(13)C in the organic layer, and atmospheric CO(2) in the same forest area were also determined. The specific activity of (14)C and Delta(14)C in the soil organic matter decreased with the increase in depth of 0-60cm, while that of soil CO(2) did not vary greatly at a soil depth of 13-73cm and was more prevalent than that of atmospheric CO(2). Peaks of specific activities of (14)C appeared at the depth of 0-4cm and Delta(14)C values were positive in the depth range from 0 to 15cm. These results suggest that the present soil at a depth of 0-4cm had been produced from the mid-1950s up until 1963, and the bomb C had reached the depth of 15cm in the objective soil area. The delta(13)C in the soil organic matter increased at the depth of 0-55cm, while that of soil CO(2) collected on 8 November 2004 decreased rapidly at the depth of 0-13cm and only slightly at the depth of 53-73cm. By combining the Delta(14)C and delta(13)C of the respective components and using the Keeling plot approach it was made clear that the entering of atmospheric CO(2) showed a large contribution to soil CO(2) at the depth of 0-13cm and a negligible contribution at the depth of 53-73cm for soil air collected on 8 November 2004. Respiration of live roots was presumed to be the main source of soil CO(2) at the depth of 53-73cm on 8 November 2004.

214Bi/214Pb radioactivity ratio in rainwater for residence time estimation of cloud droplets and raindrops.

Continuous observation of radioactivity ratio of (214)Bi to (214)Pb as tracers in rainwater was carried out with 15-min cyclic measurement at Nagoya, Japan, from August 2011 to July 2012 to characterise wet deposition. The ratio ranged from 0.55 to 3.82. Based on the observation and a simplified time evolution model of (214)Pb and (214)Bi radioactivities during rainwater formation, the 'age' of raindrops ranged from sub-10 to several 100 min. The age would have negative correlation to rainfall intensity, but its variation for lower rainfall intensities was quite large. Secular equilibrium of (214)Pb and (214)Bi radioactivities in cloud water with airborne radon did not hold for some rainfall events.

Quantification of the dependency of radon emanation power on soil temperature.

A study on the quantification of the dependency of the radon emanation power on soil temperature has been made using the accumulation method. The emanation from dry soil was studied at temperatures between -20 degrees C and 45 degrees C. A formula to calculate the radon emanation power as a function of the temperatures has been developed. The formula would contribute to the modeling of radon transport in soils and building materials.

Indoor/outdoor radon decay products associated aerosol particle-size distributions and their relation to total number concentrations.

The activity size distributions of indoor and outdoor radioactive aerosol associated with short-lived radon decay products were observed at Nagoya, Japan, for some periods from 2010 to 2012, following the indoor observation by Mostafa et al. [Mostafa, A. M. A., Tamaki, K., Moriizumi, J., Yamazawa, H. and Iida, T. The weather dependence of particle size distribution of indoor radioactive aerosol associated with radon decay products. Radiat. Prot. Dosim. 146: (1-3), 19-22 (2011)]. The tendency of smaller indoor activity median aerodynamic diameter (AMAD) after rainfalls showed in the previous study was not consistently obtained, while the consistent tendency of less indoor radioactive particles with diameters in the accumulation mode was observed again after rainfalls. The indoor aerosols showed activity size distributions similar to the outdoor ones. Non-radioactive aerosol particle concentrations measured with a laser particle counter suggested a somewhat liner relationship with AMAD.

On the characteristics of the wet deposition process using radon as a tracer gas.

A quantitatively characterised atmospheric process can provide important information on a variety of atmospheric models and help one to understand the deposition process in the atmosphere. In this study, the total activity concentrations of 214Pb and 214Bi and the activity ratios of 214Bi/214Pb in collected rain samples were measured by two-inch well-type NaI(Tl) detectors. Observed precipitations were classified by the origins of their air mass (continental/maritime) and their meteorological conditions (cold/warm front, high/low pressure). The result shows that (1) when the air mass passed over the continent of Asia 5 d prior to the precipitation, the activity concentration of radon decay products in the rainwater tended to be higher than when the air mass passed over from the Pacific Ocean; (2) higher activity concentrations were more frequently observed in precipitations accompanied with a cold front than warm/stationary front; (3) in general, the activity ratios of 214Bi/214Pb decrease with increase in rain intensity.

Present and future studies on environmental radioactivity in Japan.

In this paper, recent studies on environmental radioactivity are briefly reviewed. Considering the growing dependency on nuclear energy in Asia, emphasis has been given to (14)C cycle at the ground surface including rice paddy fields and to long-range atmospheric transport of natural and anthropogenic radioactive materials. Studies on the dynamic behaviour of (14)C are being carried out and some important experimental results are becoming available for models of (14)C migration in rice paddy. Long-range atmospheric transport models have been improved substantially by using sophisticated physical models and simulation techniques to improve emergency preparedness. Although our knowledge on radon flux distribution in Asia has been improved and radon has become a more reliable atmospheric tracer, we still need more intensive measurements of exhalation flux and atmospheric concentration of radon and radium content in soil.

Inverse modeling of Asian (222)Rn flux using surface air (222)Rn concentration.

When used with an atmospheric transport model, the (222)Rn flux distribution estimated in our previous study using soil transport theory caused underestimation of atmospheric (222)Rn concentrations as compared with measurements in East Asia. In this study, we applied a Bayesian synthesis inverse method to produce revised estimates of the annual (222)Rn flux density in Asia by using atmospheric (222)Rn concentrations measured at seven sites in East Asia. The Bayesian synthesis inverse method requires a prior estimate of the flux distribution and its uncertainties. The atmospheric transport model MM5/HIRAT and our previous estimate of the (222)Rn flux distribution as the prior value were used to generate new flux estimates for the eastern half of the Eurasian continent dividing into 10 regions. The (222)Rn flux densities estimated using the Bayesian inversion technique were generally higher than the prior flux densities. The area-weighted average (222)Rn flux density for Asia was estimated to be 33.0 mBq m(-2) s(-1), which is substantially higher than the prior value (16.7 mBq m(-2) s(-1)). The estimated (222)Rn flux densities decrease with increasing latitude as follows: Southeast Asia (36.7 mBq m(-2) s(-1)); East Asia (28.6 mBq m(-2) s(-1)) including China, Korean Peninsula and Japan; and Siberia (14.1 mBq m(-2) s(-1)). Increase of the newly estimated fluxes in Southeast Asia, China, Japan, and the southern part of Eastern Siberia from the prior ones contributed most significantly to improved agreement of the model-calculated concentrations with the atmospheric measurements. The sensitivity analysis of prior flux errors and effects of locally exhaled (222)Rn showed that the estimated fluxes in Northern and Central China, Korea, Japan, and the southern part of Eastern Siberia were robust, but that in Central Asia had a large uncertainty.