Tetsuo Yasutaka

Distribution of dissolved and particulate radiocesium concentrations along rivers and the relations between radiocesium concentration and deposition after the nuclear power plant accident in Fukushima

This study involved measurement of concentrations of dissolved and particulate radiocesium ((134)Cs and (137)Cs) in river water, and determination of the quantitative relations between the amount of deposited (137)Cs and (137)Cs concentrations in river waters after the Fukushima Daiichi nuclear power plant accident. First, the current concentrations of dissolved and particulate (134)Cs·(137)Cs were determined in a river watershed from 20 sampling locations in four contaminated rivers (Abukuma, Kuchibuto, Shakado, and Ota). Distribution characteristics of different (137)Cs forms varied with rivers. Moreover, a higher dissolved (137)Cs concentration was observed at the sampling location where the (137)Cs deposition occurred much more heavily. In contrast, particulate (137)Cs concentration along the river was quite irregular, because fluctuations in suspended solids concentrations occur easily from disturbance and heavy precipitation. A similar tendency with dissolved (137)Cs distribution was observed for the (137)Cs concentration per unit weight of suspended solids. Regression analysis between deposited (137)Cs and dissolved/particulate (137)Cs concentrations was performed for the four rivers. The results showed a strong correlation between deposited (137)Cs and dissolved (137)Cs, and a relatively weak correlation between deposited (137)Cs and particulate (137)Cs concentration for each river. However, if the particulate (137)Cs concentration was converted to (137)Cs concentration per unit weight of suspended solid, the values showed a strong correlation with deposited (137)Cs.

Rapid measurement of radiocesium in water using a Prussian blue impregnated nonwoven fabric

We developed a rapid method for concentrating and measuring radiocesium (134Cs and 137Cs) dissolved in fresh water using nonwoven fabric impregnated with Prussian blue (PB) as a radiocesium absorber in combination with gamma-ray spectrometry using a germanium (Ge) detector. Utilizing this method, dissolved radiocesium in a 20–100 L freshwater sample could be concentrated within a period of 20–60 min by passing the sample through 10–12 columns, connected in series, that had been fitted with nonwoven fabric disks impregnated with PB. Laboratory tests using water samples containing known amounts of radiocesium confirmed that the overall recovery rate of the isotope was 100%–108%, and that the first six columns recovered 84%–97% of the isotope. The detection limit of this method was determined to be 0.002 Bq/L with a sample of 100 L and measurement time of 43,200 s. In comparison with traditional methods using ion-exchange resin, co-precipitation with ammonium phosphomolybdate, etc., our method has the advantages of reduced cost and a significantly shorter concentration time. Since water samples can be treated in short periods of time, it is now possible to conduct radiocesium pre-concentration in situ, thus eliminating the need to transport large-volume water samples to laboratories.

A GIS-based evaluation of the effect of decontamination on effective doses due to long-term external exposures in Fukushima.

Despite the enormous cost of radiation decontamination, there has been almost no quantitative discussion on how much it would reduce the long-term external radiation exposure in the Evacuation Zone and Planned Evacuation Zone (restricted zone) in Fukushima. The aim of this study is to assess the effectiveness of decontamination and return options and to identify important parameters for estimating the long-term cumulated effective dose (CED) during 15, 30 and 70 year period using data on land-use, population and decontamination in the restricted zone (about 1100 km(2)) in Fukushima. Decontamination of the land is assumed to have a certain efficacy in terms of the reduction of CED. The EeCC (external exposure conversion coefficient) is the parameter having the greatest effect on the percentage of area having CED during the 30 years above 100 m Sv after decontamination, ranging from 13% (EeCC=0.2) to 55% (EeCC=0.6). Therefore, we recommend a detailed investigation of the EeCC in Japan.

Cost and Effectiveness of Decontamination Strategies in Radiation Contaminated Areas in Fukushima in Regard to External Radiation Dose

The objective of the present study is to evaluate the cost and effectiveness of decontamination strategies in the special decontamination areas in Fukushima in regard to external radiation dose. A geographical information system (GIS) was used to relate the predicted external dose in the affected areas to the number of potential inhabitants and the land use in the areas. A comprehensive review of the costs of various decontamination methods was conducted as part of the analysis. The results indicate that aerial decontamination in the special decontamination areas in Fukushima would be effective for reducing the air dose rate to the target level in a short period of time in some but not all of the areas. In a standard scenario, analysis of cost and effectiveness suggests that decontamination costs for agricultural areas account for approximately 80% of the total decontamination cost, of which approximately 60% is associated with storage. In addition, the costs of decontamination per person per unit area are estimated to vary greatly. Appropriate selection of decontamination methods may significantly decrease decontamination costs, allowing more meaningful decontamination in terms of the limited budget. Our analysis can help in examining the prioritization of decontamination areas from the viewpoints of cost and effectiveness in reducing the external dose. Decontamination strategies should be determined according to air dose rates and future land-use plans.

Rapid quantification of radiocesium dissolved in water by using nonwoven fabric cartridge filters impregnated with potassium zinc ferrocyanide

This paper presents a cartridge filter incorporating a nonwoven fabric impregnated with potassium zinc ferrocyanide (Zn-C) to effectively concentrate and quantify cesium dissolved in water. Experiments conducted with 137Cs in conditioned water showed that at a flow rate of 2.5 L/min the filter could absorb 97.9% of dissolved 137Cs from 20 L of water; high recovery efficiency was achieved over a pH range of 3–10. Test measurements of 137Cs concentrations using Zn-C in river water agreed with the results derived by using an evaporative concentration method (within the counting error of the detector). Using this method, the pre-concentration time of radiocesium in 20 L of fresh water can be reduced to just 8 minutes.

Relationship between Individual External Doses, Ambient Dose Rates and Individuals' Activity-Patterns in Affected Areas in Fukushima following the Fukushima Daiichi Nuclear Power Plant Accident.

The accident at Fukushima Daiichi Nuclear Power Plant on March 11, 2011, released radioactive material into the atmosphere and contaminated the land in Fukushima and several neighboring prefectures. Five years after the nuclear disaster, the radiation levels have greatly decreased due to physical decay, weathering, and decontamination operations in Fukushima. The populations of 12 communities were forced to evacuate after the accident; as of March 2016, the evacuation order has been lifted in only a limited area, and permanent habitation is still prohibited in most of the areas. In order for the government to lift the evacuation order and for individuals to return to their original residential areas, it is important to assess current and future realistic individual external doses. Here, we used personal dosimeters along with the Global Positioning System and Geographic Information System to understand realistic individual external doses and to relate individual external doses, ambient doses, and activity-patterns of individuals in the affected areas in Fukushima. The results showed that the additional individual external doses were well correlated to the additional ambient doses based on the airborne monitoring survey. The results of linear regression analysis suggested that the additional individual external doses were on average about one-fifth that of the additional ambient doses. The reduction factors, which are defined as the ratios of the additional individual external doses to the additional ambient doses, were calculated to be on average 0.14 and 0.32 for time spent at home and outdoors, respectively. Analysis of the contribution of various activity patterns to the total individual external dose demonstrated good agreement with the average fraction of time spent daily in each activity, but the contribution due to being outdoors varied widely. These results are a valuable contribution to understanding realistic individual external doses and the corresponding airborne monitoring-based ambient doses and time-activity patterns of individuals. Moreover, the results provide important information for predicting future cumulative doses after the return of residents to evacuation order areas in Fukushima.

Effect of the concentration of radiocesium dissolved in irrigation water on the concentration of radiocesium in brown rice

Abstract In 2011, the radiocesium concentration of brown rice grown (Oryza sativa L.) in mountainous areas with irrigation water that flowed from a nearby mountain forest, in Fukushima Prefecture, Japan, exceeded 500 Bq kg–1. Migration of radiocesium from contaminated irrigation water was suspected. Here, we investigated the migration of dissolved radiocesium (137Cs) from irrigation water into brown rice by determining the concentration of 137Cs in brown rice grown in pots irrigated with water containing dissolved 137Cs (0.1, 1.0 or 10 Bq L–1 137Cs). The treatment water was prepared by extracting 137Cs from conifer branches collected from the contaminated area and diluting the stock solution with tap water. We used two Gray Lowland soils (soils A and B) collected from two paddy fields in Fukushima Prefecture. Rice plants (Oryza sativa L. cv. Hitomebore) were grown in pots and treatment water was kept at a depth of 3–5 cm throughout the growing season. We calculated the amount of 137Cs absorbed by the plants by subtracting 137Cs concentrations in brown rice grown with tap water from that in brown rice grown with treatment water. The concentration of 137Cs in brown rice increased in proportion to that in the treatment water. The concentration of 137Cs in brown rice grown with water containing 10 Bq L–1 137Cs in soil A was 4.9 times that in brown rice grown in soil B, but the concentration of 137Cs in brown rice grown in either soil was not significantly different when grown with water containing 0.1 Bq L–1 137Cs. Before plants were grown, the exchangeable potassium in soil B was 3.8 times that in soil A. At least, even with irrigation water containing about 0.1 Bq L–1 of radiocesium, the migration of radiocesium into brown rice is negligible for the provisional regulation value for radiocesium in brown rice under the condition of our experiment.

Non-destructive detection of particulate radiocesium using a non-woven fabric cartridge filter for rapid preprocessing

Abstract A nondestructive method for monitoring 137Cs in particulate matter in natural water was developed, to expedite preprocessing time. Suspended solids collected on a nonwoven fabric cartridge filter were directly analyzed for 137Cs without preprocessing; a formula was determined for geometric correction of the radiation source in the cartridge. The ratio of the 137Cs radioactivity in the cartridge determined by the nondestructive method to the actual 137Cs radioactivity determined by a destructive method was estimated to be 1.14 for 44 water samples, and this value was also valid for various experimental conditions and soils with various particle size distributions.

Development of a copper-substituted, Prussian blue-impregnated, nonwoven cartridge filter to rapidly measure radiocesium concentration in seawater

ABSTRACT This paper presents a cartridge filter incorporating a nonwoven fabric impregnated with Prussian blue copper analog (Cu-CF) to effectively concentrate and quantify radiocesium dissolved in seawater. The recovery ratio of the Cu-CF was >95% in laboratory experiments at a flow rate of 0.5 L min−1 through the filter and >93% in field experiments. Test measurements of 137Cs concentrations in seawater using Cu-CF agreed with the results obtained by using a conventional coprecipitation method that employed ammonium phosphomolybdate (within the counting error of the detector). The proposed method can shorten the preconcentration and pretreatment times for radiocesium, to just 40 min in 20 L of seawater, which is much faster than the ∼1 week required by traditional ammonium phosphomolybdate methods.

Column percolation test for contaminated soils: Key factors for standardization

Column percolation tests may be suitable for prediction of chemical leaching from soil and soil materials. However, compared with batch leaching tests, they are time-consuming. It is therefore important to investigate ways to shorten the tests without affecting the quality of results. In this study, we evaluate the feasibility of decreasing testing time by increasing flow rate and decreasing equilibration time compared to the conditions specified in ISO/TS 21268-3, with equilibration periods of 48h and flow rate of 12mL/h. We tested three equilibration periods (0, 12-16, and 48h) and two flow rates (12 and 36mL/h) on four different soils and compared the inorganic constituent releases. For soils A and D, we observed similar values for all conditions except for the 0h-36mL/h case. For soil B, we observed no appreciable differences between the tested conditions, while for soil C there were no consistent trends probably due to the difference in ongoing oxidation reactions between soil samples. These results suggest that column percolation tests can be shortened from 20 to 30days to 7-9days by decreasing the equilibration time to 12-16h and increasing the flow rate to 36mL/h for inorganic substances.