Catálogo de publicaciones - libros

The release of radioactive cesium into the environment in the aftermath of disasters such as the Fukushima Daiichi disaster poses a great health risk, particularly since cesium easily spreads in nature. In this context, we perform solid-state nuclear magnetic resonance (NMR) experiments to study Cs ions adsorbed by clay minerals to analyze their local structure. The NMR spectra show two kinds of peaks corresponding to the clays (illite and kaolinite) after immersion in CsCl aqueous solution; the peak at −30 ppm is assigned to Cs on the clay surface while that at −100 ppm is assigned to Cs in the silicate sheet in the clay crystal. This result is consistent with the fact that Cs with smaller coordination number yields a small field shift in the NMR spectra. Moreover, after immersion in KCl aqueous solution, these peaks disappear in the NMR spectra, thereby indicating that our assignment is reasonable. This is because Cs on the clay surface and in the silicate sheet is easily subject to ion exchange by K. We believe that our findings will contribute to a better understanding of the pathway through which Cs transfers from the soil to plants and also to the recovery of the agriculture in Fukushima.

We evaluated the migration of radionuclides (I, I, Cs, Cs, Cs, and Te) in the surface soil after the Fukushima nuclear accident. The radionuclides in the soil collected late March in 2011 were barely leached with ultrapure water, indicating that these are insoluble. We observed the chemical behavior of Cs and I in soil: (1) Cs was predominantly adsorbed within a depth of 2.5 cm from the ground surface; (2) Cs was hardly released from soil by the water leaching experiments that lasted for 270 days; (3) approximately, more than 90 % of Cs was adsorbed on organic matters and the residual fractions, while I was mainly fixed on the Fe-Mn oxide and organically bounded fraction. Therefore, we conclude that Cs and I in soil seldom leach into the soil water and migrate downward because of the irreversible adsorption. The shallow groundwater which residence time is short.

The isotopic ratios of radioactive cesium derived from the Fukushima accident were determined by γ-spectrometry and thermal ionization mass spectrometry (TIMS). In order to ascertain the initial ratios at the time of the accident, environmental samples collected during 2011 were used for the analysis. Soil, litter, and seaweed were incinerated, and the cesium contained therein was adsorbed into ammonium phosphomolybdate powder. The cesium in the seawater was adsorbed into AMP-PAN resin (Eichrom Technologies, LLC); its recovery ratio was almost one without the carrier being added. Incinerated samples and the AMP-PAN resin were then measured by γ-spectrometry. The cesium solution recovered from adsorbers was subjected to TIMS measurements. The isotopic ratios of Cs/Cs and Cs/Cs were found to be independent of the type of sample in question, as well as the sampling location; the ratios were 0.07 and 0.36 (decay correction: 11 March 2011), respectively, which differ from the results of atmospheric nuclear tests (i.e., 0 and 2.7, respectively). This difference in the ratio of Cs/Cs will contribute to estimations of the origin of radioactive contamination in the future.

For the assessment of Fukushima Daiichi Nuclear Power Plant accident, the applicability of the thermal ionization mass spectrometry (TIMS), which is a type of mass spectrometry, was studied. For the study of the recovery/analysis method of cesium and strontium, at first, the radioactive cesium and strontium were generated by the irradiation of natural uranium at KUR. After this study, the applicability of this method to the environmental samples obtained in Fukushima prefecture was verified.

The TEPCO Fukushima Daiichi Nuclear Power Plant accident in March 2011 led to high amounts of emitted radioactive Cs being deposited on land by both rainwater and snowfall. In addition, a significant amount of Cs was deposited on the surface of leaves, and after the accident, both trees and grasses absorbed radioactive Cs through their roots. In order to assess the effect on water sources, it is therefore important to evaluate the amount of radioactive Cs migrating to the water from both grass and fallen leaves.

A number of samples of clover, dandelion, and mugwort were collected from the Yamakiya elementary school in Kawamata-machi, Date-gun, Fukushima-ken in May 2013 and May 2014. Fallen leaves were also sampled from the wood adjoining the school. Measurement of the Cs content in water was carried out by placing the sample in water for over 400 days at 10–30 °C. The radioactive Cs content was measured using the HPGe detector. In the case of grass, the amount of migration to water was saturated after about 120 days. The saturation levels of migration rate to water varied with kinds of grass in the range of 0.2–0.8. The migration rate for fallen leaves was not larger than 0.13. In addition, after leaching from grass or fallen leaves into water, the absorption of radioactive Cs to soil was observed, and therefore, migration would be limited to a small area.

This study investigates the source and flux of particulate I in the downstream reaches of the Niida River system in Fukushima. The upper watershed is a relatively highly contaminated zone located 30–40 km northwest of the Fukushima Daiichi Nuclear Power Plant. Samples of total suspended substance (SS) were collected continuously at Haramachi (5.5 km upstream from the river mouth) from December 2012 to January 2014 using a time-integrative SS sampler. Activity of I and the I/I ratio in SS were 0.9–4.1 mBq kg and (2.5–4.4) × 10, respectively, and were strongly correlated with the total dry weight of SS samples with R of 0.79–0.88. High SS I activity and I/I ratios were found in March, April, September, and October 2013. SS I activity and I/I ratios are considered to reflect the SS source, i.e., the more contaminated upper watershed or the less contaminated downstream area. The flux of particulate I at the Haramachi site was estimated to be 7.6–9.0 kBq month during September–October 2013. A relatively high amount of particulate I may have been transported from the upstream to the downstream reaches of the Niida River by high rainfall over this period.

The safety decontamination system of the contaminated forestry wastes by combustion was developed. Under the laboratory scale test with 10 g of cedar bark, about 35 % of radiocesium in the contaminated bark flowed out as a gaseous state by the combustion above 500 °C. The developed system consisted of three modules, the smoke extraction apparatus by water, the combustion ash coagulate apparatus and the radiocesium filtration unit from the sewage water. The demonstration combustion tests were carried out in March 2012. Forestry wastes (6.3 kg), pine needles, Japanese cedar bark and sapwood chips including radiocesium were combusted at 550–700 °C. The exhaust smoke was washed by the jet stream of water, the sewage water included small amount of soot and the radiocesium concentration of sewage water without soot was 50 Bq/kg. After the filtration of 550 L of sewage water by the radiocesium absorption filter consisting of wool dyed by Prussian blue, the radiocesium concentration decreased less than 0.2 Bq/kg. The filtrated water was recyclable in this system. No gaseous radiocesium was detected in the exhausted air from this system during the decontamination of forestry wastes. The combustion ash (140 g), consisting of cesium oxide alloy including various metal ions, was collected and packed under the reduced pressure automatically.

Remediation of materials contaminated with a radioactive material such as Cs is important for public health and environmental concerns. Here, we report the effectiveness of aqueous sodium metasilicate (SMC) prepared using a microbubble crushing process for the removal of radioactive Cs from contaminated materials. We have already reported that almost 80 % Cs removal was achieved for a nonwoven cloth sample in which multiple washings using low SMC concentrations were effective. In addition, the volume of the waste solution can be reduced by neutralizing the SMC and using gelation to remove the radioactive material. We also attempt to clarify the mechanism of SMC operation by measuring its electrical properties. Decontamination is shown to be more efficient with SMC than with sodium hydroxide, even for washing granule conglomerates.

In the aftermath of the 2011 accident at Fukushima Daiichi Nuclear Power Plant (F1 hereafter), municipal solid waste (MSW) contaminated with radioactive cesium (rad-Cs hereafter) has been generated in 12 prefectures in Japan. The Japanese Minister of Environment classified MSW that contained rad-Cs in the concentration more than 8,000 Bq/kg as “designated (solid) waste (DSW hereafter), and prescribed the collection, storage and transportation procedures. When MSW containing rad-Cs was incinerated, rad-Cs was concentrated in fly ash, and the ash often fell into the category of DSW. We have investigated a technique that can reduce the volume of the rad-Cs-contaminated fly-ash by extracting rad-Cs with aqueous solvents such as water and oxalic acid and concentrating rad-Cs in a small amount of hexacyanoferrate (or ferrocyanide, designated as Fer hereafter) precipitate. Since DSW could not be transported to the outside laboratory, we have conducted on-site tests at places where DSW were generated to investigate the applicability of the extraction – precipitation technique.

The present report is a summary of our most recent on-site test conducted in 2014. Also presented is the re-evaluation of the results of our past on-site test from the viewpoint of leaching of rad-Cs and heavy metals in the fly ash. An apparent decrease in leaching of rad-Cs from fly ash was observed by incinerating sewage sludge with soil. Fly ash from a melting furnace contained more water-soluble rad-Cs than that from a fluidized-bed incinerator. Some incinerator fly ash appeared to produce rad-Cs in colloidal form when extracted with oxalic acid, resulting in the lower removal of rad-Cs from the extract by Fer method.

A carborne survey system named as KURAMA (Kyoto University RAdiation MApping system) has been developed as a response to the nuclear accident at TEPCO Fukushima Daiichi Nuclear Power Plant. KURAMA is a -ray survey system with the global positioning system (GPS) and up-to-date network technologies developed for a primary use of carborne surveys. Based on the success of KURAMA, KURAMA-II, an improved version of KURAMA with better handling and ruggedness, is developed for the autonomous operation in public vehicles to minimize the workload of long-standing radiation monitoring required. Around two hundreds of KURAMA-II now serve for the continuous monitoring in residential areas by local buses as well as the periodical monitoring in Eastern Japan by the Japanese government. The outline and present status of KURAMA and KURAMA-II are introduced.