福島原発事故由来放射性物質による樹木の汚染状態の解析と木材資源の有効利用に 関する研究
小川 秀樹
Contamination Analysis of Woods by Radioactive nuclear Derived From Fukushima Nuclear Power Plant Accident and Utilization of Woody Biomass
Toward Future
Chapter 1: Objective and construction of this study
Various radioactive nuclear were dispersed in the environment by Tokyo Electric Power Company, Fukushima Dai-Ichi Nuclear Power Plant accident, Iodine (131I) and Cesium (137Cs,
134Cs) were major radionuclide. In Fukushima, the forestry works were influenced by the contamination of the forest that constitutes approximately 70% of Fukushima areas. In particularly, the information about the distribution and changes of 137Cs in the forest was important, because of 30 years half -life of 137Cs.
The objectives of this study were as follows. (1) Evaluation of the initial contamination of trees, (2) Clarify the process of the contamination in wood, (3) Development of the safe utilization system of the wood biomass. This tissue consisted of 7 chapters. Chapter 1 and 2 were Introduction and Conclusion. Chapter 2 discussed about (1), Chapter 3-5 discussed about (2), Chapter 6 discussed about (3).
Chapter 2: Analysis of the initial tree contamination
The distribution of 137Cs in the parts of Japanese cedar tree (Cryptomeria japonica) was investigated in the forest of Koriyama-city (Fukushima Forestry Research Center, 0.82 μSv/h) and Kawamata-town (Yamakiya area, 4.70 μSv/h), and the initial tree contamination and its changes were also clarified. Trees were felled for study in Koriyama city in August 2011 and October 2012, and in the Yamakiya area in November 2011 and December 2012. As a result,
137Cs concentration of outer contamination parts, such as leaves and barks, were higher than that of sapwood and inner barks. The vertical distribution of 137Cs concentration in outer bark and leaves showed a trend and the contamination of the surface of barks differed among the directions. The profiles of vertical 137Cs distribution in the outer parts changed from 2011 to 2012, indicating that the leaves was decontamination by the rain and the bark was contaminated again.
Chapter 3: Changes in the distribution of 137Cs of the inside-wood
Characteristics of the 137Cs distribution in the wood and its changes were clarified by estimating the two dimensional 137Cs concentration profiles in the inside-wood of Japanese cedar trees. In May 2012, 10 trees in 5 forests in the Kawamata area (0.61～0.91μSv/h) were felled and the radial distribution of 137Cs in the inside-wood was measured at different heights. Distributions differed according to the sampling height and depended particularly on
the distance from the treetop.
The 137Cs distribution changes in the inside-wood were studied in the same forest from 2011 to 2013 in the Yamakiya area. The 137Cs distribution profiles changed considerably from 2011 to 2013, it was predicted that the active transportation of 137Cs from sapwood to heartwood and the normal diffusion of 137Cs in heartwood radial and vertical occurred. In Japanese cedar, these process progressed in 3 years after the accident, 137Cs distribution in the wood in 2013 approached to that in steady state.
Chapter 4: Changes in the distribution of 137Cs in the wood using the increment borer
The changes in the distribution of 137Cs in the inside-wood of same tree were measured by using the increment borer in the Nihonmatsu area from 2012 to 2015 at every summer and winter, in the Yamakiya area from 2012 to 2015 at every summer. 137Cs concentration in sapwood decreased and that in heartwood increased at first, but 137Cs concentration in heartwood and sapwood approached to be constant year by year. Changes of 137Cs distribution profiles indicated that 137Cs concentration in heartwood became uniform due to the diffusional movement of 137Cs. Total amount of 137Cs in disk, which was estimated by the radial distribution of 137Cs concentration, was almost same from 2012 to 2015. And the vertical distribution of 137Cs in the soil showed the exponential profiles. These results indicated that the most amount of 137Cs was derived from the direct uptake just after the fall out, and small effect of the additional uptake via the root.
Chapter 5: 137Cs distribution in the wood of Konara and Hinoki
137Cs distribution in the wood of Konara (Quercus serrata) and Hinoki (Chamaecyparis obtusa) were compared with that of Japanese cedar trees. The contaminated side of the inside-wood corresponded to that of the bark, indicating the absorption of 137Cs into the inside-wood through the leaves or bark during the initial fall out. 137Cs concentration in heartwood of Konara and Hinoki were lower than that of Japanese cedar trees. There were relationships between the
137Cs concentration and the water content in heartwood among Hinoki and Japanese cedar trees. These results indicated that the water content in the heartwood was one of the factor that decided the distribution of 137Cs in the wood.
Chapter 6: Development of the utilization systems of wood biomass
The utilization systems of wood biomass were studied based on the information about the tree contamination that was clarified in Chapter 2-5. Safety decontamination system for combustion of forestry wastes was developed, and the demonstration combustion test using this system was carried out. Furthermore we tried for decontamination of the wood by the transpirational drying, expecting 137Cs in the wood to move to leaves with water. By transpirational drying, 137Cs concentration in sapwood inclined to decrease, but that in heartwood did not incline to decrease or increase apparently.
Chapter 7: Conclusion and prospect toward the future
In this study, the initial wood contamination was clarified, which was most basecal
imfomatinon about the forest contamination. Vertical and directional distrbutions of 137Cs in the leaves and bark was useful for the restart of forestry works in the evacuation area. It was important to estimate the forest contamination in the future, in order to use the contaminated forest biomass for long time. We expected that the forest contamination in the futre might be estimated depending on the result of this study.
