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Actuators of a robot that physically interacts with objects or human require backdrivability to obtain force controllability, impact resistance and compactness. An electro-hydrostatic actuator (EHA) is a servo pump type hydraulic system that has a potential to satisfy backdrivability. However, the previous mechanical design of a revolute EHA did not achieve compact design compared to the other actuators. The hydraulic pump and vane motor were separately located, which made the size of EHA system larger. To solve this problem, we propose a design methodology of a revolute EHA using a frameless motor. Commercially available frameless motor is suitable for actuating the hydraulic pump. Moreover, the pump can be built in the hollow space of the frameless motor, and this design makes the EHA downsized. We present a prototype design based on the formulation of the output power. Through the experimental validations, we show that developed EHA has higher backdrivability than the comparable harmonic drive, and obtain the knowledges on design factors that are crucial to the EHA with high power-to-weight ratio. [ABSTRACT FROM AUTHOR]

The Fukushima Daiichi Nuclear Power Plant accident caused an accumulation of 137Cs in coastal sediment. The 137Cs supply from rivers to the ocean can affect the long-term fate of 137Cs in coastal sediment. Since the Fukushima coastal river basins include large decontaminated and evacuation order areas, considering the decontamination work and resumption of agriculture is important for predicting the 137Cs supply. We conducted a 30-year prediction of the 137Cs supply from the Fukushima coastal rivers to the ocean using a distributed radiocesium prediction model, considering the effects of human activities. In river basins with decontaminated and evacuation order areas, human activities reduced the total 137Cs outflow from agricultural lands, urban lands, and forest areas to the rivers and the 137Cs supply to the ocean by 5.0% and 6.0%, respectively. These results indicated that human activities slightly impacted the 137Cs outflow and supply. The 137Cs supply from rivers impacted by the accident to the coastal sediment was estimated to correspond to 11–36% of the total 137Cs in the coastal sediment in the early phase of the accident. Therefore, the 137Cs supply from rivers to the ocean is important for the long-term behavior of 137Cs in coastal sediment. [ABSTRACT FROM AUTHOR]

To investigate the factors regulating the concentration of particulate iodine (PI) in seawater, it was measured in two Pacific coastal areas adjacent to Japan (offshore Aomori and offshore Fukushima) in two seasons (May and October) along with parameters such as particulate organic carbon (POC) and nitrogen (PON), dissolved iodine (DI), and phytoplankton pigments. The observed PI concentrations ranged from 0.0068 to 2.2 nmol L−1, with uncertain seasonal and regional differences. The atomic ratio of PI to POC (I/C) ranged from 4.4 × 10−7 to 1.1 × 10−4, in line with previous studies on marine phytoplankton, although the lower part of the range of the I/C in this study was found to be somewhat lower compared to the previous studies. The dataset was divided in three groups namely, lower (Group A), average (Group B), and higher (Group C) I/C because the I/C is an indicator of the accumulation efficiency of PI in particles in seawater and POC is the main carrier of PI. Based on the observed POC concentrations, the atomic ratio of POC to PON (C/N), and the ratio of carbon derived from chlorophyll a to POC, the three groups were characterized by phytoplanktonic physiological states as highly productive, steady, and senescent states for Groups A, B, and C, respectively. Based on the finding that PI production is associated with the phytoplankton physiological state, the seasonal and regional differences in PI concentration and I/C in the observation areas were consistently explained. Finally, we suggest that the phytoplanktonic physiological state is one of the vital factors regulating the PI concentration in seawater. [ABSTRACT FROM AUTHOR]

Cesium-rich microparticles (CsMPs) with high cesium-137 (137Cs) concentrations were released and deposited in surface soil after the Fukushima Daiichi Nuclear Power Plant accident. Radioactive materials on the soil surface layer enter rivers owing to soil erosion during rainfall. In this study, we investigated CsMPs runoff through the river via soil erosion during rainfall in the Takase River watershed in Namie Town, Fukushima Prefecture, Japan. CsMPs were rarely detected in suspended solids (SS) in water samples collected during four rainfalls between February and July 2021. Furthermore, the proportion of 137Cs concentration derived from CsMPs to 137Cs concentration in the form of SS (particulate 137Cs) in the water was ~6% on average, which suggests that 137Cs runoff in the form of CsMPs from the forest to the Takase River was not large. [ABSTRACT FROM AUTHOR]

Nakanishi, T.; Tsuruta, T.; Misonou, T.; Shiribiki, T.; Urabe, Y., and Sanada, Y., 2023. Behavior of radiocesium (137Cs) on the coastal seafloor near the Fukushima Daiichi Nuclear Power Plant inferred from radiocesium distributions in long cores. In: Lee, J.L.; Lee, H.; Min, B.I.; Chang, J.-I.; Cho, G.T.; Yoon, J.-S., and Lee, J. (eds.), Multidisciplinary Approaches to Coastal and Marine Management. Journal of Coastal Research, Special Issue No. 116, pp. 161-165. Charlotte (North Carolina), ISSN 0749-0208. Continuous data of 137Cs concentrations in surface seabed sediment at several monitoring points around the Fukushima Daiichi Nuclear Power Plant (FDNPP) from 2012 to 2022 were compiled, and exponential functions fitted its temporal change. The 137Cs concentrations are gradually declining with time. However, at some monitoring points in shallow regions with a water depth of 30 m or less, long half-lives and/or large deviations of 137Cs concentrations were noticed. Long sediment cores (∼1 m) were collected at shallow regions near the FDNPP to gain insight into radiocesium dynamics in shallow seafloor, and the vertical profiles of 137Cs concentrations and particle size distribution were determined. In the sediment cores collected in the onshore zone, the 137Cs concentrations and particle size distribution were homogenized from several tens of cm to more than 1 m depth, indicating strong vertical mixing by wind waves. Therefore, onshore sediments have significant 137Cs reserves in deeper layers at present. The 137Cs supply from the deep layers and the land area might suppress this zone's surface 137Cs concentration decline. However, in sediment cores at the offshore zone, the vertical 137Cs concentration distributions were highly heterogeneous, showing the particle size dependency. Moreover, 137Cs profiles obtained at the same point for several years differed significantly, indicating local spatial distribution and temporal deposition. Therefore, it might cause a large fluctuation in 137Cs concentrations at surface sediment over time. [ABSTRACT FROM AUTHOR]

Abstract It is a critical to examine the migration behavior of radiocesium derived from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in river systems to predict the future contamination status and propose effective countermeasures to reduce exposure. We conducted a three-year-long observation (April 2015–March 2018) of the 137Cs concentration in two rivers which located surrounding the FDNPP. The result revealed a declining trend for the dissolved and particulate 137Cs concentration in river water from four to seven years after the FDNPP accident. The dissolved and particulate 137Cs concentrations for both rivers had similar temporal patterns and showed declining trends with time. However, the dissolved 137Cs concentration had longer half-life than the particulate 137Cs concentration and large seasonal variations related to water temperature. The environmental half-life for the dissolved 137Cs concentration was longer than previous reported values within three years after the accident, suggesting that the declining trend for the dissolved 137Cs concentration is gradually decreasing with time. The temperature dependency of the dissolved 137Cs concentration became weaker year by year. From the D 10 equation we proposed, the dissolved 137Cs concentration will likely remain at the same level for several decades. The results of the present study promote our understanding of both the medium- and long-term impacts of the FDNPP accident on river systems. Highlights • 137Cs concentration in river water was continuously monitored. • The environmental half-life was different between particulate and dissolved 137Cs. • Dissolved 137Cs concentration had seasonal variation related to water temperature. • Temperature dependency of dissolved 137Cs concentration became weaker with time. [ABSTRACT FROM AUTHOR]

Particulate Cs was collected from stream water for 2 years to assess the long-term trend of Cs discharge from a forest after the Fukushima Nuclear Power Plant accident. A seasonal increase in the fluvial transport load of particulate Cs in suspended solids (SS) was observed in July-October when rainfall was abundant. The Cs load was controlled by the SS load. This control was attributed to cesium affinity for phyllosilicate clay minerals as verified by the low extractability of particulate Cs. These findings indicate the fluvial particulate Cs load is significantly related to the climate and geomorphological features of Japan. [ABSTRACT FROM AUTHOR]