Seiki Ohnishi

Distribution of local 137Cs anomalies on the seafloor near the Fukushima Dai-ichi Nuclear Power Plant

An estimated 3.5±0.7×10(15) Bq of (137)Cs is thought to have been discharged into the ocean following the melt down at Fukushima Dai-ichi Nuclear Power Plant (F1NPP). While efforts have been made to monitor seafloor radiation levels, the sampling techniques used cannot capture the continuous distribution of radionuclides. In this work, we apply in situ measurement techniques using a towed gamma ray spectrometer to map the continuous distribution of (137)Cs on the seafloor within 20 km of the F1NPP. The results reveal the existence of local (137)Cs anomalies, with levels of (137)Cs an order of magnitude higher than the surrounding seafloors. The sizes of the anomalies mapped in this work range from a few meters to a few hundreds of meters in length, and it is demonstrated that the distribution of these anomalies is strongly influenced by meter scale features of the terrain.

Development of a sonar system to estimate the seafloor subsurface burial depth of a towed gamma-ray spectrometer

A sonar system and image processing method have been developed to continuously estimate the seafloor subsurface burial depth of a gamma-ray spectrometer that is towed along the seafloor by a ship. The towed gamma-ray spectrometer, called the RESQ hose, is an instrument designed to measure radiation levels in seafloor sediments that were contaminated as a result of the accident at the Fukushima-Daiichi nuclear power plant. In situ measurements of subsurface burial depth can increase the accuracy of the radionuclide concentration levels estimated by the RESQ hose by allowing for more accurate modeling of the measurement conditions. The reliability of the system is verified through sea trials performed near the outlet of the Abukuma River. Continuous measurements of burial depth are achieved with an outlier ratio of less than 5%. The burial depths measured by the device show a strong correlation with the acoustic backscatter intensity of side-scan sonar images obtained along the surveyed transect, indicating that burial depth is dependent on sediment type. We examine the influence of burial depth on the conversion factors between detected gamma-ray count rates and the concentrations of 134Cs and 137Cs. For both nuclides, the calculated factors increase by nearly 80% as burial depth increases from 0 to 11 cm. The converted results from the measured 137Cs count rates are compared with a core sample obtained near the transect, with the nearest point within a factor of two of the sampling result.

Generation of 137Cs surface distribution maps using dense measurement transects of a towed gamma-ray spectrometer

In this paper, we describe a method to generate surface distribution maps of seafloor 137Cs levels. The method interpolates the measurements of a towed gamma-ray spectrometer, which are made along intersecting line transects, by modeling the spatial variations between each observation. Since it is known that sediment type has an influence on the distribution of 137Cs, the proposed method aims to improve the spatial auto correlation of the models by segregating the surveyed area into continuous regions in which the sediment types are judged similar. This is achieved based on the back-scatter intensity of side scan sonar images taken in the surveyed region. The method is applied to data obtained at the Abukuma river mouth located approximately 80km north from F1NPP on October 2013. The accuracy of the proposed method was evaluated by correlating the levels of 137Cs in the generated maps with samples that were obtained in the same area. The results demonstrate that the proposed method of interpolation improves the correlation compared to conventional methods that do not account for sediment type.

Continuous mapping of radionuclides on the seafloor using a towed gamma ray spectrometer

An instrument to continuously map the distribution of radionuclides on the seafloor has been developed and applied to survey radioactive discharge from the Fukushima Dai-ichi Nuclear Power Plant following the M9.0 earthquake and tsunami that struck the east coast of Japan on March 11 2011. The towed gamma ray spectrometer has been deployed off Fukushima and has to date, been used to monitor radionuclide distribution over a total distance of more than 120 km. Here, we introduce the results of measurements made along three transects within an 80km radius of the plant, focusing on a 22km long transect off Iwaki. While the levels of the natural gamma ray emitter 40K remained constant, the levels of 137Cs and 134Cs were found to vary significantly with location. The in situ measurements show good agreement with laboratory analyzed samples obtained during the survey, indicating that the technique described provides an effective solution for rapid, low cost monitoring of radioactive material on the seafloor.