Mapping of seafloor radionuclides off Miyagi using a towed gamma ray spectrometer

An instrument to continuously map the distribution of radionuclides on the seafloor has been developed to survey radioactive discharge from the Fukushima Dai-ichi Nuclear Power Plant. To date, the system has been deployed off the east coast of Japan to measure the distribution of ¹³⁷Cs over a total towed distance of more than 320 km. Here, we describe the results of measurements made along two transect north of the plant, off Miyagi, during February 2013, almost two years after the accident. There results reveal the continuous distribution of radionuclides over a total distance of 140 km. The accuracy of the measurements is verified through laboratory analysis of samples obtained during the survey. The results demonstrate that towed in situ measurement techniques provide an effective solution for rapid, low cost monitoring of radioactive material on the seafloor. It is expected that continued monitoring efforts using the device can contribute to our understanding of radionuclide distribution on the seafloor following the accident at the plant.