A REMOTE CONTINUOUS AIR MONITORING SYSTEM FOR MEASURING AIRBORNE ALPHA CONTAMINATION.

We developed a remote continuous air monitoring (RCAM) system. The RCAM system consisted of a personal air monitor and a robot. The personal air monitor (poCAMon, SARAD, Germany) had a 400-mm2 ion-injected silicon detector and a membrane air filter with 25 mmφ. The personal air monitor provides the alpha energy spectra for any measurement time interval. Demonstration measurements were taken underground at the Mizunami Underground Research Laboratory and at a poorly ventilated concrete building. The RCAM system was remotely operated and successfully measured the 222Rn progeny even though the relative humidity was almost 100%. In the measured alpha spectra, the peaks of 218Po (6.0-MeV alpha) and 214Po (7.7-MeV alpha) were clearly identified. Our developed monitor is promising for alpha dust monitoring in a high gamma-ray environment or contaminated areas where a worker cannot safely physically enter.

Detection of alpha particle emitters originating from nuclear fuel inside reactor building of Fukushima Daiichi Nuclear Power Plant.

We measured alpha emitters obtained from a reactor building in the Fukushima Daiichi Nuclear Power Plant (FDNPP) by using an alpha particle imaging detector. For developing the detector, we used a very thin (0.05-mm-thick) a cerium-doped Gd3(Ga,Al)5O12 (Ce:GAGG) scintillator and silicon photomultiplier (SiPM) arrays as the photodetector. The floor of the reactor building in FDNPP was wiped off by using smear papers, and the radioactivity of these papers was measured by the alpha particle imaging detector. In addition, we measured a Plutonium (Pu) sample (mainly 5.5 MeV alpha particles from 238Pu) obtained from a nuclear fuel facility by using of the same detector for comparison with the smear papers. The alpha spectrum was in the energy range of 5-6 MeV, which corresponds to the alpha particle energy of 238Pu (5.5 MeV). The correlation coefficient of the alpha spectra of the smear papers and the Pu sample had a strong positive linear relation. Moreover, the peak of 241Am was identified by gamma spectrum measurement. Based on these results, we report actual findings of alpha emitters in the FDNPP reactor buildings originating from nuclear fuels. The surface contamination level of alpha emitters exceeded 4 Bq/cm2.

Evaluation of ecological half-life of dose rate based on airborne radiation monitoring following the Fukushima Dai-ichi nuclear power plant accident.

Airborne radiation monitoring was conducted in order to evaluate the influence of radionuclides emitted by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident throughout Japan. Carrying out airborne radiation monitoring using manned and unmanned helicopters, the we have developed and established an analysis method concurrently with the development of this monitoring method. In particular, because the background radiation level differs greatly between East and West regions of Japan, we have developed a discrimination method for natural radionuclide and cosmic rays using the gamma energy spectra. The reliability of the airborne radiation monitoring data was validated through comparison with large amounts of ground measurement data. The ecological half-lives of short and long components for decline of the ambient dose equivalent (air dose rate) were 0.61 years and 57 years, respectively, based on the results of air dose rate of airborne radiation monitoring using manned helicopter. These results indicate the importance of airborne monitoring to evaluate and predict the radiation exposure of residents.

Evaluation of ecological half-life of dose rate based on airborne radiation monitoring following the Fukushima Dai-ichi nuclear power plant accident.

Airborne radiation monitoring was conducted in order to evaluate the influence of radionuclides emitted by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident throughout Japan. Carrying out airborne radiation monitoring using manned and unmanned helicopters, the we have developed and established an analysis method concurrently with the development of this monitoring method. In particular, because the background radiation level differs greatly between East and West regions of Japan, we have developed a discrimination method for natural radionuclide and cosmic rays using the gamma energy spectra. The reliability of the airborne radiation monitoring data was validated through comparison with large amounts of ground measurement data. The ecological half-lives of short and long components for decline of the ambient dose equivalent (air dose rate) were 0.61 years and 57 years, respectively, based on the results of air dose rate of airborne radiation monitoring using manned helicopter. These results indicate the importance of airborne monitoring to evaluate and predict the radiation exposure of residents.

Temporal variation of dose rate distribution around the Fukushima Daiichi nuclear power station using unmanned helicopter.

Aerial radiological survey using an unmanned aerial vehicle (UAV) was applied to measurement surface contamination around the Fukushima Daiichi nuclear power station (FDNPS). An unmanned helicopter monitoring system (UHMS) was developed to survey the environmental effect of radioactive cesium scattered as a result of the FDNPS accident. The UHMS was used to monitor the area surrounding the FDNPS six times from 2012 to 2015. Quantitative changes in the radioactivity distribution trend were revealed from the results of these monitoring runs. With this information, we found that the actual reduction of dose rate was faster than the one calculated with radiocesium physical half-life. It is indicated that the attenuation effect of radiation by radiocesium penetration in soil is dominant as for reason of reduction of dose rate.

Utilization of (134)Cs/(137)Cs in the environment to identify the reactor units that caused atmospheric releases during the Fukushima Daiichi accident.

The Fukushima Daiichi nuclear power reactor units that generated large amounts of airborne discharges during the period of March 12-21, 2011 were identified individually by analyzing the combination of measured (134)Cs/(137)Cs depositions on ground surfaces and atmospheric transport and deposition simulations. Because the values of (134)Cs/(137)Cs are different in reactor units owing to fuel burnup differences, the (134)Cs/(137)Cs ratio measured in the environment was used to determine which reactor unit ultimately contaminated a specific area. Atmospheric dispersion model simulations were used for predicting specific areas contaminated by each dominant release. Finally, by comparing the results from both sources, the specific reactor units that yielded the most dominant atmospheric release quantities could be determined. The major source reactor units were Unit 1 in the afternoon of March 12, 2011, Unit 2 during the period from the late night of March 14 to the morning of March 15, 2011. These results corresponded to those assumed in our previous source term estimation studies. Furthermore, new findings suggested that the major source reactors from the evening of March 15, 2011 were Units 2 and 3 and that the dominant source reactor on March 20, 2011 temporally changed from Unit 3 to Unit 2.

Aerial radiation monitoring around the Fukushima Dai-ichi Nuclear Power Plant using an unmanned helicopter.

The Great East Japan Earthquake on March 11, 2011 generated a series of large tsunami that seriously damaged the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), which resulted in the release of radioactive materials into the environment. To provide further details regarding the distribution of air dose rate and the distribution of radioactive cesium ((134)Cs and (137)Cs) deposition on the ground within a radius of approximately 5 km from the nuclear power plant, we carried out measurements using an unmanned helicopter equipped with a radiation detection system. The distribution of the air dose rate at a height of 1 m above the ground and the radioactive cesium deposition on the ground was calculated. Accordingly, the footprint of radioactive plumes that extended from the FDNPP was illustrated.

Enhanced analysis methods to derive the spatial distribution of 131I deposition on the ground by airborne surveys at an early stage after the Fukushima Daiichi nuclear power plant accident.

This paper applies both new and well tested analysis methods to aerial radiological surveys to extract the I ground concentrations present after the March 2011 Fukushima Daiichi nuclear power plant (NPP) accident. The analysis provides a complete map of I deposition, an important quantity incalculable at the time of the accident due to the short half-life of I and the complexity of the analysis. A map of I deposition is the first step in conducting internal exposure assessments, population dose reconstruction, and follow-up epidemiological studies. The short half-life of I necessitates the use of aerial radiological surveys to cover the large area quickly, thoroughly, and safely. Teams from the U.S. Department of Energy National Nuclear Security Administration (DOE/NNSA) performed aerial radiological surveys to provide initial maps of the dispersal of radioactive material in Japan. This work reports on analyses performed on a subset of the initial survey data by a joint Japan-U.S. collaboration to determine I ground concentrations. The analytical results show a high concentration of I northwest of the NPP, consistent with the previously reported radioactive cesium deposition, but also shows a significant I concentration south of the plant, which was not observed in the original cesium analysis. The difference in the radioactive iodine and cesium patterns is possibly the result of differences in the ways these materials settle out of the air.