Validated radiometric mapping in 2012 of areas in Japan affected by the Fukushima-Daiichi nuclear accident

Sanderson, D. and Cresswell, A. and Seitz, B. and Yamaguchi, K. and Takase, T. and Kawatsu, K. and Suzuki, C. and Sasaki, M. (2013) Validated radiometric mapping in 2012 of areas in Japan affected by the Fukushima-Daiichi nuclear accident. [Data Collection]

Collection description

On March 11 2011 the north-eastern region of Japan was hit by a magnitude 9 earthquake, which occurred underneath the sea-bed 70 km east of the Oshika peninsula in Tohoku. The north-eastern shore of Honshu was hit by a tsunami resulting from this earthquake. As a consequence, reactors 1-4 at the nuclear power station Fukushima-Daiichi suffered a completed loss of power and cooling causing a partial core meltdown in units 1, 2 and 3 followed by a series of explosions and the release of large quantities of radioactivity into the environment.

The accident was rated level 7 (the highest level) on the International Nuclear Event Scale. While most of the emissions were driven towards the Pacific Ocean, a significant amount of radioactive material was deposited onto the Japanese land-mass, resulting in enhanced localised radiation exposure.

This report covers measurements and detailed radiation maps conducted by a UK University team working with Japanese colleagues during a series of visits in 2012. They have been presented and shared locally in Japan at time of acquisition. Since then the data have been cross-validated relative to soil samples from a reference site established at the University of Fukushima, and analysed independently in the UK and in Japan.

This report provides detailed descriptions of the data sets, validated radiometric maps for 134Cs, 137Cs and the overall gamma dose rates, together with a full account of the establishment of the calibration site. The work reported here demonstrates the utility of detailed radiometric maps in helping to understand the distribution of radionuclides in complex environmental systems.

This information is potentially of use to help avoid unnecessary external radiation exposure in the outdoor environment, to help to visualise and target areas for remediation, to evaluate the effectiveness of clean-up and soil removal activities, to examine uptake of radioactivity from the environment through agricultural systems, and to monitor redistribution over time of the activity in the environment.

The report includes full copies of digital data sets for the demonstration surveys. Radiometric methods provide means of measuring radioactivity with recognised roles in nuclear emergency response, and environmental applications. In the aftermath of nuclear accidents they are crucial to evaluate the environmental impact of the accident and guide remediation measures.

The Scottish Universities Environmental Research Centre (SUERC) is a research centre attached to the University of Glasgow with extensive experience of radiometric mapping using airborne and ground based systems, conducting most of the UK post-Chernobyl radiation mapping using airborne systems and coordinating European projects to harmonise and cross-calibrate systems for nuclear emergency response purposes. In the early stages of major accidents the most pressing needs are for rapid, large scale, information. Airborne surveys of the affected area were conducted initially by a joint US/Japanese team and later on by The Ministry of Education Science and Technology (MEXT) and the Japanese Atomic Energy Agency (JAEA), eventually providing national scale radiation maps with a spatial resolution of several hundred meters.

In later stages of accident recovery there are increasing needs for more detailed spatial information and increasing requirements for objective, traceable and cross validated analysis. Systems for this work need to be mobile, preferably portable, efficient, robust and well calibrated, providing detailed real time information directly on location. The SUERC Portable Gamma Spectrometry system fulfils these criteria. It provides a spatial resolution of better than 10 m for mapped data and allows a real time identification of spatial features down to 10-20 cm.

In the work reported here, calibration sites have been established in Fukushima at the campus of Fukushima University and the Fukushima Prefecture Fruit Tree Research Institute to provide objective and internationally traceable validation of ground based instruments. High resolution HPGe spectrometry of soil samples conducted at laboratories at Fukushima University and SUERC was used for validation of the field instrument. Values of 137Cs activity concentration measured by the two laboratories were in full agreement to high precision. Agreement for the 134Cs activity concentration values was within 5-10%. The soil samples used in this process were analysed as a function of depth, revealing that while the majority of activity was retained in near surface layers, a small component of approximately 1% of the total activity appears to have migrated more rapidly to greater depths.

This suggests complex transport behaviour in the soil columns which should be investigated further. The calibration sites are open for future use. These data establish a traceable record between local facilities and internationally acknowledged standards for future use. Deployment of the SUERC system in vehicles has demonstrated the ability of relatively small detectors to measure regional scale deposition patterns over a wide range of radioactivity levels, varying from 10,000 Bq per square meter up to more than 10 million Bq per square meter. These activity levels are broadly consistent with the national scale airborne maps for the study areas, but the ground based maps provide very high level of local detail, which allow small scale changes to be readily observed and related to the local soils, land cover and built environment. Areas covered in two surveys in March and July 2012, respectively, cover parts of the Fukushima Prefecture including parts of the evacuation and exclusion zones as well as areas directly affected by the 2011 tsunami.

Short backpack surveys in urban areas associated with the car trips were also conducted e.g. in Fukushima City (in the heart of the prefecture), Ōkuma (within 3 km of the Fukushima Daichi reactors), Minami-Daira and Kawauchi-mura (within the initial evacuation zones), demonstrate the versatility of a system which can be rapidly moved from a vehicle to backpack where further detailed information is needed. With a fully spectrometric system as used in this study, it is possible to identify specific radioactive isotopes and to quantify the activity concentrations of natural and artificial sources spanning many orders of magnitude, from less than 10 kBq m-2 to above 10 MBq m-2 for radiocaesium. This information has been used to estimate the contribution to the dose rate from different sources, so that a comparison between the contributions from naturally occurring radioactive materials (not affected by the accident) and the different radiocaesium isotopes present following the accident can be readily made. These data provide vital information to the local population and emergency services in guiding remediation efforts, and also over the course of time, in observing the extent to which weathering and radioactivity decay processed are gradually reducing the relative contributions from the accident. In the long term it is hoped that this type of representation will help put the accident contributions into perspective, and to register improvements with time which may help to establish increased confidence in affected areas. In urbanised areas backpack systems provide the means of producing detailed surveys in locations where people spend their time.

Surveys conducted of Fukushima University and Fukushima Iizaka have demonstrated the ability of detailed radiometric mapping to identify locations with highly varying levels of contamination, from more highly contaminated areas around drain pipes to the relatively low levels of contamination on roads and other hard surfaces where rain and snow melt have removed deposited activity. The effectiveness of the remediation work conducted on the University campus was evident, showing the ability of radiometric surveys to demonstrate the effectiveness of remediation and to identify where remaining activity is located. The remediation methods employed resulted in a three- to fourfold reduction in dose rate compared to untreated areas. Remediation efforts in orchards or woodland are particularly challenging.

Given the economic importance of agriculture in the Fukushima area and the possible pathways from contaminated produce into the human body, a precise evaluation of the local radiation risk is of prime importance. The SUERC system has been demonstrated in orchards at the Fukushima Prefecture Fruit Tree Research Institute, and at other orchards in the Prefecture. Work has also been undertaken with agricultural research institutions at Tsukuba, Ibaraki Prefecture. The comparison of measurements taken at these same sites over the time of a year show a decrease in activity following the half-life of the radioisotopes deposited, superimposed by environmental factors like precipitation, illustrating self-remediation.

Ongoing work on these sites will allow an assessment of transfer of activity from to the fruit, and the impact of measures to remediate the orchards or reduce uptake of activity in the fruit, and the evaluation of external doses to workers in the orchards. The data collected during this work contain the specific activity per isotope at a given surveyed location and at a given moment in time. The data accompanying this report are transparent, open and independently validated. They are made available for reference purposes and further utilisation. The work presented here highlights many of the difficult challenges ahead in the monitoring and remediation effort in the area affected by the accident in the Fukushima Dai-ichi nuclear power plant.

It demonstrates the capabilities and methods at the disposal of the international community in aiding the Japanese efforts, demonstrating the value of international collaboration in helping to address some of the difficult problems associated with recovery from a serious nuclear accident. It is hoped that these data, and the methods which they demonstrate, will contribute, together with future work, to increased understanding of the environmental impacts of the accident, and that future cooperative work involving Japanese and international teams will contribute to recovery and restoration of confidence in affected areas and communities.

College / School: College of Science and Engineering > Scottish Universities Environmental Research Centre
Date Deposited: 24 Jul 2014 14:28
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Sanderson, D. and Cresswell, A. and Seitz, B. and Yamaguchi, K. and Takase, T. and Kawatsu, K. and Suzuki, C. and Sasaki, M. (2013); Validated radiometric mapping in 2012 of areas in Japan affected by the Fukushima-Daiichi nuclear accident

University of Glasgow


Retrieved: 2017-09-21


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