Last update on March 2016
The study of external dosimetry in the Fukushima region (EDOFU) is a current research study (2014-2016) by IRSN aiming at better quantifying the evolution (in space and time) of ambient dose rates in the Fukushima region following the nuclear accident in 2011.
Background and objectives
The Fukushima-Daiichi accident in 2011 led to massive release of radioactive substances into the atmosphere. Vast continental territories were contaminated, leading to exposure of the local populations to radionuclides by external radiation or ingestion of contaminated foods. The most heavily contaminated areas are situated within a radius of 80 km around the accident site, and are mainly contaminated by cesium radioisotopes.
There have been several successive measurement campaigns which have led to better identification of the spatial distribution and temporal evolution of the radiological state of the different land ecosystems at Fukushima. These measurements have revealed several important pieces of information:
- a high spatial variability of the deposits and ambient dose rates (in intensity and isotopic composition), mainly due to the heterogeneity of the landscape and weather conditions;
- a large reduction in ambient dose rates (annual decrease measured at around 40% in 2011 and 2012), contrasted geographically because of the relatively recent nature of the accident, but also because of other factors of natural or anthropogenic origin.
The objective of the research project is to better better quantify, understand and model the spatial and temporal evolution of the stocks (Bq/m²) of cesium and associated ambient dose rates (Bq/m²/s) in inhabited zones and agricultural or forest areas of the Fukushima region. This implies to study the spread of the contamination in the medium term, under the effect of the processes of migration and transfer of radioactive elements within ecosystems as well as rehabilitation actions.
The main challenge of this study is to improve the expertise of IRSN in a post-accident situation and to better forecast the radiological consequences of an accident.
Performance of the study and lines of research
EDOFU has developed two lines of research:
- fine analysis of the available radiological and dosimetric measurements,
- integrated modeling of radioactivity transfer and ambient dose rates at the scale of the landscape and in the medium term.
The research is concentrated on the 80 km area around the Fukushima-Daiichi site and the simulation forecasts are for the 2014-2023 period. This forecast scale (medium term) corresponds to the scope of validity of radioecological models in the Symbiose platform used for the simulations. Only the gamma emitters still currently present in the Fukushima region were taken into account in the models and in the field, i.e. Cs137 and Cs134.
The environmental factors and processes involved in the reduction of ambient dose rates have not been clearly identified. In fact, other processes apart from the physical decrease of radionuclides are significantly involved. These processes of natural and anthropogenic origin cause half of the annual decrease, i.e. 20%. Preliminary analyses show that this rate of decrease appears to be closely correlated with ground occupation; EDOFU enables these analyses to be completed.
The land areas are modeled by distinguishing forest environments (70% of the area of Fukushima), agricultural environments (mainly rice fields and also market gardens, orchards and pasture land) and highly anthropized environments (basically urban areas). Wet and dry atmospheric deposition processes and also surface water (rivers, lakes and reservoirs) are taken into account; for the sake of simplicity, it is considered that they no longer make a significant contribution to the ambient dose rate. Thus EDOFU is studying several phenomena:
- wet and dry atmospheric deposition onto ground, plants (forests, crops and grasslands) and anthropic areas (buildings, roads, etc.);
- all recycling processes in agricultural and forest environments,
- decontamination of urban environments mainly under the effect of rainwater leaching and
- exportation of contamination by runoff and erosion of contaminated watersheds.
The ambient dose rates are studied on the scale of one parcel or one site, then extrapolated to larger spatial scales.
Environmental system studied under EDOFU © IRSN
(click to enlarge)
2014 and 2015 were dedicated on the one hand to the detailed space-time analysis of the radiometric measurement campaigns, and on the other hand to the development and then the use of the EDOFU simulator in the Symbiose platform. These studies were the subject of a publication and three oral presentations delivered at conferences (cf. references below).
- Gonze M.-A., Mourlon C., Calmon P., Manach E., Debayle C., Baccou J. (2015). Modeling the Dynamics of Ambient Dose Rates Induced by Radiocaesium in the Fukushima Terrestrial Environment. Journal of Environmental Radioactivity 215: 1−13
- Gonze M.-A., Calmon P., Mourlon C., Manach E., Debayle C., Hurtevent P., Coppin F., Thiry Y. (2015). Regional Modeling of Radiocaesium Dynamics and Associated External Doses in Fukushima Forests. Communication orale invitée at ICOBTE Conference (Fukuoka, Japon, June 2015)
- Gonze M.-A., Mourlon C., Calmon P., Manach E., Debayle C., Baccou J. (2015). Assessment Study of Ambient Dose Rates Dynamics in the Fukushima Terrestrial Region. Communication orale at ERMSAR Conference (Marseille, France, March 2015).
- Gonze M.-A., Calmon P., Mourlon C., Hurtevent P., Coppin F., Simon-Cornu M. (2014). Modelling of Radionuclides Transfer and Ambient Dose Rates in Fukushima Forest Ecosystems: a Preliminary Study. Communication orale à IUFRO Conference (Salt Lake City, USA, October 2014).