title: Modelling the dynamics of ambient dose rates induced by radiocaesium in the Fukushima terrestrial environment.
abstract: Since the Fukushima accident, Japanese scientists have been intensively monitoring ambient radiations in the highly contaminated territories situated within 80 km of the nuclear site. The surveys that were conducted through mainly carborne, airborne and in situ gamma-ray measurement devices, enabled to efficiently characterize the spatial distribution and temporal evolution of air dose rates induced by Caesium-134 and Caesium-137 in the terrestrial systems. These measurements revealed that radiation levels decreased at rates greater than expected from physical decay in 2011-2012 (up to a factor of 2), and dependent on the type of environment (i.e. urban, agricultural or forest). Unlike carborne measurements that may have been strongly influenced by the depuration of road surfaces, no obvious reason can be invoked for airborne measurements, especially above forests that are known to efficiently retain and recycle radiocaesium. The purpose of our research project is to develop a comprehensive understanding of the data acquired by Japanese, and identify the environmental mechanisms or factors that may explain such decays. The methodology relies on the use of a process-based and spatially-distributed dynamic model that predicts radiocaesium transfer and associated air dose rates inside/above a terrestrial environment (e.g., forests, croplands, meadows, bare soils and urban areas). Despite the lack of site-specific data, our numerical study predicts decrease rates that are globally consistent with both aerial and in situ observations. The simulation at a flying altitude of 200 m indicated that ambient radiation levels decreased over the first 12 months by about 45% over dense urban areas, 15% above evergreen coniferous forests and between 2 and 12% above agricultural lands, owing to environmental processes that are identified and discussed. In particular, we demonstrate that the decrease over evergreen coniferous regions might be due the combined effects of canopy depuration (through biological and physical mechanisms) and the shielding of gamma rays emitted from the forest floor by vegetation. Our study finally suggests that airborne surveys might have not reflected dose rates at ground level in forest systems, which were predicted to slightly increase by 5-10% during the same period of time.
authors.affiliation: Institut de Radioprotection et de Sûreté Nucléaire (IRSN), Laboratoire de Modélisation pour l'Expertise Environnementale (LM2E), Cadarache, Bâtiment 159, 13115 St Paul-lez-Durance, France. Electronic address: marc-andre.gonze@irsn.fr.
authors.forename: Marc-André
authors.initials: MA
authors.lastname: Gonze
authors.affiliation: Institut de Radioprotection et de Sûreté Nucléaire (IRSN), Laboratoire de Modélisation pour l'Expertise Environnementale (LM2E), Cadarache, Bâtiment 159, 13115 St Paul-lez-Durance, France.
authors.forename: Christophe
authors.initials: C
authors.lastname: Mourlon
authors.affiliation: Institut de Radioprotection et de Sûreté Nucléaire (IRSN), Laboratoire d'Etudes Radioécologiques en milieux Continental et Marin (LERCM), Cadarache, Bâtiment 151, 13115 St Paul-lez-Durance, France.
authors.forename: Philippe
authors.initials: P
authors.lastname: Calmon
authors.affiliation: Institut de Radioprotection et de Sûreté Nucléaire (IRSN), Laboratoire de Surveillance Atmosphérique et d'Alerte (LS2A), Le Vésinet, Bâtiment C8, 78116 Le Vésinet, France.
authors.forename: Erwan
authors.initials: E
authors.lastname: Manach
authors.affiliation: Institut de Radioprotection et de Sûreté Nucléaire (IRSN), Laboratoire de Surveillance Atmosphérique et d'Alerte (LS2A), Le Vésinet, Bâtiment C8, 78116 Le Vésinet, France.
authors.forename: Christophe
authors.initials: C
authors.lastname: Debayle
authors.affiliation: Institut de Radioprotection et de Sûreté Nucléaire (IRSN), Laboratoire Incertitude et Modélisation des Accidents de Refroidissement (LIMAR), Cadarache, Bâtiment 700, St Paul-lez-Durance 13115, France.
authors.forename: Jean
authors.initials: J
authors.lastname: Baccou
mesh_terms: D000383:Agriculture; D000396:Air Pollutants, Radioactive / Q000032:analysis*; D002588:Cesium Radioisotopes / Q000032:analysis*; D002947:Cities; D065928:Forests; D062406:Fukushima Nuclear Accident; D007564:Japan; D008962:Models, Theoretical; D011891:Rain
chemical_list: D000396:Air Pollutants, Radioactive; D002588:Cesium Radioisotopes
keywords: Air dose rate; Forests; Fukushima; Modelling; Terrestrial ecosystems