The IRSN’s earliest assessments of the Fukushima accident's consequences for the terrestrial environmentin Japan

Abstract

In 2011 the IRSN conducted several assessments of atmosphericradioactive releases due to the Fukushima Daiichi NPP accident (March11, 2011) and of their impact on Japan’s terrestrial environment.They were based on the IRSN’s emergency management tools and onthe abundant information and technical data gradually publishedin Japan. According to these assessments, the main release phaselasted from March 12 to 25, 2011 and impacted Japanese land in twoevents, the first on 15 and 16 March, in which the main radioactivedeposits were formed, and the second from March 20 to 23, whichwas less significant. The highest amounts of radioactive deposits werefound in an area extending upwards of several tens of kilometersnorthwest of the plant. Lower amounts were discontinuously scatteredin an area extending up to over 250 km away. Initially composedmainly of short-lived radionuclides, the deposits’ activity sharplydecreased in the subsequent weeks. Since the summer of 2011, cesium-134and cesium-137 have become the residual deposits’ main components.According to IRSN estimates, in the absence of protection, the dosesdue to exposure to the radioactive plume during the atmosphericrelease phase may have been potentially higher for people who remainedin coastal areas up to several tens of kilometers north and southof the damaged plant. Thereafter, people living up to 50 km northwestof the plant, outside the 20-km emergency evacuation zone, were potentiallymost vulnerable to residual radioactive deposits over time.