In 2011 the IRSN conducted several assessments of atmospheric
radioactive releases due to the Fukushima Daiichi NPP accident (March
11, 2011) and of their impact on Japan’s terrestrial environment.
They were based on the IRSN’s emergency management tools and on
the abundant information and technical data gradually published
in Japan. According to these assessments, the main release phase
lasted from March 12 to 25, 2011 and impacted Japanese land in two
events, the first on 15 and 16 March, in which the main radioactive
deposits were formed, and the second from March 20 to 23, which
was less significant. The highest amounts of radioactive deposits were
found in an area extending upwards of several tens of kilometers
northwest of the plant. Lower amounts were discontinuously scattered
in an area extending up to over 250 km away. Initially composed
mainly of short-lived radionuclides, the deposits’ activity sharply
decreased in the subsequent weeks. Since the summer of 2011, cesium-134
and cesium-137 have become the residual deposits’ main components.
According to IRSN estimates, in the absence of protection, the doses
due to exposure to the radioactive plume during the atmospheric
release phase may have been potentially higher for people who remained
in coastal areas up to several tens of kilometers north and south
of the damaged plant. Thereafter, people living up to 50 km northwest
of the plant, outside the 20-km emergency evacuation zone, were potentially
most vulnerable to residual radioactive deposits over time.