Post-Accident Sporadic Releases of Airborne Radionuclides from the Fukushima Daiichi Nuclear Power Plant Site

The
Fukushima nuclear accident (March 11, 2011) caused the widespread
contamination of Japan by direct deposition of airborne radionuclides.
Analysis of weekly air filters has revealed sporadic releases of radionuclides
long after the Fukushima Daiichi reactors were stabilized. One major
discharge was observed in August 2013 in monitoring stations north
of the Fukushima Daiichi nuclear power plant (FDNPP). During this
event, an air monitoring station in this previously scarcely contaminated
area suddenly reported 137Cs activity levels that were
30-fold above the background. Together with atmospheric dispersion
and deposition simulation, radionuclide analysis in soil indicated
that debris removal operations conducted on the FDNPP site on August
19, 2013 are likely to be responsible for this late release of radionuclides.
One soil sample in the center of the simulated plume exhibited a high 90Sr contamination (78 ± 8 Bq kg–1)
as well as a high 90Sr/137Cs ratio (0.04); both
phenomena have usually been observed only in very close vicinity around
the FDNPP. We estimate that through the resuspension of highly contaminated
particles in the course of these earthmoving operations, gross 137Cs activity of ca. 2.8 × 1011 Bq has been
released.