Abstract
Radioactive Cs isotopes (137Cs, t1/2 = 30.07 y and 134Cs, t1/2 = 2.062 y) occur in severely contaminated soils within a few km of the Fukushima Dai-ichi nuclear power plant at concentrations that range from 4×10^5 to 5×10^7 Bq/kg. In order to understand the mobility of Cs in these soils, both bulk and submicron-sized particles elutriated from four surface soils have been investigated using a variety of analytical techniques, including powder X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), and analysis of the amount of radioactivity in sequential chemical extractions. Major minerals in bulk soil samples were quartz, feldspar, and minor clays. The submicron-sized particles elutriated from the same soil consist mainly of mica, vermiculite, and smectite and occassional gibbsite. Autoradiography in conjunction with SEM analysis confirmed the association of radioactive Cs mainly with the submicron-sized particles. Up to ~3 MBq/kg of 137Cs are associated with the colloidal size fraction (98% of Cs within top ~5 cm of the soil. These results suggest that the mobility of the aggregates of submicron-sized sheet aluminosilicate in the surface environment is a key factor controlling the current Cs migration in Fukushima.
Highlights
Four years have passed since the Fukushima Daiichi nuclear power plant (FDNPP) event released ~520 PBq of radionuclides (Steinhauser et al, 2014) in the vicinity of FDNPP
Mukai et al (2014) recently conducted transmission electron microscopy (TEM) analysis of Fukushima soils and showed that most particles associated with radioactive Cs are aggregates of clays and other phases, such as organic matter
It is well-known that submicron-sized particles behave as colloids in surface and subsurface aquifers (McCarthy and Zachara, 1989); these colloids can potentially control the migration of trace radionuclides
Summary
Four years have passed since the Fukushima Daiichi nuclear power plant (FDNPP) event released ~520 PBq of radionuclides (Steinhauser et al, 2014) in the vicinity of FDNPP. Mukai et al (2014) recently conducted transmission electron microscopy (TEM) analysis of Fukushima soils and showed that most particles associated with radioactive Cs are aggregates of clays and other phases, such as organic matter. This aggregation makes it difficult to characterize the submicron phases associated with Cs or to estimate their contribution to the total Cs radioactivity. It is well-known that submicron-sized particles behave as colloids in surface and subsurface aquifers (McCarthy and Zachara, 1989); these colloids can potentially control the migration of trace radionuclides. The present study investigated that the soil particles collected within a few kilometer of FDNPP utilizing a variety of analytical techniques including conventional sequential extraction methods and electron microscopy in order to describe the mineralogy, identify the Cs speciation, and determine the dose contribution from Cs sorbed onto submicron-sized particles
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