Radioactive Cs in the Severely Contaminated Soils Near the Fukushima Daiichi Nuclear Power Plant

Abstract

Radioactive Cs isotopes (137 Cs, t 1/2 = 30.07 years and 134 Cs, t 1/2 = 2.062 years) occur in severely contaminated soils within a few kilometer of the Fukushima Daiichi 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, scanning electron microscopy (SEM), transmission electron microscopy (TEM), 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 occasional gibbsite. Autoradiography in conjunction with SEM analysis confirmed the association of radioactive Cs mainly with the submicronsized particles. Up to~3 MBq/kg of 137 Cs are associated with the colloidal size fraction (<1 μm), which accounts for~78% of the total radioactivity. Sequential extraction of the bulk sample revealed that most Cs was retained in the residual fraction, confirming the high binding affinity of Cs to clays, aluminosilicate sheet structures. The chemistry of the fraction containing submicron-sized particles from the same bulk sample showed a similar distribution to that of the bulk sample, again confirming that the Cs is predominantly adsorbed onto submicron-sized sheet aluminosilicates, even in the bulk soil samples. Despite the very small particle size, aggregation of the particles prevents migration in the vertical direction, resulting in the retention of >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.

Isotopic signature and nano-texture of cesium-rich micro-particles: Release of uranium and fission products from the Fukushima Daiichi Nuclear Power Plant

“…The OTZ3 and OTZ10 were separated from the same soil collected from the top ~1 cm of the paddy soil at Ottozawa located ~4 km west from the FDNPP in Okuma Town, Futaba County, Fukushima, on March 16, 2012. The soil was mainly composed of clay minerals, quartz, and feldspar 9 . Because it is still prohibited to enter the area due to the high radiation dose, the locality remained untouched and was not disturbed by decontamination or restoration.…”

“…The gravel samples were carefully collected from the ground surface using a hand shovel and placed in plastic bags. The soil was also mainly composed of clay minerals, quartz, and feldspar 9 . The third sample (OMR1) was collected under the drainpipe of the warehouse in Omaru located in Namie Town, Futaba County, Fukushima, ~10.5 km northwest from the FDNPP during the sampling campaign on December 20, 2012.…”

“…Previous studies of the distribution and migration of radioactive Cs in the surface environment around Fukushima 4 – 6 , 8 – 12 indicated that an initially soluble form of radioactive Cs was released from the damaged reactors and spread over the Fukushima Prefecture and the surrounding area through dry and wet deposition 3 – 5 , 8 . Subsequently, a soluble form of Cs was tightly bound to the interlayers of clays, such as vermiculite, and remained within the top ~5 cm of the soil 3 , 9 , 10 . However, Cs in the contaminated soils is heterogeneous and concentrated locally at the micron scale as hot spots, as revealed through autoradiography 9 – 11 , and this heterogeneity has not been fully described.…”

“…Subsequently, a soluble form of Cs was tightly bound to the interlayers of clays, such as vermiculite, and remained within the top ~5 cm of the soil 3 , 9 , 10 . However, Cs in the contaminated soils is heterogeneous and concentrated locally at the micron scale as hot spots, as revealed through autoradiography 9 – 11 , and this heterogeneity has not been fully described. A possible cause of the heterogeneity is the formation of Cs-rich microparticles (CsMPs), with a high Cs radioactivity per unit mass, found at a range of distances from the FDNPP 12 .…”

Radioactive Cs in the estuary sediments near Fukushima Daiichi Nuclear Power Plant

“…Because of this it is easily detected by gamma ray emission during the isomeric transition. Many researchers have suggested that the occurrence and geochemical behavior of radioactive Cs (mainly 134 Cs and 137 Cs) in the surface environment is critical to predicting its distribution and subsequent radiation exposures (Ueda et al, 2013;Kato et al, 2012;Aoyama et al, 2012;Tazoe et al, 2012;Kaneko et al, 2015).…”