The ease in which radionuclides move through the environment and are taken up by plants
and animals is governed by their chemical forms and by site-specific environmental
characteristics. The objective of this paper is to review basic mechanisms of the behavior of
radiocesium and radiostrontium in the environment after the nuclear accident. Our
understanding of radionuclide’s speciation and migration processes seems to be adequate and
explains similarities and differences of radiocesium (r-Cs) behavior in the environment after
Fukushima and Chernobyl accidents.
Climate and geographical conditions in Fukushima Prefecture of Japan and
Chernobyl’s near-field zone are obviously different. In particular, precipitation differs
substantially, with the annual average for Fukushima being about 3 times higher
than at Chernobyl. The landscapes and soils also differ significantly. What is more,
the speciation of r-Cs in the releases was distinct (large fraction of radionuclides
was deposited as fuel particles in 30-km zone around Chernobyl NPP, while in
Fukushima radiocesium is mostly part of condensation particles including glassy hot
particles).
Radiocesium (r-Cs) in the environment is strongly bound to soil and sediment particles
containing micaceous clay minerals (illite, vermiculite, etc.), which is associated with two
basic processes – high selective reversible sorption and fixation. The r-Cs distribution
coefficient Kd in Fukushima rivers was found to be 1-2 orders of magnitude higher than
corresponding values for rivers and surface runoff of Chernobyl area. This is indicative of
higher ability of Fukushima soils and sediments to bind r-Cs. Dissolved r-Cs wash-off for
Fukushima river watersheds is essentially slower than those for Chernobyl. However, steeper
slopes and higher precipitation in Fukushima area cause higher erosion and higher particulate
r-Cs wash-off. For a comparable time after the accident the total r-Cs wash-off from
contaminated catchments in Fukushima is up to one order of magnitude higher than in
Chernobyl.
Long-term dynamics of radionuclide concentrations in rivers is approached from the
standpoint of basic mechanisms of radionuclide sorption-desorption, fixation, vertical
migration in catchment soils. Corresponding semi-empirical models are presented and
discussed. For the Chernobyl case, radiostrontium (r-Sr) was shown to be more mobile and
moving faster in dissolved state with surface runoff and river water in comparison
with r-Cs. Similar pattern was observed for Mayak area in South Ural (Russia),
where r-Sr was traced up to 1500 km away from the release point migrating through
Techa-Iset’-Tobol-Irtysh-Ob’ river system. On the other hand, r-Cs bound to clay
particles settles down in Techa river reservoirs and is transported with river water only
insignificantly.
For the first 3 years after the accident vertical migration of r-Cs in soils of Fukushima
catchments was found to be faster than in Chernobyl due to higher air temperature, higher
precipitation and higher biological activity in top soil. However, with time this process slows
down because of higher r-Cs retardation in Fukushima soils.
In Fukushima case, extreme floods during typhoons lead to substantial reduction in dose
rate on floodplain areas due to sedimentation of relatively clean material and burial of
contaminated top soil layer.
In general, due to higher precipitation, higher temperatures and higher biological
activities in soils, self-purification of the environment and natural attenuation in Fukushima is
essentially faster than in Chernobyl area. |