At a clean up site above Oruda primary school on
the outskirts of Fukushima
The clean-up strategy for Fukushima and Date City
was devised by Shunichi Tanaka, the former acting head of Japan
"Usually the contamination happened in a nuclear facility, inside a
controlled area, but this type of contamination is global environmental
contamination - it's completely different," he says.
Following the explosions at the Fukushima Daiichi nuclear plant on 11
March last year, wind carried a cloud of radioactive debris off towards the
north-west.
Some places have very high radioactivity; some people may be growing rice
there without realising”
This airborne debris fell on populated areas beyond the 20km exclusion zone that would later be set
up around the plant.
Soon after the disaster, scientists at Tokyo University
Prof Tomoko Nakanishi presented their initial findings in the Japanese
journal Radioisotopes in August 2011. She says the radioactive caesium does not
move very far once it bonds with the soil particles.
"The rainfall was 198mm
during the first three months and the caesium... moved 21.6mm (downwards).
"Then in the next three months the caesium moved only 5.6mm - despite the rainfall being three
times higher. This shows with time, the caesium was more firmly adhered to the
soil."
The radioactive fallout from the nuclear plant contained a number of
radioactive isotopes - products of the nuclear reactions - which differ widely
in how long they persist in the environment.
Radioactive iodine decayed to near background levels within a few days,
but the bigger concern was over isotopes of caesium, some with half lives of
more than 30 years.
The Tokyo University
This gave Shunichi Tanaka something to work on for his clean up strategy;
he concluded only the upper contaminated soil layer need be removed.
"The most important thing is to remove the surface, for soil one or
two centimetres; 5mm to 1cm for concrete and asphalt," he says.
While the caesium forms a tight bond with clay particles in the soil
surface, wherever caesium landed on hard surfaces, such as concrete or asphalt
it presented a different problem.
"Its not chemically bonded, it's in a physical trap," says
Shunichi Tanaka.
The caesium finds its way into tiny pores on the surface, and the only way
to remove it effectively is to grind off the surface, a difficult procedure
which can create contaminated dust.
Nevertheless, as the soil, asphalt and concrete is removed overall
radiation levels should fall. But this does not necessarily mean the threat of
exposure has completely gone.
Prof Tomoko Nakanishi acknowledges there might be a small future risk from
food, in particular from less well studied areas
"We have to continue these measurements for a long time, we do not
know what will happen to the next harvest or the harvest after that," he
explains.
"This year perhaps we can get less radioactivity in rice… It takes a
year to harvest rice. Contamination is not uniform, some places have very high
radioactivity; some people may be growing rice there without realising.
"There is a possibility there might be some contaminated rice coming
out."
While the caesium is hard for plants and animals to take up when it is
tightly bound to the soil, once in solution (dissolved in water) it can be more
readily absorbed.
The scientists are watching closely the effect of melting snow and
rainfall leaching down from mountainous areas. They are also concerned this
water may bring more dissolved caesium into rice fields in the future.
