Perilous
Times
Japan: Nuclear pollution of sea from Fukushima was world's
biggest
by Staff Writers
Paris (AFP) Oct 27, 2011
France's nuclear monitor said on Thursday that the amount of
caesium 137 that leaked into the Pacific from the Fukushima
disaster was the greatest single nuclear contamination of the sea
ever seen.
But, confirming previous assessments, it said caesium levels had
been hugely diluted by ocean currents and, except for near-shore
species, posed no discernible threat.
From March 21 to mid-July, 27.1 peta becquerels of caesium 137
entered the sea, the Institute for Radiological Protection and
Nuclear Safety (IRSN) said.
One peta becquerel is a million billion bequerels, or 10 to the
power of 15.
Of the total, 82 percent entered the sea before April 8, through
water that was pumped into the Fukushima's damaged reactor units
in a bid to cool them down, it said.
"This is the biggest single outflow of man-made radionuclides to
the marine environment ever observed," the agency said in a press
release.
Caesium is a slow-decaying element, taking 30 years to lose half
of its radioactivity.
The IRSN said large quantities of iodine 131 also entered the sea
as a result of the disaster, caused by the March 11 9.0-magnitude
quake that occurred off northeastern Japan.
But iodine 131 decays quickly, having a half-life of eight days,
and the contamination "swiftly diminished," the report said.
The IRSN said that, for the Pacific generally, caesium levels
would ultimately stabilise at 0.004 becquerels per litre thanks to
the diluting effect of powerful ocean currents.
This is twice the concentration that prevailed during atmospheric
nuclear testing in the 1960s.
"These levels should not have an impact in terms of radiological
safety," the IRSN said.
However, "significant pollution of seawater on the coast near the
damaged plant could persist," because of continuing runoff of
contaminated rainwater from the land, it said.
"Maintaining monitoring of marine species taken in Fukushima's
coastal waters is justified," it said.
The IRSN cited deep-water fish, fish at the top of the marine food
chain and molluscs and other filtrating organisms as "the species
that are the most sensitive" to caesium pollution.