The Institute’s numerical models predict that within two years, the radioactivity will fall to “about 10 becquerel per cubic metre” (Bq/m3) within two years, and 1-2 Bq/m3 within four-seven years.
The US Environmental Protection Agency says that contamination of drinking water with caesium radioisotopes would still be considered safe below 100 Bq/m3.
But the study may do little to assuage public fears in California, where it was reported last week that local prunes and almonds had detectable levels of caesium-134, “a fingerprint for radiation from Fukushima Daiichi,” according to Fairewinds Energy Education.
Local media also report that radiation levels in the Los Angeles and Santa Monica areas are high and increasing, while fears abound that “buckyballs” of uranium-oxide fuel could be washed ashore in the next two years.
These football-shaped structures – the largest matter shown to exhibit a wave-particle duality – may have been created when millions of tonnes of freshwater were used to try to cool down the Fukushima reactors three molten cores, and then flushed to sea.
The Fukushima reactors were severely damaged by a tsunami in March 2011.
A University of California Davis study published earlier this year first warned that uranium buckyballs of up to 60 uranyl ions were likely to form in the Pacific.
“Being thermodynamically stable and kinetically persistent in the absence of peroxide, they can potentially transport uranium over long distances,” the study said.
Uranyl ion contamination has been noted on and around sites where weapons tipped with depleted uranium have been used. In sufficiently high concentrations, it is hazardous to public and environmental health.
However, the Geomar Helmholtz Centre concludes that because the Paciﬁc Ocean is so vast, and is stirred by “energetic, ﬂuctuating currents”, it will result in “an effective dilution of the contaminated body of seawater arising from the ... short-term discharge from the stricken Fukushima-Daiichi reactors”.
'Below the level of concern'
While the total peak radioactivity levels after four to seven years would still be about twice the pre-Fukushima values, it says this is “below the level of concern”.
The German study used a sequence of global ocean circulation models to estimate the long-term dispersion by ocean currents of a slow decaying tracer, comparable to that of caesium-137. The tracer was continuously injected into coastal waters over several weeks and its spreading and dilution in the Pacific Ocean was then simulated for 10 years.
An abstract of the German report found that two to three years on, the tracer cloud had reached depths of more than 400 metres, leading to “a rapid dilution of concentrations”.
The study accepts that some fraction of the long-lived isotopes such as caesium-137 (with a half-life of more than 30 years) will accumulate in the food chain, with as yet unknown consequences for marine organisms.