Fukushima radioactivity diluted in the Pacific makes tracing ocean currents possible

ocean currents
Sea-water samples and samples with planktons and suspended matter underwent measurements deep (225m) underground to avoid interference from cosmic rays in the sensitive instruments. © Fotolia, GreenVector

Very little is known about ocean currents and generally about dynamics in the oceans. But radioactivity released into the Pacific by the Fukushima nuclear accident, which was quickly diluted to harmless levels, has allowed scientists to trace the ocean’s currents.

The JRC’s expertise in nuclear measurements was instrumental in detecting and quantifying the radioactivity of sea-water samples. The study was carried out with a team of researchers from two Japanese Universities, following a campaign of sampling and measuring anthropogenic radionuclides in the North Pacific. It allowed natural processes using radionuclides as tracers to be studied.

The most important oceanographic conclusion from the study is that most of the surface water transported to the east towards the USA is submerged to a depth of 400 m near to the International Date Line and then turns towards south-west. This movement of the currents was not known prior to this study and will have an impact on e.g. computer models calculating global warming.

The accident in the Fukushima Dai-ichi nuclear power plant in March 2011 led to the release of huge amounts of radioactivity into the Pacific Ocean. During 2011 and 2012, Japanese scientists collected some 800 water samples and about 80 samples of plankton and suspended particles. In collaboration with the JRC, the samples were analysed, revealing very low levels of radioactivity. To measure such low radiation, the samples were placed deep (225m) underground to avoid interference from cosmic rays in the sensitive instruments.

Three radionuclides from Fukushima were detected in the samples from the Pacific: Caesium-134 (134Cs, half-life: 2.1 years), Caesium-137 (137Cs, half-life: 30 years) and the Silver isotope 110mAg (half-life 0.68 years). The zoo-plankton contained higher amounts of radiocaesium than particulate matter as it consumes organic matter and thereby accumulates caesium. The study of plankton is useful to understand the uptake in the food chain and estimate impact on biosystems of future releases. The measurements of the plankton showed that in all sampling locations the level of radiocaesium was in the order of 30 mBq/g (May/June, 2011) whilst only those samples collected up to 70 km from Fukushima (near to the epicentre of the earthquake) had measurable amounts of 110mAg.