Decommissioning works are seen continuing at the Fukushima Daiichi Nuclear Power Station, in Okuma, Fukushima Prefecture, in this image taken from a Mainichi Shimbun helicopter on Feb. 14, 2019. A multitude of treated water storage tanks can be seen behind the reactor buildings. (Mainichi/ Koichiro Tezuka)

TOKYO -- Radioactive cesium released into the Pacific Ocean due to the March 2011 meltdowns at Fukushima Daiichi Nuclear Power Station is returning to Japanese shores via a shorter route than expected, according to a joint research initiative.

The findings were revealed by a team from the University of Tsukuba, the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and Kanazawa University.

Until now, it was thought that cesium from the Tokyo Electric Power Co. (TEPCO)-operated plant would be circulated around the Pacific by subtropical gyre currents for several decades before returning to Japanese waters. But in 2012, a year after the reactor core meltdowns, tests on seawater samples collected by the team showed increased cesium concentrations in East China Sea waters off Japan. Researchers say that the concentrations observed are too low to impact sea life.

The rate increased, peaking in 2014, and a year later high concentrations were also reported in the Sea of Japan. The team believes the cesium is now flowing around the Pacific Ocean again.

It is thought that seawater sank deeply into the sea after its density increased due to cooling by winter winds, causing the cesium to travel on a western-flowing underwater route.

Michio Aoyama, a visiting professor at the University of Tsukuba, said, "That the cesium would come back in such a short time was unexpected. We've found a previously unknown route."

Senior JAMSTEC research scientist Yuichiro Kumamoto said of the project's potential benefits, "Because it has visualized ocean circulation, the results could be used in the future for predictions on issues such as climate change."

(Japanese original by Mayumi Nobuta, Science & Environment News Department)