Plume from Japan's Fukushima nuclear plant tracked across Pacific
Scientists use ocean currents to follow plume from damaged nuclear plant
How long did it take a radioactive plume to travel the waters of the Pacific from Fukushima, Japan, to the shores of North America?
The answer, according to a new study published in the Proceedings of the National Academy of Sciences, is about 2.1 years.
After an earthquake-triggered tsunami damaged the Fukushima nuclear power plant in March 2011, a team of Canadian scientists saw an opportunity to put models of Pacific Ocean current speeds to the test.
After the tsunami struck, the plant released cesium 134 and cesium 137 into the ocean. The scientists knew a small percentage of the radioactive material would be carried by currents across the Pacific, eventually reaching the west coast of North America.
Computer models could predict when this might happen, but by taking actual samples of the ocean water and testing them for cesium 134 and cesium 137, the scientists could see for certain.
"We had a situation where the radioactive tracer was deposited at a very specific location off the coast of Japan at a very specific time," said John Smith, a research scientist at the Bedford Institute of Oceanography in Dartmouth, Nova Scotia, and the lead author of the paper.
"It is unambiguous - you either see the signal or you don't, and when you see it you know exactly what you are measuring."
Just three months after the tsunami, Smith and his team began sampling ocean water from as far as 1,500km off the coast of British Columbia. They took measurements from the same sites every June from 2011 to 2013, collecting 60 litres of water and then analysing it for traces of cesium 134 and cesium 137.
In June 2011 they detected no signature from the Fukushima disaster at any of the test sites. The following June they found small amounts of the Fukushima radiation at the westernmost station, but it had not moved any closer to shore.
By June 2013, however, it had spread all the way to the continental shelf of Canada.
The amount of radiation that finally made it to Canada's west coast by June 2013 was very small, less than one becquerel - the number of decay events per second per 260 gallons of water - per cubic metre. That is more than 1,000 times lower than acceptable limits in drinking water.
Computer models that match fairly closely with the hard data Smith collected suggest the amount of radiation will peak during this year and next in British Columbia, but it will never exceed about 5 becquerels per cubic metre. "Those levels of cesium 137 are still well below natural levels of radioactivity in the ocean," Smith said.
Radiation levels in southern California are expected to peak a few years later, although by then they will be even smaller than the highest levels in Canada.