The 2011 Fukushima nuclear disaster has caused a trace spike in radiation levels in Californian wines from the 2012 vintage onward according to a new report from the University of Bordeaux’s Nuclear Studies Center of Bordeaux Gradignan (CNRS).

The study found that the level-7 nuclear reactor accident at the Fukushima Daiichi plant in Japan caused levels of cesium-137 in the atmosphere to rise, which has affected grapes grown in Northern California. However, the radiation levels found in the wine are still within a normal threshold.

A natural level of radioactivity is always present in the earth’s atmosphere, soil, seas and rivers, causing trace levels of radiation in food and drinks, but it very rarely hits levels that could cause health concerns.

The aim of the study was to discover whether the level of natural and artificial radioactivity in Californian wines had been affected by the Fukushima disaster in the same way that the Chernobyl disaster affected wines from the south-west of France in the 1980s. Making such a comparative determination allows researchers to assess the impact distance of nuclear incidents.

“The Fukushima incident, which took place on March 11, 2011, resulted in a radioactive cloud that crossed the Pacific Ocean to reach the west coast of the United States,” the research stated. “In Northern California, there is Napa Valley. The idea was then to see if, as was the case in Europe following the Chernobyl accident, if we could detect a variation in the cesium-137 level in these wines.”

Following the 1986 Chernobyl disaster, radiation was found to have spiked within wines from the south-west of France. Similarly, the CNRS study found that the Fukushima Daiichi plant meltdown released a radioactive cloud that travelled across the Pacific and reached California’s Napa Valley, where the highly soluble cesium isotope was absorbed by the grapes.

A typical bottle of Californian Cabernet Sauvignon produced in 2012 was found to have double the cesium-137 activity, from about 7.5mBq (megabecquerel, a measurement of the activity of radioactive material) of per litre to around 15mBq per litre. The resarchers noted, however, that this barely compares to levels registered at the height of the Cuban missile crisis.

The researchers tested bottles of Californian Cabernet Sauvignon from 2009 to 2012 vintages and found that the levels of cesium-137 varied before and after the Fukushima disaster. The found slightly higher levels of radiation in red wines than white and rosé variants. The Chernobyl-caused spike in French wines was comparatively stronger, but all levels of detected radiation have been declared to be within safe bounds for public health.

“As was the case in France’s white or rosé wine, Californian rosé bottles lead to significantly lower values than red wines,” the researchers noted, adding: “It seems there is an increase in activity in 2011 by a factor of two.”