Japanese researchers have found genetic mutations in three generations of butterflies -- a phenomenon they say is a direct result of the Fukushima Dai-ichi nuclear disaster.

The butterflies have deformities that include enlarged eyes, shrunken wings and abnormal spot patterns, and the frequency of their mutations has increased with each of the last three generations, according to the study.

"We conclude that artificial radionuclides from the Fukushima Nuclear Power Plant caused physiological and genetic damage to this species," stated the introduction to the report, published in the online journal Scientific Reports, a property of the journal Nature.

The researchers from Ryukyu University collected the first batch of pale grass blue butterflies in May 2011 in the Fukushima area shortly after the March 11 nuclear disaster, which was triggered by an earthquake and tsunami.

The first batch of butterflies were incubating as larvae during the disaster, and showed a 12 per cent rate of abnormality when they hatched.

Those insects were then mated in a laboratory in Okinawa, far outside the Fukushima-affected area, and their offspring showed an even higher frequency -- 18 per cent -- of the same mutations.

Then, in a third generation, the mutation rate rose to 34 per cent even though only one parent from each mating pair came from a Fukushima-affected population.

"An important finding was that certain traits observed in the (first) generation were inherited by the (second) generation. Colour-pattern modifications were relatively frequent. Wing-wide spot enlargement was evident," the study states.

The researchers were also able to reproduce the abnormalities by intentionally exposing previously unaffected butterflies to radiation doses similar to those from Fukushima.

And when they went back to the Fukushima area in September of 2011 and collected 240 fresh butterflies, deformities or abnormalities were recorded in 52 per cent.

The researchers said the pale grass blue butterfly is particularly suitable for testing the effects of radiation because “it is widespread in Japan, including the Fukushima area, and because its wing colour patterns are sensitive to environmental changes.”

In the past, the species has been used to evaluate the ecological risk associated with genetically modified maize pollen.

Japan has been working to clean the radiation from the area affected by the 2011 nuclear meltdown, with the eventual goal of allowing evacuated residents to return to their homes.

However, the task is massive, with workers attempting to scrape roughly five centimetres of contaminated soil from an area the size of Connecticut.

Where that soil will be stored, and whether the area can ever truly be deemed safe, is still unknown.