A nuclear accident at a power plant in South Korea could cause wider radiation contamination in western Japan than on its home soil, a study by a South Korean scientist has shown.

If a cooling system fails at the spent-fuel storage pools at the Kori power plant's No. 3 reactor in Busan, massive amounts of cesium-137 would be released that could potentially reach western Japan, according to a simulation by Jungmin Kang of the Natural Resources Defense Council, a U.S. think tank.

In the worst-case scenario, up to 67,000 sq. km of Japanese soil would be contaminated and 28.3 million people would be forced to evacuate, the study said, though the fallout's spread would depend on the season.

As for South Korea, an accident at the plant could taint more than half of the nation by contaminating up to 54,000 sq. km, it said.

A total of 818 tons of spent nuclear fuel were stored in pools at the site as of the end of 2015, Kang said. He said an accident could be triggered not only by natural disasters but by terrorism or a missile from North Korea.