A new study published in the Proceedings of the National Academy of Sciences on Monday suggests that Russia suffered a major nuclear accident in 2017.

Led by George Steinhauser, a nuclear chemist at Leibniz University, the study uses atmospheric data to report a likely accident at the Mayak facility in Russia’s Ural Mountains. Speaking to Live Science, Steinhauser said the radioactive material released was “between 30 and 100 times the level of radiation released after the Fukushima accident in Japan in 2011.”