The Russian Academy of Sciences's Nuclear Safety Institute set up an international committee to investigate the incident. The French Institute of Radioprotection and Nuclear Security (IRSN) provided a report to the committee in January 2018. The report concluded that the most likely source of the pollution is a spent fuel treatment facility located in a region between Volga and Ural. A possible reason for the release of radioactivity at Mayak PA might be an unsuccessful attempt to produce cerium-144 for the European scientific project Borexino. Mayak PA had agreed to deliver cerium-144 in early 2018, but canceled the contract in December 2017. Mayak PA was the only facility capable of producing cerium-144 from "fresh" spent nuclear fuel 2–3 years old. Usually spent fuel is not processed earlier than five years after extraction. The Ru106/Ru103 ratio in analyzed environmental samples during the pollution spread was characteristic of "fresh" spent fuel. While the non-Russian members of the committee accepted the conclusions of IRSN's report, the Russian members maintain that an inspection by Rostechnadzor of the Mayak PA facility in November 2017 showed no anomalies and a rare meteorological event may have transported the ruthenium-106 from somewhere else to the apparent region of origin.
