French nuclear security watchdog, IRSN, detected the cloud of radioactive ruthenium 106 over Europe at the end of September.
Monitoring weather patterns, the scientists say the most plausible source is an area south of the Ural Mountains between Russia and Kazakhstan.
Ruthenium 106 is used in nuclear medicine because of its short half-life.
The lack of other radioactive nuclide contaminants suggests the leak came from a nuclear fuel treatment site.
The cloud of radioactive pollution high in the atmosphere over most of Europe was first detected on the 30th September indicating that the accident occurred in the last week of September. The radioactive levels steadily decreased from 6 October onwards.
The exact source is still unknown with a spokeswoman for the Russian emergencies ministry saying she could not immediately comment and attempts to contact authorities in Kazakhstan and the Kazakh embassy in Moscow have been unsuccessful. according to Reuters.
Jean-Marc Peres, director of the Institut de Radioprotection et de Sûreté Nucléaire (IRSN) estimates that the quantity of ruthenium 106 released was major and that if the accident had happened in France it would have required the evacuation of people in a radius of a few kilometres around the accident site.
Radioactive elements used in nuclear power generation such as uranium 236 have an incredibly long half-life of some 4.5 billion years meaning they lose half their radioactive concentration over that period. Ruthenium 106 has a half-life of around a year making it much more suitable for medical applications.
The fact that no other radioactive nuclides were detected probably indicates the source of the contamination to be a nuclear fuel treatment site or centre for nuclear medicine, according to Monsieur Peres.
Peres reassured the public that the concentrations of ruthenium recorded in Europe were of no consequence for human health and the environment.
He also ruled out the crash of a ruthenium-powered satellite as an IAEA investigation concluded no ruthenium-containing satellite had fallen back to earth during this period.