PARIS (Reuters) - A cloud of radioactive pollution over Europe in recent weeks indicates that an accident has happened in a nuclear facility in Russia or Kazakhstan in the last week of September, French nuclear safety institute IRSN said on Thursday. 
 
 FILE PHOTO: An employee looks at equipment in a new facility at a nuclear waste disposal plant in the town of Fokino in Russia's far-eastern Primorsky region June 24, 2014. REUTERS/Yuri Maltsev 
 
 The IRSN ruled out an accident in a nuclear reactor, saying it was likely to be in a nuclear fuel treatment site or center for radioactive medicine. There has been no impact on human health or the environment in Europe, the IRSN said. 
 
 IRSN, the technical arm of French nuclear regulator ASN, said in a statement it could not pinpoint the location of the release of radioactive material but that based on weather patterns, the most plausible zone lay south of the Ural mountains, between the Urals and the Volga river. 
 
 This could indicate Russia or possibly Kazakhstan, an IRSN official said. 
 
 “Russian authorities have said they are not aware of an accident on their territory,” IRSN director Jean-Marc Peres told Reuters. He added that the institute had not yet been in contact with Kazakh authorities. 
 
 A spokeswoman for the Russian Emergencies Ministry said she could not immediately comment. It was not immediately possible to reach authorities in Kazakhstan or the Kazakh embassy in Moscow. 
 
 Peres said that in recent weeks IRSN and several other nuclear safety institutes in Europe had measured high levels of levels of ruthenium 106, a radioactive nuclide that is the product of splitting atoms in a nuclear reactor and which does not occur naturally. 
 
 IRSN estimates that the quantity of ruthenium 106 released was major, between 100 and 300 teraBecquerels, and that if an accident of this magnitude had happened in France it would have required the evacuation or sheltering of people in a radius of a few kilometers around the accident site. 
 
 The ruthenium 106 was probably released in a nuclear fuel treatment site or center for radioactive medicine, Peres said. Because of its short half-life of about a year, ruthenium 106 is used in nuclear medicine. 
 
 The IRSN ruled out an accident in a nuclear reactor, as that would have led to contamination with other radionuclides too. It also ruled out the crash of a ruthenium-powered satellite as an IAEA investigation has concluded that no ruthenium-containing satellite has fallen back on earth during this period. 
 
 Measurement from European stations showed high levels of ruthenium 106 in the atmosphere of the majority of European countries, at the beginning of October, with a steady decrease from Oct. 6 onwards. 
 
 The IRSN said that the concentrations of ruthenium 106 in the air that have been recorded in Europe were of no consequence for human health and the environment. 
 
 The institute also said that the probability of importation into France of foodstuffs, notably mushrooms, contaminated by ruthenium 106 near the site of the accident is extremely low. ||||| Clues In That Mysterious Radioactive Cloud Point Toward Russia 
 
 Enlarge this image toggle caption Sovfoto/UIG via Getty Images Sovfoto/UIG via Getty Images 
 
 The tiny nation of Denmark has just three stations for monitoring atmospheric radiation. Each week, scientists change out air filters in the detectors and take the used ones to a technical university near Copenhagen. 
 
 There, Sven Poul Nielsen and other researchers analyze the filters. They often snag small amounts of naturally occurring radioactivity, radon for example. 
 
 Then about a month ago, Nielsen was startled to find something far stranger: a radioactive isotope known as ruthenium-106. 
 
 Ruthenium-106 has a half-life of just one year, which means that it isn't naturally found on Earth. It is, however, created in the glowing cores of nuclear reactors — and usually only detected in the atmosphere when something goes terribly wrong. 
 
 The ruthenium was detected far beyond Denmark. It showed up all across Europe, from Cyprus to Spain. The levels seen in dozens of nations were far too low to pose a health risk. Within weeks, the thin cloud had vanished. 
 
 So far, no nation has announced a recent nuclear incident or accident involving ruthenium-106. But some Western scientists say they suspect the source might have been a Russian research institute near the city of Dimitrovgrad, west of the Ural Mountains. The reason for their suspicion comes down to atmospheric calculations and the type of work being done at the facility, known as the Research Institute of Atomic Reactors. 
 
 Asked about activity at RIAR in the past six months, spokeswoman Anna Volkova told NPR by email that no incidents had occurred at the facility and that "ruthenium was not produced in that period." Russian atomic energy officials have previously said there has been nothing unusual detected at any of their nation's nuclear facilities. 
 
 Nielsen acknowledges that the theory of a Russian origin is an educated guess. "We don't know for sure," he says. But in the absence of more information, this is perhaps the most complete explanation for the radioactive haze he picked up in his filters. 
 
 Enlarge this image toggle caption IRSN IRSN 
 
 Clue No. 1 
 
 Ruthenium-106 has been detected in the atmosphere before. After the Chernobyl nuclear disaster in 1986, the isotope was a part of the radioactive soot that blanketed Europe. The Chernobyl meltdown happened inside the secretive Soviet Union, and some of the first signs there had been an accident were the detection of ruthenium along with other radioactive isotopes. 
 
 This time is different. Starting in late September, detectors throughout Europe began picking up trace amounts of ruthenium — but they didn't pick up other radioactive elements associated with a nuclear power accident. 
 
 "We've never seen ruthenium-106 alone," Nielsen says. "This is the strange thing we see now." 
 
 To see ruthenium-106 without the other isotopes seen inside a reactor suggests that it might have been isolated chemically. 
 
 There is one very particular process that makes ruthenium-106, says Kenneth Czerwinski, a radiochemist at the University of Nevada, Las Vegas. Ruthenium-106 is created as a byproduct during the production of a medical isotope known as molybdenum-99. 
 
 The world needs molybdenum-99. It's an important ingredient used in some medical scans of the heart and other organs. But very few locations actually make it. "There aren't many facilities," Czerwinski says. "There are some reactors in Canada, Belgium, South Africa, Australia." 
 
 And in Russia. In 2013, the state atomic energy corporation ROSATOM announced that RIAR in Dimitrovgrad had successfully begun producing and purifying molybdenum-99. There is a global shortage of the isotope, and Russia has ambitions of becoming a supplier to hospitals all over the world. According to a report by the U.S. National Academies of Sciences, Engineering, and Medicine, it eventually hopes to snag a 20 percent share of the international market. 
 
 To meet Russia's ambitions, RIAR researchers must learn how to make molybdenum-99 according to international standards. It's conceivable they might have made a mistake in that process and released some ruthenium, says Thomas Ruth, an emeritus nuclear chemist at TRIUMF, in British Columbia, Canada. "There is a learning curve," he says. 
 
 But Ruth isn't certain that RIAR was the source. He thinks that other radioactive isotopes might be present if such an accident had occurred. For that reason he thinks an accident involving old nuclear fuel, perhaps at another facility, might be to blame. 
 
 Clue No. 2 
 
 The second clue pointing to Dimotrovgrad has to do with where the ruthenium was found. According to the International Atomic Energy Agency, 43 nations reported detecting ruthenium-106. Scientists with various governments have collected that data, along with weather information, to try to work backward and find the cloud's origin. 
 
 A French analysis released last week suggested the most likely region for the release was somewhere in between the Volga River and the southern Ural Mountains, the precise region where Dimitrovgrad is located. A German statement also pointed to the southern Urals, though it said other areas in the south of Russia could also be candidates. 
 
 Scientists speak in probabilities, and none are prepared to say they know what happened for sure. But many now believe the Research Institute of Atomic Reactors is the "obvious candidate," Nielsen says. "That institute has a number of reactors that do interesting things including isotope production," Czerwinski agrees. 
 
 That being said, RIAR isn't the only nuclear facility in the region. This stretch of Russia is dotted with nuclear facilities. Among them is the Mayak Production Association, once a major nuclear weapons plant that now does things like recycling vast quantities of old nuclear fuel. Mayak has had a spotty safety record over the decades. 
 
 In the end, it's educated speculation. The IAEA has issued a brief statement saying that all member states that detected the ruthenium have also said there were no nuclear incidents in their territory, including in Russia. 
 
 Russia's ROSATOM did not respond to NPR's inquiry, but it issued a statement on Oct. 11 broadly calling the Russian-origin theory untenable. The organization cited radiation monitoring that had showed no ruthenium-106 over Russia itself, except in minute quantities over St. Petersburg. 
 
 Without confirmation of an incident or accident, Nielsen says the true story may never be known. "We're waiting for somebody to step forward and say, 'Yes, we did it.' "
– A nuclear accident happened somewhere in Russia or Kazakhstan during the last week of September and was kept very quiet, the French nuclear safety institute IRSN has concluded after analyzing a mysterious radioactive cloud that appeared over Europe last month. IRSN researchers say they detected high levels of ruthenium 106, a type of atom that does not occur in nature and is only found when atoms are split in reactors, Reuters reports. The cloud was at its peak during the first week in October and dissipated until it could not be detected in France by Oct. 13, according to IRSN, which says its analysis of weather patterns points to a source between the Ural mountains and the Volga river. The institute, which says other European nuclear regulators have made similar findings, has ruled out the crash of a radioactive satellite or an accident at a nuclear reactor as the source of the cloud. It says the most likely source was either a facility dealing with spent nuclear fuel or a facility for radioactive medicine. Russian authorities say they're not aware of any accident in the area, while Kazakh authorities haven't been reached for comment yet, IRSN says. The institute says the cloud itself is harmless and is unlikely to contaminate food, though it says there must have been a very large amount of ruthenium 106 released at the accident site, enough that would have theoretically warranted an evacuation of a radius of several miles.