The botched production of a powerful neutrino source is the most likely cause of a radioactive cloud that enveloped much of Europe in the autumn of 2017. That is the conclusion of a group of radiation experts from across the continent who have used isotope monitoring and chemical analysis to try and understand where the leak came from. The researchers think that the isotope involved – ruthenium-106 – was probably released during an accident, possibly an explosion, involving spent fuel at the Mayak reprocessing plant in southern Russia.
Aerosols containing ruthenium-106 were detected in countries as far apart as Greece and Norway at the end of September and beginning of October 2017. The radioactive substance, not found in nature, was spread too thinly to pose a risk to public health but its geographical spread pointed to a major release somewhere. According to the latest work, published in the Proceedings of the National Academy of Sciences, the leak might have come about after workers at Mayak had been trying to make an extremely intense source of cerium-144 for a neutrino experiment in Italy.
Sourcing the leak
The Mayak plant, located in the southern Urals, has a long and chequered history. Built after the Second World War to produce plutonium for the Soviet Union’s atomic bomb programme, it was the site of one of the world’s worst nuclear disasters in 1957 when a chemical explosion in a radioactive waste tank spread vast quantities of radionuclides over a large area. According to Georg Steinhauser of Leibniz Universität in Hanover, Germany, the incident in 2017 would be second only to that in terms of the amount of ruthenium-106 released – some 250 tera becquerels.
Mayak came under suspicion after the French Institute of Radioprotection and Nuclear Security (IRSN) said in October 2017 that after feeding air-sampling data and weather patterns into a computer model it found that the radioactive cloud probably originated in the southern Urals. The Russian meteorology agency Roshydromet then said it had detected ruthenium-106 in the same area in late September. Russian authorities, however, denied that Mayak was the source of the leak.
For a worker who is standing in the plume that would mean very, very unpleasant doses
Georg Steinhauser
Yet in early December 2017 representatives of the plant told scientists from the Gran Sasso National Laboratory in central Italy that they were having difficulties making the cerium-144 source. It was designed to allow the Borexino detector, housed at Gran Sasso, to search for hypothetical particles known as sterile neutrinos. The source had to be extremely radioactive – emitting at least 3.7 peta becquerels – but small enough to function as a powerful point source for antineutrinos. Mayak was the only facility capable of producing it, but in the end was unable to deliver a high enough density of cerium-144 (within stable cerium). So in February 2018, the project’s funders – the Italian National Institute of Nuclear Physics and the French Atomic Energy Commission – axed the experiment, known as SOX.
Following the clues
According to Steinhauser, who led the latest research alongside Olivier Masson of the IRSN, there is good reason to think that the failed fabrication of the SOX source was responsible for the release of ruthenium-106. One of the most important clues, he says, is the age of the ruthenium, and by implication, the age of the spent fuel used to extract the cerium.
Spent fuel is usually left for at least three years to cool down after being taken out of a reactor before it is reprocessed. But a comparison of the radioactivity of the ruthenium-106 with that of a shorter-lived isotope (ruthenium-103) detected by a small number of European labs showed that the spent fuel involved in the leak had been removed less than two years earlier. Mayak had taken that very unusual step, Steinhauser suggests, to ensure that the material was as radioactive as possible. “They were forced to reduce the cooling time to squeeze in as much cerium-144 as the Italians wanted,” he says. “That is still a hypothesis but it makes perfect sense.”
Handling such young spent fuel would have carried significant risks, according to Steinhauser. The higher levels of ionising radiation would have affected the chemical reactions involved in reprocessing, while the extra heat generated by the spent fuel could have warmed gaseous ruthenium tetroxide, which is generated during reprocessing, to the point where it exploded.
Steinhauser and his student Dorian Zok have carried out experiments that suggest an explosion might well have taken place inside the Mayak complex. The ruthenium dioxide produced by highly reactive tetroxide gas within the atmosphere is very insoluble, he explains, but half of the material collected by air filters dissolved in water. That means the aerosols probably contained several compounds including perhaps ruthenium chloride, which can be formed during reprocessing to stabilise the gas by bubbling it through hydrochloric acid. “This is only speculation,” he says, “but ruthenium chloride needs higher temperatures to vaporise. And that goes hand in hand with an explosion or a fire.” It’s also possible that people working in the plant were killed, reckons Steinhauser. “For a worker who is standing in the plume that would mean very, very unpleasant doses,” he says. Controversial Italian neutrino experiment gains support online
The Russian authorities themselves set up a commission to investigate the leak. Comprising scientists from several European countries, it met in January and April last year but there was disagreement between its Russian and non-Russian members over the location of the source. Panel organiser Leonid Bolshov of the Russian Academy of Sciences says he is unconvinced by the latest research, arguing it ignores soil measurements taken around the Mayak plant that he says showed a “low level of contamination”. There is, he argues, “no reason to revise” the panel’s conclusions.
However, Steinhauser and colleagues have now dismissed two hypotheses considered by the panel as alternatives to that of an accident at Mayak. They argue that the ruthenium-106 could not have been released by a nuclear-powered satellite burning up in the atmosphere, given that its half life – 372 days – is too short to power a satellite over its expected lifetime, while pointing out that no satellite appears to have gone missing at the time of the radioactive release. They also say that what appeared to be very high levels of ruthenium in Romania at the end of September 2017 were due to the way air was sampled, rather than actual concentrations.
