Alarming events may not be what they initially seem. When an enormous explosion created a mushroom cloud over Beirut on August 4, some people immediately jumped to the wrong conclusion, spreading rumors on social media that a nuclear bomb had gone off. It hadn’t.
Eventually it became clear that the explosion was caused by chemicals stored improperly in warehouses at Beirut’s port. But weapons experts knew from the start that the powerful explosion was not nuclear, because it did not produce a blinding flash of light, or a blast of heat intense enough to set a city on fire. In the hours that followed the explosion, the Comprehensive Test Ban Treaty Organization (CTBTO), which operates a network of monitoring stations around the world, did not detect a telltale spike in atmospheric radiation.
That last clue is how experts are able to narrow down the location of nuclear events, from the smallest accidents to major disasters like Chernobyl, and to make educated guesses about what happened. A release of nuclear material spreads its signature on the wind. But that signature is often incomplete or garbled.
Nuclear experts are still puzzling over a mysterious event that happened in June, when several monitoring stations in northern Europe detected extremely small quantities of radionuclides in the atmosphere. That event was not a nuclear weapons test, because the CTBTO stations did not detect any seismic activity. So what was it? Experts have scrutinized the radiation signature and narrowed down the possibilities. The finger points to Russia.
Radiation alerts. Radioisotope monitoring stations cover most of the globe. The CTBTO runs the biggest network. National radiation safety agencies—for example, in Finland and Sweden—operate other stations. Universities also operate monitoring stations, often in cooperation with the CTBTO or national agencies. Independent monitoring organizations, like the volunteer-driven Safecast, also report radiation measurements.
In early June, Norwegian monitoring stations and a CTBTO station detected iodine 131 in far northern Norway. On June 16 and 17, Finland’s Radiation and Nuclear Safety Authority (STUK) detected cobalt 60, ruthenium 103, cesium 134, and cesium 137 in Helsinki. On June 22 and 23, a CTBTO station in Sweden detected ruthenium 103, cesium 134, and cesium 137.
Radiation is easy to detect at low levels. The iodine 131 readings were around 1 microbecquerel per cubic meter of air. (A microbecquerel is one atomic disintegration per second in one million cubic meters of air.) But our knowledge of the Chernobyl explosion began with measuring small amounts of radionuclides, so any detection raises an alert.
Interpreting the detections. The types of radionuclides detected also provide information. The radionuclides detected in June, except cobalt 60, are produced by nuclear fission. The half-lives of iodine 131 and ruthenium 103 are 8 days and 39 days, respectively, so they must be from recent fission events. These are common fission products from a nuclear reactor. A wider suite of radionuclides would help to pin down what kind of reactor.
The iodine 131 detection is ambiguous, though. It is produced by fission, but it is also used fairly commonly to treat hyperthyroidism, in pet cats as well as people. It is easily sent into the air. So it may come from sewage plants or other sources. The fact that it showed up without the other fission products means that its source may be something other than a nuclear accident.
Cobalt 60 is not a fission product, but rather an activation product of steel that has been in or very close to a nuclear reactor. It doesn’t usually show up with fission products. It could mean that something was broken in the reactor that released the fission products, or it could be that the Finnish reading was in error.
Narrowing down the location. Airborne radionuclides by themselves cannot tell us how or precisely where they were released. The CTBTO tweeted a map indicating the region in northern Europe where the June release may have occurred. This map was probably constructed by tracing the winds during the period just before the radionuclides were detected. A couple of Russian nuclear power plants are located in the area identified, which also covers the location where the United States believes a Russian experimental reactor exploded last summer while it was being raised from the seabed.
Last month’s release was probably a minor incident, like a breach in a filter at a nuclear power plant. But Russia has said that there were no incidents at its nuclear power plants. The presence of cobalt 60 and the location have led some to suggest that the release may have been from a new attempt to raise that experimental reactor.
It’s not possible, with the limited additional data available so far, to do more than guess the origin of the June radionuclides. The source of a similarly ambiguous release of ruthenium 106 in 2017 took two years to identify. In that case, the release of a single fission product suggested a processing facility, and the wind patterns suggested the Mayak facility in Russia. A detailed study of the stable ruthenium isotopes collected with the ruthenium 106 confirmed those early provisional conclusions.
Pinning down the source. Last year’s deadly accident seems to have occurred when a reactor for an experimental nuclear-powered cruise missile was being lifted from the seabed. Again, radionuclide readings from monitoring stations some distance from the source gave early warning to the rest of the world. The amounts and numbers of radionuclides were larger than in the recent release, and reporting on the accident, as people were brought to hospitals, provided more information, including radioisotope detection closer to the site, along with satellite photos and, later, photos of the damaged barge. The radioisotopes indicated that a fission source was involved, although early reports from Russia described an “isotopic source,” a phrase usually reserved for single-isotope heat sources, which would be unsuitable for propulsion.
In all three of these cases over the past three years, the radionuclides initially detected by air sampling were ambiguous. Additional information was needed to pin down a source. The best information would come from the country responsible for the release – in all three cases, Russia, which is a signatory to the Convention on Early Notification of a Nuclear Accident.
Somebody knows what happened in these cases. We need to hear from them.
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