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Getting critical

作者:牧傍旒    发布时间:2019-03-07 08:05:05    

By Rob Edwards AT 10.50 am on Tuesday, 17 June 1997, Aleksander Zakharov made a fatal mistake. Alone, behind concrete walls three metres thick in an underground bunker at the secretive Arzamas-16 nuclear research centre in Russia, he placed a thin shell of copper on a sphere of highly enriched uranium. Suddenly, a huge burst of radiation turned the air blue as the uranium went critical. Zakharov, an internationally respected expert on nuclear criticality, would have realised immediately what had happened. He left the bunker, closed its hatch, notified a manager and lost consciousness. Three days later, aged 42, he died in a Moscow hospital, his nervous system destroyed by the radiation. Before the accident at the uranium processing plant at Tokaimura in Japan on 30 September this year, Zakharov was the most recent person to die in what the nuclear industry euphemistically calls an “excursion”—an uncontrolled chain reaction that happens when uranium-235 or plutonium-239 atoms split. Scientists know how to use fission to drive nuclear reactors with fuel pellets enclosed in zirconium alloy sheaths. But in fuel-processing plants such as Tokaimura, where people are handling the materials, the opportunity for disaster is much greater. Two new international studies show that mistakes made in the past three years in Russia and Japan mirror those made decades ago during the Cold War, suggesting that nuclear engineers have learnt nothing over the years. “That is the tragedy,” says Enrico Sartori, a criticality analyst from the OECD’s Nuclear Energy Agency (NEA) near Paris, which co-ordinates international nuclear policy. “The technology is difficult and each generation has to relearn it.” The day the Tokaimura accident ended, the French nuclear safety agency, IPSN, coincidentally published an authoritative analysis of the world’s accidental criticalities. Including Tokaimura, 17 people have been killed and 104 more irradiated by 60 accidents since 1945. Of these accidents, 33 happened in the US, 19 in Russia, two in Canada and one each in Britain, France, Belgium, Yugoslavia, Argentina and Japan. Because the history of Soviet criticalities was a secret until now, the IPSN report comes as a big surprise. Accidents happened regularly in the 1950s and 1960s, but although the frequency of incidents decreased as expertise increased, they still kept on happening. There were five in the 1970s, including three in the former Soviet Union—one in the 1980s, two in 1997 (both in Russia) and now one in Japan in 1999. “We didn’t expect that,” observes Sartori. “We almost expected them to stop.” A forthcoming study for the US Department of Energy points out that Zakharov’s accident bore remarkable similarities to two events in the US more than half a century earlier. While constructing the experimental critical assembly, Zakharov seems to have miscalculated the critical mass of uranium in the configuration he was using. Copper played the crucial role of reflecting neutrons from splitting uranium-235 atoms back into the uranium sphere to trigger the chain reaction. In a paper to be published in the November issue of Health Physics (vol 77, p 505), George Vargo from the Pacific Northwest National Laboratory in Richland, Washington, compares this accident to two at Los Alamos National Laboratory, New Mexico, during the Manhattan Project to build the atomic bomb. In one accident in 1945, Harry Daghlian was killed after he caused a criticality in a sphere of plutonium by dropping a block of tungsten on top of it. The following year, Louis Slotin suffered the same fate when a screwdriver he was using to stop a beryllium shell from covering a sphere of plutonium slipped. Like the copper at Arzamas-16, the tungsten and the beryllium reflected neutrons back into the plutonium spheres and started the chain reaction. But it is not only neutron reflectors that nuclear engineers have to be wary of. In an analysis of 13 other previously unreported criticality accidents at nuclear plants in Russia, Vargo concludes that nine of them could have been prevented by using containers whose shape made it impossible for fissile uranium or plutonium to form critical masses. Cylinders and pipes, he says, should be long and thin rather than short and fat. Unfortunately, determining critical masses is not that simple. It depends not only on the availability of neutron reflectors and the shape of the fissile material but also on the ratio of its isotopes. According to the NEA, the critical mass of uranium enriched so that 3 per cent of it contains the uranium-235 isotope is 101 kilograms in an aqueous solution. The critical mass of 20 per cent enriched uranium in the same solution, however, is just 5.39 kg. The presence of moderators, which slow down escaping neutrons and make them more likely to cause further fissions, is also important. Vargo points out that water was a moderator in 11 of the Russian accidents, and its role as both a moderator and a reflector was key in the Tokaimura accident (New Scientist, 9 October 1999, p 4). “Any use of water in the processing of fissile material has to be carefully engineered and controlled,” he argues. But Vargo’s account shows how difficult it is to plan for every eventuality. In one of the seven criticalities at the Mayak nuclear complex in the Urals in 1957, he describes how three workers died after a tank of highly enriched uranium solution they were trying to empty went critical. The reaction started because the workers themselves became “human neutron reflectors”. The IPSN study also points out that 20 of the 21 criticalities that have occurred worldwide in nuclear facilities other than reactors involved plutonium or uranium in solution. “You have to be more careful when dealing with solutions because they can change their shape,” warns Cassiano De Oliveira, a physicist investigating criticality at Imperial College in London. Many of the accidents were caused, as at Tokaimura, by excessive amounts of solutions being mixed together. The effects of last month’s accident on public health is also a matter for concern. For nearly 11 hours after the criticality, 4.5 millisieverts per hour of radiation washed over the nearest house 100 metres from the fabrication plant. People exposed to this would have received 10 times as much in one day as nuclear power workers might ordinarily expect to receive in one year. Exactly what happened at Tokaimura a month ago is still uncertain. But Western experts investigating the accident are now coming to the conclusion that poor management and organisation were as much to blame as occasional safety breaches. London-based independent nuclear consultant John Large is scornful of suggestions that the Tokaimura accident resulted from inexperienced workers slopping buckets around. “The plant is under International Atomic Energy Agency supervision; there were even thought to be IAEA inspectors on site at the time of the accident.” He adds: “Whenever the nuclear industry has an accident or needs to pass the buck they say the people involved were inexperienced. But even if that is the case, surely it’s their responsibility to ensure sufficiently experienced staff are doing the job in the first place.” Publicly, nuclear operators outside Japan make politely critical remarks about the lack of training and the need for better safety procedures. Privately, however, they are scathing. “Management broke all the rules,” says one. “It was criminal.” The police are investigating whether such assertions are justified. Regardless of their investigation, it seems inevitable that nuclear safety agencies will have to take on board the lessons from Tokaimura. After the Japanese accident, the Nuclear Inspectorate quizzed the operators of all the British sites that handle fissile material, including Sellafield, the Aldermaston nuclear weapons establishment in Berkshire and the Dounreay nuclear plant in Scotland, to make sure they were safe. Elsewhere around the world, nuclear regulators are making similar checks, all hoping the next nuclear excursion will not be in their back yards. As if to underline the problem,

 

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