Repeating History

by Melissa Lovin

Following two recent accidents in Japan and Russia causes some concern over why history seems to be repeating itself.

On June 17, 1997 Aleksander Zakharov, an internationally respected expert on nuclear criticality, made a fatal mistake. Alone in the secretive Arzamas-16 research center in Russia, he placed a thin shell of copper on a sphere of highly enriched uranium resulting in a huge burst of radiation turning the air blue as the uranium went critical. Zakharov, knowing the severity of the experiment gone awry, immediately left the bunker, sealed off the facility, notified a manager and lost consciousness. He died in a Moscow hospital three days later, his nervous system destroyed by the radiation. His was the most recent nuclear accident until Tokaimura.

The near catastrophic accident on September 30, 1999 at Japan's Tokaimura uranium processing plant near Tokyo was merely the latest in a long chain of nuclear accidents dating back to the 1940s. 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 higher.

These two accidents are so similar in nature to some that occurred during the Cold War suggesting that it is possible nuclear engineers have learned nothing over the years, according to two recently released international studies. But should these scientists have learned from their mistakes or, at the very least, been more careful? The answers are not always simple. According to Enrico Sartori, a criticality analyst from the OECD'a Nuclear Energy Agency (NEA) near Paris which coordinate international nuclear policy, "That is the tragedy. The technology is difficult and each generation has to relearn it."

Purely by coincidence, the French nuclear safety agency, IPSN, published an analysis of the world's accidental criticalities on the day the Tokaimura accident ended. Including Tokaimura, 17 people were fatally injured and 104 more irradiated by 60 accidents since 1945. Of these accidents, 33 happened in the U.S., 19 in Russia, two in Canada, and one each in Britain, France, Belgium, Yugoslavia, Argentina and Japan.

This report comes as a huge surprise to some due to the fact that the history of the Soviet Union's criticalities was a secret until now. Accidents happened fairly regularly in the 1950s and 1960s, but although their frequency decreased as the expertise increased, they still continued to occur. Including three incidents in the former Soviet Union, there were five incidents in the 1970s, one in the 1980s, two in 1997 (both of which were in Russia), and now one in Japan in 1999. Most experts indicate that they did not expect the accidents to continue. On the contrary, they expected them to stop altogether.

According to a study to be released some time in 2000 by the US Department of Energy, Zakharov's accident was very similar to two events that occurred in the US more than half a century earlier. Zakharov seems to have miscalculated the critical mass of uranium while constructing his experiment. The copper played a critical role of reflecting neutrons from splitting uranium-235 atoms back into the uranium sphere to trigger the chain reaction.

This accident has been compared 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. In the following year, Louis Slotin fell victim to the same fate when a screwdriver he was using to stop a beryllium shell from covering a sphere of plutonium slipped. Like the copper in Zakharov's study, the tungsten and the beryllium reflected neutrons back into the plutonium spheres and started the chain reaction.

But, according to George Vargo from the Pacific Northwest National Laboratory in Richland, Washington, neutron reflectors are not the only thing nuclear engineers have to be wary of . In his analysis of 13 other incidents in Russia previously unreported, 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. He says that the cylinders and pipes should be long and thin, not short and fat.

Unfortunately, it is not that simple to determine critical mass. 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. The presence of moderators, which slow down escaping neutrons and make them more likely to cause further fissions, is also important. According to Vargo, water was a moderator in 11 of the Russian incidents, and its role as both moderator and reflector was critical in the Tokaimura accident.

But Vargo also shows how nearly impossible it is to plan for every eventuality. The IPSN study shows that 20 of 21 criticalities that have occurred worldwide in nuclear facilities other than reactors involved plutonium or uranium in solution. Many of these accidents were caused by excessive amounts of solutions being mixed together, as it was at Tokaimura.

The health effects of last month's accident are also a major cause of concern. For nearly 11 hours after the criticality, 4-5 millisieverts per hour of radiation washed over the nearest house 100 meters from the plant. People exposed to this much radiation would have received 10 times as much as a nuclear power worker would expect to receive in one year.

While it is not known exactly what happened at Tokaimura in September most experts involved in the investigation have concluded that poor management and organization as well as safety breaches are to blame. To say that operator error was all that occurred is to neglect the fact that it was still the responsibility of the management to put safe procedures in place and insure that they are followed properly.

All over the world the effects of Tokaimura and those that have gone into the history books before it will be felt for some time. Safety procedures are being looked at once and again to ensure that an accident like this will not happen again, but is it enough. Will history continue to repeat itself or have we finally learned our lesson in safety?