A Chernobyl in the US? Not too likely, say scientists. Reactor containment buildings and US design reduce the risk
Santa Barbara, Calif.
Nuclear-power safeguards adopted in the United States have made the chance of a Chernobyl-type accident happening here small, although there is room for improvement and no guarantee that a major accident won't happen. There are 100 commercial reactors in the US that produce electricity. The government operates another eight reactors to make plutonium for nuclear warheads and for research purposes.Skip to next paragraph
Subscribe Today to the Monitor
The Chernobyl reactor produces electricity but its design is substantially different most from US nuclear power plants. In American designs the atomic chain reaction automatically stops when the cooling water is removed. In the Soviet reactor this is not the case. Instead, loss of water increases the intensity of the chain reaction, adding substantially to the danger of overheating.
Another difference is the Soviet reactor's use of graphite. Only one commercial and several government reactors use this material in the US. If graphite gets hot enough, it will burn like coal and is extremely difficult to put out. Also, when graphite is exposed to steam, it can produce a highly flammable gas.
Graphite's flamability would not have been such a liability if the Chernobyl reactor had been covered by a confinement dome, as are all commercial reactors in the US. If the Soviet reactor had been similarly equipped, it would likely have been able to withstand the explosion and smother the fire by cutting off its air supply. It would also have bottled up most of the radioactivity released by the accident, US nuclear experts argue.
(The US government's plutonium production reactors, run by the Department of Energy, do not have containment structures. But they are operated in a much different manner from power reactors and their safety features are appropriate for the manner in which they are operated, Department of Energy officials say. Nevertheless, safety inspections of at least one of these reactors have reportedly been initiated as a result of the accident.)
According to Robert Bernero of the Nuclear Regulatory Commission (NRC), the decision 30 years ago to require containment structures represents a major, philosophical difference between the US and the Soviet programs. ``We decided that the risk of an accident that would destroy part of a reactor's core and so release substantial quantities of radioactive material posed a serious public danger. So we decided to enclose them in airtight `containment' structures,'' he explains.
These structures were planned using something called the ``design basis accident.'' The idea is similar to the 50- or 100-year storms and floods used for designing bridges and dams. One of the most severe accidents experts could dream up was chosen, its consequences conservatively estimated, and the containment designed to withstand them with a prescribed safety margin.
This approach proved adequate for the 1979 accident at Three Mile Island (TMI). Although the core was destroyed and large amounts of radioactivity were released from the reactor, only minute amounts were measured in the environment. As a result, no one was seriously injured.
However, this design approach came in for severe criticism. A reactor's safety features might not be as effective against other kinds of accidents, its critics argued. In recent years, nuclear safety analysts have been trying to physically model a range of severe accidents. This is known as ``source term'' research and, according to NRC's Bernero, it is showing that the safety margins in containment structures ``are very large, but irregular.''
Until the 1979 accident, the Soviets had not thought containment buildings necessary. Their basic philosophy, explains Don Winston of the Atomic Industrial Forum, was that if you design the reactor right and build all the parts with sufficient care, then major accidents should not occur. It was a view shared by many in the US nuclear industry who felt reactors were so well designed that a serious accident was virtually impossible. But TMI punctured this conceit.
While events have shown that the US course was the prudent one, knowledgeable observers generally agree that, sooner or later, a US reactor will have another serious accident, one in which a significant portion of the reactor core melts.
``This is a dangerous technology,'' comments Bruce Babbitt, governor of Arizona and a member of the presidential commission that reviewed the TMI accident. ``It is more likely than not that there will be a core meltdown in the US within the next 15 to 100 years. That is the reality we have to deal with,'' he comments.
Whether such an incident, should it happen, bears a greater resemblance to Three Mile Island or Chernobyl will depend primarily on the adequacy of the reactor's containment systems.
Although more work remains to be done, recent source-term research strongly suggests that current containment systems have a greater margin of safety than had been previously estimated. Last year, the American Physical Society issued the first independent overview of this work. Chairman of the study Richard Wilson, of Harvard University, characterized it as ``slow-developing good news for nuclear safety.''