Nuclear safety after Chernobyl Response from a scientist to Flavin’s earlier view By Richard Wilson In his article “Nuclear Safety after Chernobyl” (Environ. Sci. Tecknol. 1987, 21, 624-25). Christopher Flavin of the Worldwatch Institute makes several statements that would have been better phrased as questions. Had such questions been asked a few weeks after the accident, they would have been considered insightful. But one year later, after a great deal of information has been made available, it is clear that many of Flavin‘s statements are wrong. 1 invite your readers to examine a more accurate account, “A Visit to C h e r n o byl,” written by me and published in the lune 26, 1987, issue of Science. Flavin incorrectly pooh-poohs the argument that RBMK reactors, like the one at Chernobyl, are different from the light-water reactors used in the West, and goes on to say that “both designs have weaknesses,” which is of course true, “and have the same capacity for catastrophic accidents.” This last statement is completely wrong. Flavin asserts that the accidents at Three Mile Island and Chernobyl can be traced back to human mistakes. Although this is partially true, he incorrectly implies that this was the sole cause in each case. He fails to point out two fundamental differences in design that make the consequences of human mistakes very different. The accident at Chernobyl happened within seconds; that at Three Mile Island developed over the course of one hour. Moreover, 99.9999% of the dangerous radionuclides-the cesium and iodine-were contained at Three Mile Island, whereas 20-50% were released at Chernobyl. This was not mere chance, but an important feature of design. Even if the engineers at Three Mile Island had not acted in time to stop a complete meltdown, it is now clear that the containment would probably have held. In discussing containment, Flavin incorrectly states that the “less complete system [of containment with the RBMK reactors] is similar to those used in boiling-water reactors in the United States.” The safety provided by the
Richard Wilson
containments of boiling-water reactors is not as easy to relate to fundamental principles, such as conservation of energy, as the containments of pressurized-water reactors (PWR). But contrary to Flavin’s implication, this does not make them similar to the RBMK reactors. In the RBMK reactors a pressure suppression pool would protect against failure of part of the primary coolant circuit; there is no strong structure around the reactor itself and no protection against the simultaneous failure of many channels. Nor is there a flow path-with a large aperture and therefore a low resistance-to carry steam from the top of the reactor to the pressure suppression pool, In contrast, if the core melts in a PWR, large pipes would convey steam to a pressure sup pression pool where it is condensed; in most cases the containment will hold. Flavin also states that “no one knows whether any containment could have survived the Chernobyl explosion.” We can and have calculated the energy released. Such calculations show that it is probable that a large dry containment of a PWR would have survived the initial explosion. It might have failed later, but there would have been considerable time for coagulation and deposition of radionuclides in particulate form. More important, the issue is barely relevant. A PWR will never have to contain a Chernobyl explosion because that type of explosion is only possible
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in an RBMK reactor. The important issue is whether it will contain the radionuclides in the failure of a PWR, where the pressure rise comes from the accumulated fission-product energy. Flavin points out that “nuclear engineers have identified scenarios in which a nuclear reactor could go quickly and dangerously out of control.” The context implies that he is talking about a reactor losing control as swiftly as in the Chernobyl accident. U.S. designers make every effort to avoid positive power coefficients and, in particular, prompt critical ways in which a reactor can go because of a rapid increase of radioactivity of a percent or more. Such scenarios remain, of course, but they are extraordinarily unlikely. For example, the whole core may drop away from the control and shut-down rods. Mechanical means exist to stop this from happening, even though the probability of such an event is generally considered to be about once in the age of the Earth. This illustrates an important difference between the Soviet safety philosophy and that in the West: In the West we endeavor to discover all possible scenarios and reduce their probability. This procedure, referred to grudingly by Flavin, is called probabilistic risk assessment (PRA). Norman Rasmussen received the Fermi Prize in 1985 for its development. Although Flavin emphasizes the calculation of probabilities, an important advantage of preparing a PRA is that someone thinks through the problem. No PRA has yet been seen for a Soviet reactor, and that is one of the reasons the weaknesses of the RBMK remained unnoticed by the Soviet hierarchy. I was unaware that Gov. Richard Thornburgh of Pennsylvania was told that the TMI has little relevance to the Soviet nuclear program. But Anatoliy Alexandrov, former president of the Soviet Academy of Sciences, said something similar to both Pravda and Isvesria. While lecturing in the U.S.S.R. in 1979, I expressed the hope that safety experts would not take such political statements seriously. Although my plea was not completely heeded, the Envirm. Sci. Technol.. Vol. 21, NO. 11, 1987 1051
Soviets did note the fact that TMI had a containment and their reactors did not. They have not started a new RBMK 1000 since that time. All new starts have been PWRs with containments. Contrary to Flavin’s assertion, steam explosions or superheat explosions have been considered seriously by reactor safety experts at least since the Rasmussen report of 1975. They are now believed less likely to occur than Rasmussen thought, notwithstanding the Chernobyl accident. Although the 1000-ton cover plate of the Chernobyl reactor was probably lifted by the pressure of steam, this was not a steam explosion in the sense used in the West-it was not a superheat explosion. Although the driving force at Chernobyl was steam, it has no relevance to the probability of the superheat explosions we consider. One important way to make a technology safe is to record and analyze every malfunction, because a combination of separate malfunctions occurring simultaneously can lead to an accident. But the mere totaling of recorded malfunctions, which Flavin emphasizes, tells us little. Such totals may only reveal that the recording authorities have been active, as they are increasingly in the United States. It is similar to damning an automobile because the state has demanded an increased number of inspections. In one respect, however Flavin is correct. Present analysis suggests that a core meltdown, or a partial core meltdown, is likely to occur once in every 5000 to 10,000 reactor-years. Many experts believe it is even less likely. as a result of recent safety improvements, but analysis has not justified this confidence. Such estimates suggest that with the I 0 0 reactors present in the United States there might be an accident once every 50 years. This probability, however, refers to an accident with the consequences of Three Mile Island: no immediate casualties, even among the plant workers, and a negligible increase in cancer incidence in the area, comparable to the effect of smoking one or two cigarettes in a lifetime. Our ability as a nation to cope calmly and efficiently with industrial accidents will determine our future as an industrial society. In this respect, I note that it took us six years to restart the undamaged and uncontaminated TMI unit I, whereas the Soviets decontaminated and restarted Chernobyl units 1 and 2 within six months. Chernobyl does bring to light some questions about nuclear safety that the nuclear industry would do well to ponder-and possibly answer. A 1985 study group of the American Physical 1052 Environ. Sci. Technol.. Vol. 21. NO. 11. 1987
Society documented claims that the consequences of accidents in many scenarios are smaller than previously believed. These claims, however, depend in part upon the expectation that radioactive iodine would be released in particulate form and would coagulate and deposit inside the containment, even though the containment may fail at a later time. Yet it is reported that 80% of the Chernobyl iodine measured in Sweden was gaseous. The release conditions are different; at Chernobyl it was almost a dry release. But this needs careful argument and documentation. Although Chernobyl throws little light on the probability of superheat explosions in a light-water reactor, it raises the question of whether, or in what fractions of accidents, such an explosion can occur. Finally, there is the human factor. In 1975 Rasmussen argued-and the accident at Brown’s Ferry (part of TVA) supported his argument-that operators, as thinking human beings, can find ways that were not analyzed to mitigate accidents. However, at Three Mile Island the operators thought hard but made the wrong decision and turned a malfunction into an accident. At Chernobyl it became clear that oper-
ators can, and will if it is convenient for them, break rigid safety rules. With an RBMK the effect was catastrophic. I believe that the design of light-water reactors makes such rule-breaking less serious, but I know of no detailed study. Flavin is also correct in his “motherhood” statement that “there must be greater preparation for the consequences of the inevitable failures.” But it would be wrong to evacuate people when it is unnecessary, such as when a dose of only 1-5 rem is anticipated. It also would be wrong and counterproductive if undue emphasis on nuclear accidents prompted society to revert to more dangerous technologies such as burning coal. Flavin’s argument applies with much greater force to most technologies other than nuclear power, whether they involve grade-crossing accidents as the coal train comes rumbling past, or the midair collision of two loaded aircraft. Any institute that claims to watch the world should pay special attention to this.
Richard Wilson is Mallinrkrmfr Profeww U I rhe Deparrmenr of Pliy.wcs and Energy and Eni,ironmenral Polirv Center ai Harvard Univerrirr of Physics
Estuaries and coastal waters need help By Howard Levenson For years, our marine environments estuaries, coastal waters, and the open ocean - have been used extensively by coastal communities and industries for the disposal of various wastes. The term disposal includes two types of activities: dumping and discharging. Dumping occurs when wastes, such as municipal sewage sludge, certain industrial sludges and slurries, and
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dredged material, are transported by ships or barges to designated marine sites and dropped overboard. Discharging involves the release of wastewater from municipal and industrial facilities through pipelines. Historically, marine waste disposal has been relatively cheap and has solved some short-term waste-management problems; however, its conseauences include a general trend toward ehronmental degradation, particularlv in estuaries and coastal waters. A r e p k t on “Wastes in Marine Environments,” released by the congressional Office of Technoloev Assessment (OTA), documents thgtrend in detail (I). The report concludes that, without additional protective measures, the next few decades will witness new or continued degradation in many estuaries and some coastal waters around the country, even in some that exhibited improvements in the past. The extent of current degradation varies greatly around the country. High levels of organic chemicals, metals,
