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A far more serious accident occurred seven years later at Chernobyl, in what was then still the Soviet Union. At the time of the accident--April 26, 1986--the Chernobyl nuclear power station consisted of four operating 1,000-megawatt power reactors sited along the banks of the Pripyat River, about sixty miles north of Kiev in the Ukraine, the fertile grain-producing region of the southwestern USSR. A fifth reactor was under construction.
All the Chernobyl reactors were of a design that the Russians call the RBMK--natural uranium-fueled, water-cooled, graphite-moderated--a design that American physicist and Nobel laureate Hans Bethe has called "fundamentally faulty, having a built-in instability." Because of the instability, an RBMK reactor that loses its coolant can under certain circumstances increase in reactivity and run progressively faster and hotter rather than shut itself down. Nor were the Chernobyl reactors protected by containment structures like those required for U.S. reactors, though they were shielded with heavy concrete covers.
Without question, the accident at Chernobyl was the result of a fatal combination of ignorance and complacency. "As members of a select scientific panel convened immediately after the...accident," writes Bethe, "my colleagues and I established that the Chernobyl disaster tells us about the deficiencies of the Soviet political and administrative system rather than about problems with nuclear power."
The immediate cause of the Chernobyl accident was a mismanaged electrical-engineering experiment. Engineers with no knowledge of reactor physics were interested to see if they could draw electricity from the turbine generator of the Number 4 reactor unit to run water pumps during an emergency when the turbine was no longer being driven by the reactor but was still spinning inertially. The engineers needed the reactor to wind up the turbine; then they planned to idle it to 2.5 percent power. Unexpected electrical demand on the afternoon of April 29 delayed the experiment until eleven o'clock that night. When the experimenters finally started, they felt pressed to make up for lost time, so they reduced the reactor's power level too rapidly. That mistake caused a rapid buildup of neutron-absorbing fission by products in the reactor core, which poisoned the reaction. To compensate, the operators withdrew a majority of the reactor's control rods, but even with the rods withdrawn, they were unable to increase the power level to more than 30 megawatts, a low level of operation at which the reactor's instability potential is at its worst and that the Chernobyl plant's own safety rules forbade.
At that point, writes Russian nuclear engineer Grigori Medvedev, "there were two options: increasing the power immediately, or waiting twenty-four hours for the poisons to dissipate. [Deputy chief engineer Dyatlov] should have waited...But he [had an experiment to conduct and he] was unwilling to stop...He ordered an immediate increase in the power of the reactor." Reluctantly the operators complied. By 1 a.m. on April 26, they stabilized the reactor at 200 megawatts. It was still poisoned and increasingly difficult to control. More control rods came out. A minimum reserve for an RBMK reactor is supposed to be 30 control rods. At the end, the Number 4 unit was down to only six control rods, with 205 rods withdrawn.
The experimenters allowed this dangerous condition to develop even though they had deliberately bypassed and disconnected every important safety system, including the emergency core-cooling system. They had also disconnected every backup electrical system, down to and including diesel generators, that would have allowed them to operate the reactor controls in the event of an emergency.
At 1:23 in the morning, the engineers proceeded with their experiment by shutting down the turbine generator. That reduced the electrical supply to the reactor's water pumps, which in turn reduced the flow of cooling water through the reactor. In the coolant channels within the graphite-uranium fuel core, the water began to boil.
Graphite facilitates the fission chain reaction in a graphite reactor by slowing neutrons. Coolant water in such a reactor absorbs neutrons, thus acting as a poison. When the coolant water in the Number 4 Chernobyl unit began turning to steam, that change of phase reduced its density and made it a less effective neutron absorber. With more neutrons becoming available and few control rods inserted to absorb them, the chain reaction accelerated. The power level in the reactor began to rise.
The operators noticed the power surge and realized they needed to reduce reactivity quickly by inserting more control rods. They hit the red button of the emergency power-reduction system. Motors began driving all 205 control rods as well as the emergency protection rods into the reactor core.
But the control rods had a design flaw that now proved deadly: their tips were made of graphite. The graphite tips attached to a hollow segment one meter (3.28 feet long), which attached in turn to a five-meter absorbent segment. When the 205 control rods began driving into the surging Number 4 reactor, the graphite tip went in first. Rather than reduce the reaction, the graphite tips increased it. The control rods displaced water from the rod channels as well, increasing reactivity further. All hell broke loose--The reactor exploded.
The explosion was chemical, driven by gases and steam generated by the core runaway, not by nuclear reactions; no commercial nuclear reactor contains a high enough concentration of U-235 or plutonium to cause a nuclear explosion. Medvedev, who had once worked at Chernobyl and who was on the scene within days, describes the explosion from the testimony of eyewitnesses.
Flames, sparks, and chunks of burning material went flying into the air above the Number 4 unit. These were red-hot pieces of nuclear fuel and graphite, some of which fell onto the roof of the turbine hall where they started fires...About 50 tons of nuclear fuel evaporated and were released by the explosion into the atmosphere...In addition, about 70 tons were ejected sideways from the periphery of the core, mingling with a pile of structural debris, onto the roof...and also onto the grounds of the plant...
Some 50 tons of nuclear fuel and 800 tons of reactor graphite...remained in the reactor vault, where it formed a pit reminiscent of a volcanic crater. (The graphite still in the reactor burned up completely in the next few days.)
Coolant water in such a reactor absorbs neutrons, thus acting as a poison. When the coolant water in the Number 4 Chernobyl unit began turning to steam, that change of phase reduced its density and made it a less effective neutron absorber. With more neutrons becoming available and few control rods inserted to absorb them, the chain reaction accelerated. The power level in the reactor began to rise.
The resulting radioactive release, Medvedev estimates, was equivalent to ten Hiroshimas. In fact, since the Hiroshima bomb was an airburst--no part of the fireball touching the ground--the Chernobyl release polluted the countryside much more than ten Hiroshimas would have done.
No commercial reactor in the United States is designed anything like the RBMK reactor. Cohen summarizes several of the differences:
1. A reactor which is unstable against a loss of water could not be licensed in the United States.
2. A reactor which is unstable against a temperature increase could not be licensed here.
3. A large power reactor without a containment [structure] could not be licensed here.
The absence of a containment structure is especially important. As Cohen point out about Chernobyl, "Post-accident analyses indicate that if there had been a U.S.-style containment, none of the radioactivity would have escaped, and there would have been no injuries or deaths."
But if the design of Russian and U.S. reactors is critically different, broad similarities between the two countries' management of nuclear-power development led both national programs into difficulty. In the U.S.S.R., writes Medvedev, "the ordinary citizen was made to believe that the peaceful atom was virtually a panacea and the ultimate in genuine safety, ecological cleanliness, and reliability." He quotes Soviet scientists and managers who waxed as enthusiastic in the heyday of nuclear power development as the U.S. AEC's Lewis Strauss. "Nuclear power stations are like stars that shine all day long!" academician M.A. Styrikovich claimed in 1980. "We shall sow them all over the land. They are perfectly safe!" The deputy head of the State Committee on the Utilization of Nuclear Energy, notes Medvedev, told the Soviet people that "nuclear reactors are regular furnaces, and the operators who run them are stokers"--an image corresponding to the glib coinage in the United States that nuclear power is "just another way to boil water."
Given such uninformed enthusiasm for technology, it isn't surprising that both the Soviet and U.S. nuclear power programs ran into difficulties, or that the difficulties in both cases were predominantly managerial. Nuclear power came to terrible disaster in the former Soviet Union because authority dominated there to the exclusion of informed technical discussion and judgment. "Accidents," writes Medvedev, "were hidden not only from the general public and the government but also from the people who worked at Soviet nuclear power stations. This latter fact posed a special danger, as failure to publicize mishaps always has unexpected consequences: it makes people careless and complacent."
Authority dominated in the early days of nuclear power in the United States as well. "The AEC and the JCAE," James Jasper notes, "placed themselves outside normal political accountability." Fortunately, both public and private sectors of the U.S. nuclear power industry learned the lessons of Three Mile Island and launched a major effort of improvement and regulation.
Three Mile Island and Chernobyl represent extreme instances of the problem that seems to trouble the American public more than any other about commercial nuclear power: its apparent danger. But risk is always relative. Friend and foe have estimated the relative risk of operating commercial nuclear power plants in the United States; their conclusions are instructive.
The most serious example of public exposure to radiation from a nuclear power plant is, of course, Chernobyl. The explosion at Chernobyl blew radioactive gas and dust high into the atmosphere, where winds dispersed it across Finland, Sweden, and central and southern Europe. "The sum if [Chernobyl] and exposures to people all over the world," writes Bernard Cohen, "will eventually, after about fifty years, reach 60 billion millirems, enough to cause about 16,000 deaths." (Millirem-mrem-is a measure of radioactivity; 1 mrem is estimated to increase one's risk of dying from cancer by about 1 in 4 million, corresponding to a reduction in life expectancy of about 2 minutes.) Cohen, a professor of physics and radiation health at the University of Pittsburgh, was responsible in the late 1980s for supervising the measurement of radon levels in some 350,000 U.S. homes. He puts Chernobyl's danger in context by pointing out that 16,000 deaths caused every year by air pollution from coal-burning power plants in the United States alone.
The rest of the world didn't choose to be irradiated by a badly designed and criminally misoperated Soviet nuclear power plant. Cohen's comparison is instructive but inappropriate. On the other hand, nuclear power serves useful purposes in the United States, and millions of Americans willingly buy the electricity that nuclear utilities generate. It ought to be appropriate to put nuclear-generated electricity in the context of other acceptable risks Americans take in the name of productivity, comfort, and convenience. Cohen does so, to startling effect:
Everything we do involves risk...There are dangers in every type of travel, but there are dangers in staying home--25 percent of all fatal accidents occur there. There are dangers in eating--food is one of the most important cause of cancer and of several other diseases--but most people eat more than is necessary. There are dangers in breathing--air pollution probably kills 100,000 Americans each year, inhaling radon and its decay products is estimated to kill 14,000 a year, and many diseases like influenza, measles, and whooping cough are contracted by inhaling germs...There are dangers in working--12,000 Americans are killed each year in job-related accidents, and probably ten times that number die from job-related illnesses--but most alternatives to working are even more dangerous. There are dangers in exercising and dangers in not getting enough exercise. Risk is an unavoidable part of our everyday lives.
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