By Zhores A. Medvedev

Norton. 352 pp. $24.95

SHORTLY AFTER 1 a.m. on April 26, 1986, Leonid Toptunov, a young, tired and inexperienced reactor operator conducting a safety test, moved the control rods of Chernobyl Reactor No. 4 to only sightly below the correct position. Thirty-six seconds after the beginning of the test, he pushed the special panic button for emergency shutdown of the reactor, but it was already too late. Twenty seconds later, there was a steam explosion, followed by a hydrogen explosion that spewed out part of the reactor's red-hot contents and ignited a graphite fire that lasted for 10 days. A full meltdown of the core and further explosions were barely averted during that time.

Three weeks later, Toptunov died in great pain from the radiation. Fortunately, only about 5 percent of the reactor's total inventory of radioactive materials was released to the environment before the fire was extinguished, but the release was large enough to reach every country in the Northern Hemisphere. In the Soviet Union, some 5,000 square kilometers (3,000 square miles) were rendered uninhabitable; 130,000 people had to be relocated, and while only 31 people died in the event and its immediate aftermath, some 600,000 people were so significantly exposed to radiation that they will require constant monitoring of their health. The word "Chernobyl" quickly became a global symbol of the most expensive and frightening accident in industrial history.

How could such an event occur? Zhores Medvedev, a renowned Soviet biologist who has previously written about a secret 1957 disaster in the Soviet military's nuclear reprocessing program, does an excellent job of telling us not only how, but why. As a cartoon Communist might say, "it was no accident, comrades." Although a 1987 in camera trial found local operators and supervisors guilty of criminal negligence, Medvedev views the trial as an effort to shift blame away from larger issues and higher officials. In his view, the Chernobyl disaster was caused not just by an oversight of the plant managers, "but by the way in which nuclear energy is administered in the Soviet Union." If one looks more deeply, the accident starts not on April 26, 1986, but in December 1983 when the reactor is certified without its full range of safety tests. According to Medvedev, "this kind of practice is not unusual in Soviet industrial construction . . . Everyone, including the government, is unhappy if the plan is registered as unfilled."

Reactor No. 4 was a new unit of a graphite-moderated type that (unlike American water-moderated reactors) was built without any airtight containment structure. It was nearing the end of its first fuel cycle -- a time when its core was most contaminated with radioactive fission products -- when the operators attempted one of the safety experiments that should have been performed in 1983. The terrible timing was probably related to an impending shutdown for maintenance when possible faults could be fixed without exposing the earlier coverup. WHEN THE disaster occurred, the initial responses were clumsy and accentuated the problem. Ill-equipped firemen, unprotected against radiation, poured water on the graphite. At those temperatures, the water quickly turned to hydrogen and oxygen, which fanned the flames. Physicists were not brought to the site until 18 hours later. Bombing the reactor core with sand and clay dropped from helicopters trapped the heat within the remaining core and increased the threat of a second meltdown. Finally liquid nitrogen was pumped into the ground beneath the reactor to lower temperatures and reduce the risk of molten materials reaching groundwater and creating a massive steam explosion.

Glasnost was in its infancy in 1986, and throughout the 10-day battle to contain the disaster, Soviet citizens were kept in the dark by their government. The world was notified by Swedish scientists who detected the arrival of the radioactive cloud on April 27. By April 29 the United States had pictures of the reactor taken by its KH-11 satellite. But for the entire 10 days, the Soviet government told residents of Kiev that there was no danger. Then on May 15, it announced the evacuation of children from Kiev to a safer environment.

The silver lining that Medvedev detects in the atomic cloud of Chernobyl is the greatly accelerated process of glasnost. Chernobyl was too big to hide. Reversing course, Gorbachev invited inspection by the director general of the International Atomic Energy Agency and subsequently presented a broad but (according to Medvedev) incomplete range of information to a special conference convened by the agency. In addition, the accident forced the Soviet Union to move away from its graphite reactors and to review its overly ambitious plans for the expansion of nuclear energy. Medvedev concludes his fascinating story of Chernobyl with the view that "the only way to move forward now is to integrate with those Western economies which have already been restructured, even if this means sacrificing political and ideological dogmas."

Joseph S. Nye Jr., the director of the Center for Science and International Affairs at Harvard University, is the author of "Bound to Lead: The Changing Nature of American Power."