Amid United States intelligence reports that another reactor at the Chernobyl nuclear power plant may now be involved in the accident, Western analysts are still trying to explain how it began. Two schools of thought have emerged.
The first and most widely shared view holds that the reactor's cooling system failed, allowing the reactor core to overheat, ignite, and melt the highly radioactive fuel rods.
The second theory holds that reactor operators lost control of a process that releases excess energy that builds up in the core's graphite moderator. The graphite ignited and burned away the casings on the uranium-oxide fuel rods but did not melt the fuel itself.
The most specific Soviet explanation reportedly came from an Intourist guide briefing a US high school teacher traveling in Kiev with a group of students. The guide is said to have told the teacher that a generator at the power plant caught fire. The heat cracked a wall separating the unit from the reactor, allowing radiation to escape.
But Reagan administration sources said Wednesday that based on US intelligence data (including satellite photos), the first affected reactor apparently experienced a ``major problem'' last Friday. By Saturday, the core began a meltdown. By Sunday, an explosion, perhaps caused by an accumulation of hydrogen gas in the reactor area, ripped the reactor building apart.
As for the second, nearby reactor, the administration source said: ``We estimate that there could be a meltdown at the second.'' The source said -- without elaborating -- that US intelligence agencies had reached this conclusion ``because of the close association of the building and other indications.''
But Allan Bromley, a professor of physics at Yale University, said Tuesday at a meeting of the American Physical Society in Washington that available facts point to an accidental fire. Dr. Bromley said the continued radiation bombardment of the graphite moderator in the reactor core causes it to store energy, which must be released periodically. This energy is released by heating the graphite to high temperatures. ``We believe our Soviet colleagues were attempting to remove [this] energy, as must be done about once a year for a reactor this size, when a fire occurred,'' he said.
Dr. William W. Havens Jr., executive secretary of the physics group, added that it would take temperatures of around 5,000 degrees Fahrenheit to melt the low-grade uranium-oxide fuel. It would be hard to reach even 3,000 degrees in a graphite carbon fire, he said.