The Chernobyl Nuclear Meltdown: What Happened?

David Schoonmaker shares technical facts of how the nuclear reactor was different from other reactors, and how weaknesses in the structure and human error lead to the Chernobyl nuclear meltdown.

| May/June 1987

Chernobyl unit 4

An aerial view of the damaged unit 4 after the tragedy.


The international nuclear power industry was quick to distance itself from the Chernobyl nuclear meltdown. Officials in nearly every nuclear nation assured the public that 1) the Soviet plant's design was unique, 2) it didn't have a protective containment around its reactor and 3) it lacked sophisticated safety systems. We were to believe that deficient design was responsible for the largest release of long-lasting radioactive material ever. The palliative was, "It can't happen here."

The Chernobyl Nuclear Meltdown: What Happened?

Until the Soviets released a detailed report at an international meeting in Vienna in late August 1986, most pronouncements about Chernobyl were idle speculation. Very little was known about what actually happened on April 26, 1986. Now we know that two of the power industry's initial contentions were false.

1. Chernobyl No. 4 did differ significantly from most nuclear plants in the West. It combined common elements in an uncommon way. The nuclear reactor was graphite-moderated and cooled by pressurized water, which means that the uranium fuel rods were arrayed in a matrix of graphite that slowed (but didn't absorb) neutrons to facilitate self-sustaining fission, and that heat was removed by pressurized water flowing around the core.

There are, in fact, many graphite-moderated reactors in use in the West, but most are gas-cooled. Pressurized water reactors (PWRs), which use water for both moderation and cooling, are more common, but most of the plants in England are graphite-moderated and gas-cooled. Graphite-moderated reactors are also used in France, Italy, Japan and the United States (at Hanford, Washington, and Savannah, Georgia), and a few of these are cooled by pressurized water. The reason for the popularity of graphite-moderated reactors is simple: Their internal geometry is particularly good for producing plutonium for bombs. (Surprisingly, Chernobyl doesn't seem to have been used for this purpose.)

Graphite core reactors aren't inherently more hazardous than PWRs—each type has advantages and disadvantages—but the combination of graphite moderation with pressurized-water cooling may have been a crucial factor at Chernobyl.

Chernobyl No. 4's design has what is called a positive void coefficient. If the core overheats and actually boils the cooling water, neutron absorption declines and the reactor may become hotter yet. In a pressurized- or boiling-water reactor, the opposite happens because the water is also the moderator. As the moderator/coolant boils, fission declines despite reduced absorption, because neutrons begin to move too fast. Gas-cooled reactors, an entirely different can of worms, are designed to be self-limiting. Had the Chernobyl reactor been designed with a negative void coefficient, the accident might not have happened and certainly would have been less serious.

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