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.

Chernobyl unit 4

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


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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.

2. Despite official statements made in the U.S. right after the accident, Chernobyl No. 4 did have a reinforced-concrete containment—one that was installed in 1980. Whether the shell was comparable to what you'd find on the average U.S. reactor isn't clear. In any event, Chernobyl No. 4's outer shell was probably breached by a powerful hydrogen explosion, which, you may recall, was the greatest fear in the days following the Three Mile Island accident. The power released in such an explosion could be great enough to destroy any existing reactor's containment.

3. Chernobyl No. 4 was one of the Soviet Union's best nuclear power plants. Prior to the events of late April 1986, the Soviets had planned to add units 5 and 6 to give a total output of 6,000,000,000 watts, enough to light up all the homes in England. No. 4 had a many-tiered safety system that differed from Western versions mainly in that its control was not fully automatic. Since the Soviets judged completely automatic safety systems to be unreliable, they made it possible for operators to deactivate or override the systems—a fatal miscalculation.

A Tragedy of Human Errors

The Chernobyl nuclear meltdown appears to have been caused almost entirely by human error—errors, actually, an incredible string of them. The plant was, by design, more vulnerable to an accident than most, but it still took bungling to bring one about.

Reactor No. 4 was being powered down for an annual fuel change and maintenance. During power reduction, which takes more than a day, the operators planned to do a test to see how long the generators would continue to produce electricity after steam had been cut off to the turbines. Emergency systems were dependent on the reactor's own power production until back-up diesel generators could be brought on line, so it was important to know how long the reactor could support its own safety net.

Operators began to reduce power in the wee hours of the morning on Friday, April 25. By 1 o'clock that afternoon, output had dropped to half of normal, and the operators switched of the emergency cooling system. An hour later they received an urgent request to maintain power until later in the day, so they stopped the shutdown. Unfortunately, they failed to turn the emergency cooling system back on-the first of six violations of operating rules.

Near midnight on the 25th, the plant's electricity was no longer needed, so the operators continued the shutdown. However, a disconnected automatic control allowed the power to drop too rapidly to perform the required test. Rather than abandon the experiment and face the music with their superiors, they decided to try to restore enough power to do the test.

At this stage a reactor can be very difficult to get going again, but the operators continued undaunted. They pulled a number of the control rods all the way out of the core in an effort to restore power. When inserted between the fuel rods, the control rods slow fission by absorbing neutrons. Normally, it's against Chernobyl's rules to leave fewer than 30 control rods in the core; the best guess now is that they left in only six or eight.

With so much of the fuel exposed, the reactor began to behave very unstably, heating more in some areas than others and running at a critically low coolant level. To counteract this, the operators switched on more pumps to circulate coolant. This actually worsened the instability, and the reactor quickly reached the point where it was ready to shut itself down automatically. Rather than give up the test, the operators switched off the automatic shutdown system.

At 1:23 a.m., April 26, they started the experiment. As the turbines spun down, water flow declined, and Chernobyl's engineers were quickly faced with a crucial lack of cooling. Within half a minute they realized that the reactor was running out of control, and they tried to shut it down by dropping all the control rods into the core. Probably because the fuel rods had already overheated and distorted, some of the control rods failed to go all the way into place.

Within seconds, power in a small part of the core went from less than 10% to perhaps hundreds of times normal. In fact, the first explosion may actually have been a slow-motion version of an atomic bomb going off. The fuel in the Chernobyl reactor didn't melt; it shattered when the reactor reached "prompt critical"-something that nuclear engineers had considered all but impossible. The blast blew apart the top of the reactor's core and destroyed the service crane above it. The zirconium on the fuel rods then reacted with steam from ruptured cooling lines to produce hydrogen. The subsequent explosion shattered the containment, sending radioactive material into the air and spreading fire about the plant. Only acts of heroism by firefighters (many of whom died) prevented the flames from destroying reactor No. 3.

At this stage, the graphite in the reactor's core had a profound effect on Chernobyl No. 4's release of radioactive isotopes. It caught fire and, with the hydrogen, burned with intense heat, carrying radioactivity straight up into the windless night. Flames may have reached a height of 500 meters. The prevailing wind at altitude (to the northeast) happened to be over relatively unpopulated areas, and there was no rain in the Chernobyl area to drop the radioactivity on nearby inhabitants. Though even the Soviets estimate that 30,000 to 40,000 of their citizens will eventually die as a result of the Chernobyl accident, the casualties were (and will be) far lower than could have been the case. Of course, the radioactivity that didn't fall in the immediate area was carried on the wind to places such as the Forssells' small organic farm to the northeast in Sweden.