The Chernobyl Disaster

By Nick Touran, Ph.D., P.E., , Reading time: 5 minutes

On April 26th, 1986, a nuclear power plant in Chernobyl, Ukraine failed violently during a scheduled test of the turbine system. The power of the reactor increased out of control from a low-power state, rapidly boiling water and causing a steam explosion that blew the roof off the the reactor. A graphite fire was ignited, spewing toxic radioactive substances into the atmosphere. 31 people were killed during the accident, and it has been estimated that the released radiation has caused an additional 4,000 cancer deaths.

Here, you can find out what caused the Chernobyl accident and why we are convinced that nothing like this could happen again. Check the links to the right for our collection of information on Chernobyl.

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What happened at Chernobyl? (short version)

Because Xenon built up due to a rapid shut-down mainly caused by nearby coal plant problem, operators at Chernobyl retracted the control rods all the way out of the core only to notice power instabilities due to the inherent positive coolant void coefficient of the RBMK reactor and reinsert them. But the tips of the rods were graphite, which displaced water without introducing neutron poison, thus increasing the power rather than decreasing it and the positive void coefficient allowed the reactor to undergo a runaway power excursion, resulting in a catastrophic steam explosion and graphite fire that killed 30 workers and released radiation in levels that were soon detected in Finland.

What happened at Chernobyl? (longer version)

The Soviet government ordered the operators of Chernobyl to perform a test on their reactor. The test was supposed to measure how long the spinning generator would continue producing electricity after the reactor was shut down before it slowed and stopped. The test was to happen with the reactor at low power. As the operators were about to start slowly and carefully bringing the reactor to low power, the utility company called and said that a nearby coal plant just shut down unexpectedly and asked Chernobyl to stay at full power for a while longer to keep the nearby homes lit. After peak hours, the operators brought the plant to low power rapidly. Nuclear engineers can explain to you that this kind of shut-down would result in an unusually large amount of Xenon build-up in the reactor fuel. Xenon eats up neutrons, thus making the chain reaction more difficult to maintain. Because of this, the operators were surprised when the reactor went to a lower power than they had expected.

To get the power of the reactor to its expected value, the operators pulled the control rods out further than they would ever do in any normal situation. Finally, the reactor went up to the power level needed for the test. Unfortunately, the Chernobyl reactor was designed in a dangerous manner, allowing the reactor to get hotter if bubbles formed in the water coolant flowing past the fuel. As water boiled in certain locations of the reactor, this power instability started rearing its head. Apparently, one of the operators noticed this highly unstable situation and pressed the button to insert the control rods and shut down the reactor. But, an even more serious design flaw was in the control rods. The neutron poison in most of each rod had a small graphite tip on the bottom. This graphite is not a neutron poison (and is usually beneficial to chain reactions). So with the control rods all the way out, inserting them for the first few inches displaced some water without introducing any neutron poison, and this actually increased the power of the reactor. This led to more boiling of the water, which resulted in even more power and then positive feedback took the reactor power sky-high, immediately boiling all the water to steam. The steam pressure was so great that it blew the lid right off the reactor and through the roof of the reactor building (which was not one of those steel containments, by the way. It was just a concrete building).

With no more coolant, the fuel heated up and became molten. Standing graphite rods in the reactor ignited into a very hot graphite fire that began spewing bits of the radioactive fuel into the open air. The fire took over a week to extinguish, costing the lives of about 30 emergency responders due to acute radiation poisoning.

Can this happen again?

Modern reactors have reinforced steel-concrete containments domes that would not be so easily penetrated by a steam explosion. Control rods are designed such that all parts of them are neutron poisons, including the bottom. Reactors are designed to have NEGATIVE void coefficients, meaning boiling water would result in power decreasing automatically rather than increasing. Also, graphite is not typically put in reactor cores. Short answer: no.

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