Benchmarks: April 26, 1986: Nuclear explosion at Chernobyl

by Carolyn Gramling
Thursday, January 5, 2012

A rusting Ferris wheel dominates the skyline of Prypiat, Ukraine’s ghost town. A few kilometers away, within a massive concrete structure called the Sarcophagus, are the remnants of the worst nuclear disaster in history: the ruins of the Chernobyl Nuclear Power Plant’s reactor number four — and about 200 tons of highly radioactive material.

The Chernobyl disaster was the result of a rapidly cascading chain reaction of human error and faulty reactor design. On the morning of April 25, 1986, Chernobyl’s fourth reactor was nearing the end of its first fuel cycle, with two years' worth of spent fuel ready to be removed from the reactor. The plant’s operators saw the scheduled shutdown as an opportunity to correct a longstanding safety flaw — an emergency cooling feature for the reactor’s core that should have been operating since the reactor first went online two years earlier, but had not. They planned to check the feature during the shutdown procedure.

At the end of its fuel cycle, the reactor’s cooling system was already handling large amounts of heat from the decay products of the spent fuel. The situation soon became more ominous: The daytime shift received instructions to carry out the test — but a regional power station unexpectedly went offline. The test was delayed until 11:00 p.m. so the reactor could continue to provide evening power. By then, the daytime workers had left.

The night shift was wholly unprepared to run the cooling test. A transcript of a phone call between the operators “makes one’s flesh creep,” wrote one member of the government commission that later investigated the catastrophe. “One operator rings another and asks, ‘What shall I do? In the programme there are instructions of what to do, and then a lot of things are crossed out.’ His interlocutor thought for a while and then replied, ‘Follow the crossed out instructions.'”

The errors began to build: To reduce power in preparation for the test, the workers inserted control rods into the reactor’s core to control the rate of fission. But the rods were inserted too far, reducing the power by too much. Also, at that low power, one nuclide, xenon-135, absorbs large numbers of neutrons, slowing the reaction rate further. The operators did not know this, and they needed more power to run the test. So they pulled the control rods back out — too far for safety.

Due to the xenon, the power was still too low, but the operators began the test anyway, increasing water flow to cool the core. This created new problems: Water also absorbs neutrons, inhibiting reaction and decreasing power further. To be able to continue the test, the operators decided to remove almost all of the control rods. At this point, only the xenon prevented a runaway reaction.

Unaware, the workers continued the test, now decreasing the water flow rate. Accordingly, the reaction in the core quickly began to speed up. At 1:23 a.m. on April 26, 1986, with the power rapidly increasing, the operators began an emergency shutdown of the reactor, inserting all the control rods at once.

But the design of the control rods was faulty; they briefly increased the reaction before slowing it. The core overheated. Twenty seconds later, with steam building in the rapidly heating reactor, there was one explosion, then another. But the reactor crew still thought the core was intact — defective meters suggested radiation levels were not lethal. Most of the crew died of radiation exposure within three weeks.

Following the explosions, fires broke out in nearby buildings, spreading radioactive material to outlying areas. Many of the firefighters also died of radiation poisoning.

The radioactive cloud traveled swiftly across Europe. Forced to acknowledge the accident, the Soviet government tried to downplay it. On April 27, Prypiat’s 50,000 residents were evacuated. They were told only that an accident had occurred, and to take what they would need for the next three days. They never returned.

About 50 people died of acute radiation sickness following the disaster, but the fallout was much greater. Ukraine and Belarus received the majority of the contamination, which seeped into rivers and groundwater, and affected animals and plants. Six hundred thousand people were considered highly exposed. About 4,000 cases of thyroid cancer were reported in children and adolescents in Ukraine, Russia and Belarus.

A 30-kilometer radius around the reactor called the Chernobyl Exclusion Zone remains empty today, and the damaged reactor was sealed off beneath 200 meters of concrete. The Sarcophagus, however, is structurally unsound. With cooperation among the Ukraine, the European Union and the United States, a new structure, called the New Safe Confinement, is scheduled to be in place by 2011. Officials say it will contain the radioactive remains of Chernobyl for 100 years — short compared to the lifetime of the radioactive materials it is meant to hold.

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