Missing Link: 40 years ago, the Chernobyl reactor exploded

Contents

On Saturday, April 26, 1986, at 1:20 a.m., the world was still in order. For a few more minutes: In the control room of Unit 4 of the Chernobyl power plant in the Ukrainian Soviet Socialist Republic, the night shift was preparing for a test. It was supposed to show that the reactor was safe even after a power failure. Three minutes later, at 1:23:44 a.m. (April 25, 11:23:44 p.m. our time), the reactor exploded. It was the worst nuclear accident in history.

The test was intended to simulate an external power failure. The power plant crew wanted to check whether, in such a case, the coasting turbine would still supply enough energy for the emergency cooling of the reactor until the emergency power generators started. One minute needed to be bridged – otherwise there would be a risk of core meltdown.

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The simulation had already begun 24 hours earlier, on April 25 at 1 a.m. local time, with the gradual shutdown of the reactor for its annual inspection. However, because electricity was needed in Kyiv on Friday afternoon, the shutdown was halted. The reactor was running at half power at the time.

At 11:10 p.m., the personnel resumed the shutdown. About an hour later, it had reached a quarter of its power. But suddenly, the power dropped sharply to about 1 percent. The cause was so-called xenon poisoning.

Xenon absorbs neutrons

Because the reactor had been running at half power for hours, the isotope xenon-135 had formed. Xenon absorbs neutrons and hinders the chain reaction, which is why the reactor's power dropped. It takes about one to two days for the isotope to decay. Only then is the reactor ready for operation again and can be restarted.

Just under an hour before the catastrophe, at 0:32 a.m., the crew made the first of several wrong decisions: To increase the reactor's power, they pulled out some of the control rods used to regulate the reactor. But what they intended – to increase the power again – did not succeed: the reactor only reached 7 percent. At a power level below 20 percent, it must not be operated, but only shut down. Nevertheless, the decision was made to keep the reactor running and carry out the test as planned.

Over the next half hour, the operating crew withdrew further control rods from the reactor to stabilize it. By 1:03 a.m., 193 out of 211 control rods had been withdrawn. The plant became increasingly unstable, and pressure fluctuated. The catastrophe could still have been prevented by an emergency shutdown. However, the automatic emergency shutdown had been deactivated 20 minutes earlier.

Twenty minutes later, fate took its final course: At 1:23 a.m., the deputy chief engineer ordered the test to begin. As planned, the power was cut off. The pumps were now only driven by the coasting turbine. This resulted in less cooling water being pumped into the reactor core. The temperature there rose rapidly. The world was only seconds away from catastrophe.

The shift supervisor presses the emergency button

At 1:23:40 a.m., the shift supervisor pressed the emergency shutdown button. It was the spark on the fuse. The AZ-5 button caused all control rods to be inserted. But instead of stopping the chain reaction, it was accelerated due to a design flaw in the reactor. In fractions of a second, the power increased to a hundred times the nominal value. Due to the temperature increase, the rods deformed and got stuck. At a temperature of about 2000 degrees Celsius, the fuel elements melted. The zirconium in the fuel rod cladding reacted with the water vapor, as did the graphite. Large amounts of hydrogen and carbon monoxide were released.

Two explosions occurred in quick succession, one of which blew away the reactor's approximately 2000-ton lid. A fire raged inside the reactor, releasing huge amounts of radioactive material. Burning, highly radioactive graphite was ejected. The hot air rising from the fire carried the radioactive substances high into the atmosphere.

The plant fire brigade arrived within minutes and had already extinguished the fires outside the reactor building about four hours after the explosions. However, it continued to burn inside. To extinguish the fire and prevent an uncontrolled chain reaction in the molten reactor core, it was decided to pour lead, boron, dolomite, sand, and clay into the reactor.