Chernobyl disaster

A big power plant had an accident.

It was in a place called Chernobyl. It happened long ago. Workers were running a test when things went wrong. The reactor exploded and caught fire.

This fire sent bad dust into the air. Many people had to leave their homes. A big zone was made to keep people safe. Many workers helped to fix the mess. They even used robots to help clean up.

On April 26, 1986, a big accident happened at Chernobyl.

This power plant was in the Soviet Union. Workers were running a test on reactor number four. They wanted to see how it would work during a blackout.

During the test, the power in the reactor dropped too low. To fix this, workers moved many control rods. This caused a sudden power surge. The reactor exploded and a fire started.

The fire sent radioactive dust into the air. This dust spread across Europe. Many people had to leave their homes. A large area was made to keep people safe. This is called an exclusion zone. It was first 10 km wide. Later, it grew to 30 km. Workers used robots to help clean up.

They also built a big cover called a sarcophagus. Later, a new safe cover was built to help clean up the debris. This work will last until 2065.

The Chernobyl disaster was a very serious accident at a nuclear power plant.

This plant was located near a town called Pripyat in the Soviet Union. It is one of only two accidents ever rated at the highest level of severity.

The accident happened because of a test that went wrong during a power plant operation. This event changed how people think about nuclear energy safety. It remains the most expensive disaster in human history.

To understand the accident, we must look at how the reactor worked.

The reactor used water to keep the fuel cool. This water was moved by large pumps. If the power goes out, the pumps must keep running to prevent a meltdown.

During the test, workers wanted to see if a spinning turbine could provide power. They wanted to see if it could bridge the gap until backup generators started. The test involved turning off some safety systems. This made the situation much more dangerous when things went wrong.

The trouble began on April 26, 1986.

Workers were trying to lower the power of reactor number four. However, the power dropped much lower than they expected.

This happened because of a buildup of a substance called xenon-135. To fix the power drop, the workers moved many control rods. This caused a sudden and massive surge in power. The reactor building was destroyed by steam explosions and a fire. The fire spread radioactive material across the Soviet Union and Europe.

Many people were affected by the radiation from the explosion. About 49,000 people were moved out of a 10 km zone. Later, the zone was expanded to 30 km. This meant 68,000 more people had to leave their homes.

Some workers became very sick from radiation. Among the 237 workers in the hospital, 134 had acute radiation syndrome. Many more people in the area later faced health problems like thyroid cancer. The city of Pripyat was abandoned and people had to move to a new city called Slavutych.

People have worked very hard to clean up the site.

In December 1986, they finished a structure called a sarcophagus to cover the reactor. This helped stop more radiation from spreading.

Later, between 2016 and 2018, workers built the New Safe Confinement. This is a huge structure built around the old sarcophagus. It will help workers safely remove debris until at least 2065.

This work helps keep the environment safer for the future.

The Chernobyl disaster was a catastrophic nuclear accident that occurred on April 26, 1986.

It took place at reactor no. 4 of the Chernobyl Nuclear Power Plant. This plant was located near the city of Pripyat in the Ukrainian SSR, part of the Soviet Union. The event is one of only two accidents rated at the maximum severity on the International Nuclear Event Scale. The other event was the 2011 Fukushima accident. It remains the worst nuclear disaster and the most expensive in history.

The total cost is estimated at US$700 billion.

To understand the disaster, one must understand how RBMK reactors function.

These reactors use water as a coolant to manage heat. This water is moved through 1,661 individual fuel channels by electrically driven pumps. Even after a reactor shuts down, it produces decay heat. This heat comes from radioactive decay and accounts for over 6% of thermal power.

Continuous coolant circulation is essential to prevent a core meltdown. If a total power loss occurs, backup diesel generators must start. These generators take 60 to 75 seconds to reach the 5.5 MW needed for a main pump.

The disaster happened during a specific safety test.

Engineers wanted to see if the spinning steam turbine could provide enough energy to run coolant pumps. This energy would bridge the gap during a blackout before generators reached full load. The test required reducing reactor power to between 700 MW and 1,000 MW. To perform the test, operators disabled the emergency core cooling system (ECCS). This is a system designed to pump water into the core during a loss-of-coolant accident. The test was delayed due to electricity demands from the Kiev electrical grid. This delay caused the test to be performed by the night shift instead of the day shift.

During the test, the reactor experienced an unexpected power drop.

This happened because of xenon-135, a substance that absorbs neutrons. When power levels dropped, iodine-135 decayed into xenon-135 faster than the reactor could "burn it off." This process is known as reactor poisoning. The power dropped to 30 MW thermal or less, which was only 5% of the required level. To increase the power, operators removed many control rods from the core. This caused a dramatic power surge. The resulting steam explosions and a meltdown destroyed the reactor building. A reactor core fire then spread radioactive contaminants across the Soviet Union and Europe.

The immediate human impact was severe.

Two engineers died during the explosion, and two others were severely burned. Of 237 workers hospitalized, 134 showed symptoms of acute radiation syndrome (ARS). Twenty-eight of these individuals died within three months. This is the only instance in commercial nuclear history where radiation-related fatalities occurred. In the following years, 14 more workers died from various causes. As of 2005, there were 6,000 cases of childhood thyroid cancer in affected populations. The United Nations estimates fewer than 100 deaths resulted directly from the fallout. However, a 2006 World Health Organization study projected 9,000 cancer-related deaths in Ukraine, Belarus, and Russia.

Authorities responded by creating an exclusion zone to protect people. Initially, a 10 km zone was established, evacuating 49,000 people. The zone was later expanded to 30 km, leading to the evacuation of 68,000 more people. The city of Pripyat was abandoned entirely. A new, purpose-built city called Slavutych was created for the displaced residents. The cleanup involved more than 500,000 personnel. This massive operation cost an estimated 18 billion rubles, which is about US$84.5 billion in 2025 value.

Long-term containment has required massive engineering projects.

In December 1986, workers completed a sarcophagus to cover the reactor. This structure reduced the spread of contamination and protected other reactors. Between 2016 and 2018, the Chernobyl New Safe Confinement was constructed.

This new structure was built around the old sarcophagus. It was designed to allow for the future removal of reactor debris. Clean-up activities at the site are scheduled to continue until 2065. These efforts aim to manage the radioactive legacy of the site for many decades.