What happened at Chernobyl?

26 April marks the 40th anniversary of the Chernobyl disaster, in the then Soviet-controlled country of Ukraine.

In 1986, one of the power plant's reactors suffered an explosion, sending a radioactive plume across Europe.

The effects were devastating and the disaster's impact was felt across the world.

According to the UN, the event affected more than 3.5 million people and contaminated nearly 50,000 square kilometres of land.

To this day, it remains the world's worst nuclear accident in history.

In the early hours of Saturday 26 April 1986, reactor number four at the Chernobyl nuclear power plant suffered a catastrophic explosion, throwing clouds of radioactive material over the surrounding area.

The accident happened after engineers at the plant were carrying out a test to see what would happen during a power blackout.

They cut the power to some systems, but the engineers did not know that the reactor was already unstable.

The reduced power slowed turbines that pushed cooling water to the reactor.

With less water to cool the system, what was left turned to steam, building up huge amounts of pressure.

By the time operators realised what was happening, it was too late.

The explosion was so powerful that it blew the 1,000-ton steel lid off the reactor - the same weight as three 747 passenger planes.

The explosion also created a fire that burned uncontrollably.

In the days that followed, the scale of the disaster would become clear.

At the time of the disaster in 1986, Ukraine was part of the Soviet Union.

From 1922 to 1991, Russia and the countries it controlled were named The USSR - the Union of Soviet Socialist Republics.

You might sometimes hear it called the Soviet Union. The USSR was a communist state.

During its time as the Soviet Union, Russia kept tight control of the different countries under its power, and would limit the freedoms and information available to citizens who lived there.

In the days after the disaster, the Soviet Union government attempted to cover up the incident, and they refused to report what had happened.

However, it wasn't long before other countries could tell something wasn't right. Monitor stations in Sweden reported abnormally high levels of radiation in the wind.

Officials pressed the Soviets for an explanation and the Soviet government eventually had to admit there had been an accident at Chernobyl.

It sparked an international outcry from countries all over the world about the dangers posed by the radioactive emissions.

The fallout had huge political and economic repercussions. In the following years the Soviet Union found itself under great pressure and underwent some massive changes.

The disaster at Chernobyl made people living in the Soviet Union question the government more, and demand more transparency about how the USSR was run.

Many people believe the Soviet government was forced to become less secretive as a result of the accident.

In 1991, the Soviet Union came to an end.

Fewer than six years had passed between the meltdown at Chernobyl and the break-up of the USSR.

When the Chernobyl power plant exploded it wasn't just the immediate surrounding area in the then Soviet Union that was affected.

The poisonous radiation that spewed into the atmosphere drifted over lots of European countries.

Many parts of the UK were affected. The government banned the sale of sheep from thousands of farms because they could have eaten radioactive material that had been absorbed by the grass they fed on.

North Wales was hardest hit, with sheep in the area failing radioactive tests 10 years after the accident.

Children in Ukraine and other nearby countries became ill. Many still face consequences today.

In the immediate aftermath of the explosion, scientists had to try and figure out how to contain both the heat and the radioactivity leaking from the exposed reactor core.

The reactor was covered with a giant 'sarcophagus' made from steel and concrete to stop the escape of more radiation.

The sarcophagus was built quickly to try to contain the problem straight away, and was never intended to last very long.

In the following years it was deemed to be in danger of collapse, and a new solution was needed.

Construction began on a new, larger steel cover in 2010, known as the New Safe Confinement structure.

In 2016, work on the new cover was complete, and the huge steel arch was moved into place over several weeks.

Unlike the original sarcophagus, the New Safe Confinement is designed to last for a long time - around 100 years - and will allow the reactor to be safely dismantled from within, using remotely operated equipment.

It cost around 1.5 billion euros to build.

There still remains huge amounts of dangerous radioactive material in the reactor, and it will take many more years before it's ever fully safe.

Today, Chernobyl has become a popular place for tourists who are intrigued by its history.

Companies offer guided tours and some people even enter illegally.

Radiation levels are low enough for people to spend short times in the area safely.

In February 2022, Chernobyl was seized by Russian troops on the first day of the invasion of Ukraine.

Although no longer a working power station, Chernobyl was never fully abandoned and still requires constant management.

According to the former nuclear plant's staff, the troops left around a month later.

In February last year, Ukraine accused Russia of targeting the power plant with a drone strike - a claim that Russia has denied.

Russia said its military does not strike Ukrainian nuclear infrastructure and "any claims that this was the case do not correspond to reality".

According to a UN watchdog, the protective shield covering the Chernobyl nuclear reactor was damaged following the strike, and can no longer provide its main containment function.

They added that repairs were "essential" to "prevent further degradation" of the nuclear shelter.

However, environmental experts have told the BBC that "it is not something to panic about."

Professor Jim Smith from the University of Portsmouth in the UK, who has studied the aftermath of the Chernobyl disaster, said that "the risk is low" because contaminated dust is contained within a thick concrete "sarcophagus" which is covered by the protective shield.