Chernobyl, Ukraine. Ukraine commemorates on Sunday the 40th anniversary of the explosion in the Chernobyl nuclear power plant that unleashed the worst civil nuclear catastrophe in history.
This date is remembered more than four years after the Russian invasion, which has once again endangered the plant and increased the risk of another radioactive tragedy.
Below are five things you need to know about this disaster and the current situation at the nuclear facility:
Chernobyl explosion
At 1:23 a.m. on April 26, 1986, human error during a safety test caused an explosion at reactor number four at Chernobyl in northern Ukraine, which was then part of the Soviet Union.
The blast tore apart the interior of the building and sent a cloud of radioactive smoke into the atmosphere, while the nuclear fuel burned for more than 10 days.
Thousands of tons of sand, clay and lead ingots were dropped from helicopters to contain the radioactive leak.
The International Atomic Energy Agency (IAEA) determined that the main cause of the disaster was “serious deficiencies in the design of the reactor and the shutdown system,” combined with “non-compliance” with operating procedures.
Chernobyl radioactive cloud
In the following days, the radioactive cloud severely contaminated Ukraine, Belarus and Russia before spreading throughout Europe.
The first public alert was issued just two days later, on April 28, when Sweden detected a spike in radiation levels on its territory.
The IAEA was officially notified of the accident on April 30, but Soviet leader Mikhail Gorbachev did not publicly acknowledge it until May 14.
It is estimated that thousands of people died as a result of radiation exposure, although estimates of the exact number of victims vary.
A 2005 UN report estimated the number of confirmed or expected deaths in the three most affected countries at 4,000. Greenpeace estimated in 2006 that the disaster caused nearly 100,000 deaths.
According to the United Nations, some 600,000 people who participated in the cleanup and containment operations, known as “liquidators,” were exposed to high levels of radiation.
This catastrophe increased public fear of nuclear energy, fueling a rise in anti-nuclear movements across Europe.
Russian occupation of Ukraine
Russian forces occupied the plant on the first day of the invasion of Ukraine that began in 2022.
They took the plant without combat after sending tens of thousands of soldiers and hundreds of tanks to Ukraine from Belarus, a close ally of Moscow.
Russian soldiers dug trenches and established camps in areas such as the so-called Red Forest, named after the color the trees acquired after the radioactive explosion.
Their takeover of the idle plant raised intense fears that a military incident could trigger another nuclear disaster at the site.
The Russian army withdrew about a month after the start of the war, following its failure to take the capital, kyiv, located about 130 km from Chernobyl, and where there was fierce Ukrainian resistance.
New nuclear threats
The remains of the plant are covered by an internal steel and concrete structure known as the sarcophagus, built in haste after the 1986 emergency.
Between 2016 and 2017, a new high-tech outer shell was installed, called the New Safe Confinement and designed to finally replace the sarcophagus, which was not intended as a permanent solution.
This huge exterior metal structure was pierced by a Russian drone in February 2025, thus losing its ability to contain radiation.
In a report published in April, Greenpeace noted that since the cover “cannot be repaired at the moment, and cannot function as designed, there is a possibility of radioactive leaks.”
The repairs are expected to last three to four years.
Another Russian attack could cause the radiation shelter to collapse, the director of the plant told AFP in December 2025.
exclusion zone
The area around the plant was evacuated and became an exclusion zone, with abandoned villages, fields and forests.
In total, more than 2,200 square kilometers in northern Ukraine and 2,600 square kilometers in southern Belarus are effectively uninhabitable.
People will not be able to live there safely for the next 24,000 years, according to the IAEA.
The city of Pripyat, three kilometers from the plant and with a population of 48,000 inhabitants in 1986, was completely evacuated.
It remains abandoned, with its empty and dilapidated buildings, including a rusting amusement park and a Ferris wheel, making it look like a post-apocalyptic ghost town.
Before the Russian invasion in 2022, it was possible to take guided tours of the place, but for almost three years the area has been closed to tourists.
Without human presence, the region has practically become a vast nature reserve, where in 1998 the Przewalski’s horse, a rare and endangered species, was reintroduced.
