Tuesday marks the 25th anniversary of the Chornobyl nuclear disaster. On April 26, 1986, staff members at the Chornobyl nuclear power plant near the Ukrainian town of Pripyat were conducting an experiment at reactor No. 4 when they turned off the reactor's safety systems.

The reactor overheated, and an explosion and fire sent steam and a huge radioactive cloud across Ukraine, Belarus, Russia and the rest of Europe.

The accident released enormous amounts of radiation into the atmosphere and rendered Pripyat uninhabitable for generations.

The Chornobyl disaster was the first Level 7 incident on the International Nuclear Event Scale at that time. The second is the March 11 accident this year at the Fukushima Daiichi nuclear plant in Japan that was triggered by a massive earthquake and tsunami.

The Japanese disaster is still being contained and it's unclear how severe the situation will ultimately become, but at the moment, Chornobyl remains the worst nuclear accident in history.

Here are some of the notable numbers connected to the Chornobyl disaster.

120 millisieverts: The dose of radiation that 530,000 recovery operation workers were exposed to after the accident, according to the United Nations Scientific Committee on the Effects of Atomic Radiation. In contrast, individuals can be exposed to 9 mSv from a typical X-ray.

116,000:  The number of people relocated from within a 30-kilometre radius of the plant after the accident.  

1,000: The number of people who came back to live within that contaminated exclusion zone, according to the World Nuclear Association.

50,000:  The number of people living in Pripyat at the time of the accident.  

220-ferris-chornobyl-rtr2iy

Rusting amusement rides are seen in the abandoned city of Prypiat, near the Chernobyl nuclear power plant in Ukraine. ((Gleb Garanich/Reuters))

350,000: The number of people who relocated after the accident from Belarus, Ukraine and Russia.

5.5. million: The number of people who remained in Belarus, Ukraine and Russia.

Up to 320 years: The length of time it will take for the area around to reactor to become livable again, according to state authorities.

60,000: The number of people who have died in Russia in the last 15 years because of the Chornobyl accident, according to Greenpeace International.

4,000: The number of cases of thyroid cancer, mainly in children and adolescents at the time of the accident, that have resulted from the accident's contamination, according to a UN report in 2005.

$785 million US: The amount of money that governments from around the world pledged on April 19, 2011, to build a new container around the remains of reactor No. 4 to prevent radioactive dust from getting into the atmosphere, according to Ukrainian President Viktor Yanukovych.

28: The number of governments that have pledged money to help cover the bill for securing the reactor.

$1.8 billion US: The amount spent to date securing the reactor, according to an article published by The Guardian.

20,000: The number of tonnes of steel shielding required for the new reactor cover.

200,000: The number of individuals from the Soviet Union who took part in the recovery and cleanup of the disaster.

$400 million US: The amount of money spent on improving the remaining reactors at the Chornobyl plant, according to the World Nuclear Association. 

100: The average number of tourists who visit Chornobyl each week, according to the government agency that manages the area, up from about a dozen per week last year.

$500: The approximate cost of an afternoon tour of Chornobyl. The price drops to less than $150 per person for groups.