Workers are constructing a massive sarcophagus-like cover for the destroyed Chernobyl reactor.Valentyn Ogirenko / Reuters

Thirty years after an explosion at a Soviet nuclear power plant spewed radiation across Europe and forced the evacuation of tens of thousands of people, workers continue the long process of securing the site of the worst nuclear disaster in history.

Decades after the accident, dangerous radioactive materials remain at the site. Since 2010, workers have been building a massive, husk-like structure that will be moved over reactor 4, the unit that blew up in 1986, at the end of 2017. The shell—which will be 328 feet high and 541 feet long and weigh 30,000 tons—will separate the destroyed reactor from the environment for at least the next 100 years.

The structure is expected to cost more than $2.2 billion, according to the European Bank for Reconstruction and Development, the London-based bank that manages the Chernobyl Shelter Fund. The fund was set up in 1997 to help facilitate donor money to the Ukrainian, Russian, and other affected governments. It is funded by contributions from more than 40 countries and organizations.

As of November 2015, the Chernobyl fund received close to $1.5 billion from more than 40 countries, including members of the fund:

Austria, Belgium, Canada, China, the Czech Republic, Denmark, the European Community, Finland, France, Germany, Greece, Ireland, Italy, Japan, Kazakhstan, Kuwait, Luxembourg, the Netherlands, Norway, Poland, Russia, Spain, Saudi Arabia, Sweden, Switzerland, Ukraine, the United Kingdom, and the United States.

And non-member nations:

Argentina, Australia, Azerbaijan, Croatia,Estonia, Hungary, Iceland, India, Israel, Korea, Liechtenstein, Lithuania, Portugal, Romania, the Slovak Republic , Slovenia, and Turkey.

The Soviet Union at first sought to handle the fallout of the explosion on its own. About 200,000 people from across the USSR, emergency workers dubbed “liquidators,” arrived at Chernobyl between 1986 and 1987 to participate in the cleanup process, becoming exposed to high doses of radiation, according to the World Nuclear Association. By 1990, just as the Soviet Union was on the verge of collapse, the Soviet government acknowledged the need for outside help in the cleanup process. A United Nations resolution for “international cooperation to address and mitigate the consequences at the Chernobyl nuclear power plant” was drawn up. Between March 1990 and June 1991, 200 experts from the Soviet Union and 24 other countries conducted 50 field missions to examine the area. A special task force—under which the Chernobyl fund would be established—was created in 1992 to coordinate international efforts.

The accident killed some 30 people in the weeks after and exposed thousands of others to toxic radiation. About 120,000 people were evacuated from the surrounding area, including 43,000 from the Ukrainian city of Pripyat, famous for the photos that have emerged from the radioactive ghost town. The health effects of exposure for millions of people have been hotly debated since.

Memorials in Ukraine, Russia, and elsewhere were held Tuesday to mark the anniversary of the tragedy. Relatives and friends of the “liquidators,” thousands of whom have died since the explosion, held photos of the workers and placed flowers on their graves.

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