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We’ve just passed the 35th anniversary of the Chernobyl disaster – when one of the nuclear reactors in Ukraine exploded and caught fire.

Much of the fuel stayed inside the damaged building, which was covered by a new shelter in 2017. But scientists say the nuclear material is becoming more active.

Neil Hyatt, professor of nuclear materials chemistry at Sheffield University, has been following the situation. He says the increase in radioactivity is small, but needs to be monitored.

He begins by explaining what happened to the reactor’s molten core when disaster struck in 1986.

(Image: Chernobyl nuclear power plant in 2019, Credit: svedoliver/Getty Images.)