The Russian president called the disaster "a harsh lesson to humanity."
On April 26, 1986, a botched test at the nuclear plant in then-Soviet Ukraine triggered a meltdown.
Wolves, foxes and other mammals abound in contaminated areas.
Greenpeace tests show overall contamination from key isotopes has fallen somewhat, but lingers, especially in places such as forests.
A new study suggests that humans are worse for animal populations than nuclear disasters.
A documentary that premiered at Sundance sheds an eerie light on a possible cause for the Chernobyl nuclear disaster.
Energy minister says the problem should be resolved by Friday.