It's been 30 years since the nuclear disaster at Chernobyl.

Commemorations are underway in Ukraine to mark the day of the meltdown at the Soviet plant that was the worst nuclear disaster in history.

An uncontrolled reaction blew the roof off, spewing out a cloud of radioactive material which drifted into other parts of the USSR, including Russia and Belarus, and northern Europe.

Sky's John Sparks reports.

"There's monitoring equipment and a massive new roof, built to replace a poorly constructed sarcophagus thrown on by the Soviets. It should seal up the 200 tonnes of molten uranium inside, but the surrounding area will remain out of bounds for tens if not hundreds of years."

Today the Ukrainian president Petro Poroshenko said full extent of this disaster will never be known:

"Statistics can not give us a number of victims but direct and indirect losses of human life as well as those that became disabled equal hundreds of thousands."