Ukraine is preparing to mark 30 years since the Chernobyl disaster, the world's worst nuclear accident whose death toll remains a mystery and which continues to jeopardise the local population's health.

More than 200 tonnes of uranium remain inside the reactor that exploded three decades ago at the Chernobyl nuclear power plant, raising fears there could be more radioactive leaks if the ageing concrete structure covering the stricken reactor collapses.

International donors are meeting on April 25 to discuss a funding plan for the installation of a more modern and safe sarcophagus that could last a century and keep generations from living in fear.

But despite the international community's commitment to funding the project, it remains unclear who will pay for the new dome's operations and upkeep after 2017, when it is scheduled to become operational.

At 1:23 am on April 26, 1986, reactor number four of the Chernobyl nuclear power plant, located about 100 kilometres (60 miles) north of Kiev, exploded during a safety test.

For 10 terrifying days, the nuclear fuel kept burning, spewing clouds of poisonous radiation that contaminated up to three-quarters of Europe, with Ukraine and neighbouring Belarus and Russia hit especially hard.

As the horror unfolded, the Soviet authorities said nothing publicly, in keeping with a tradition of preventing people from learning of tragedies that could tarnish the image of the Cold War-era superpower.

They evacuated the 48,000 inhabitants of the town of Pripyat, located just three kilometres from the plant, only the following afternoon.

- Soviet silence -

The first alarm was raised on April 28 by Sweden, which detected an unexplained rise in its own radiation levels.

Only in his second year on the job, Communist Party Secretary General Mikhail Gorbachev -- winner of the 1990 Nobel Peace Prize for championing democratic and economic reforms -- did not publicly admit the disaster until May 14.

With the scale of what had happened now out in the open, the authorities in 1986 relocated 116,000 people from the 30-kilometre exclusion zone that surrounds the now-dormant plant.

Subsequent years saw 230,000 others experience the same fate. Yet five million Ukrainians, Belarusians and Russians still live in areas where radiation levels are high.

Some 600,000 people who became known as "liquidators" -- comprised mostly of the military, police, firefighters and state employees -- were dispatched by Moscow with little or no protective gear to help put out the toxic fire.

They were also responsible for erecting a concrete sarcophagus over the remains of the damaged reactor to prevent further radiation leaks, and for cleaning up the surrounding area.

- Disputed toll -

Thirty years later, the number of people who died in those chilling days and subsequent years from radiation poisoning remains a matter of intense dispute.

A controversial UN report published in 2005 estimated that "up to 4,000" could eventually die in Ukraine, Russia and Belarus from the after effects of the reactor's meltdown.

Yet a year later, Greenpeace environmental protection group estimated the number of deaths already caused by radiation poisoning at a staggering 100,000.

The United Nations Scientific Committee on the Effects of Atomic Radiation officially recognised around 30 deaths among those urgently sent to fight and contain the disaster in the days following the blast.

Somewhat extraordinarily, Chernobyl continued producing electricity until December 2000, when an independent Ukraine was pressured by the West to shut down the last active reactor for good.

- Monster cage -

With the concrete structure hastily erected around the devastated site cracking and in danger of collapsing, work begun in 2010 on a 25,000-tonne steel protective barrier.

About twice the area of a football pitch and soaring 110 metres (360 feet) above ground, the structure is slightly taller than Big Ben in London and weighs three times more than the Eiffel Tower.

The funding for the 2.1-billion-euro ($2.4 billion) monster cage has come from more than 40 countries and the European Bank for Reconstruction and Development, and 165 million euros more are expected from the G7 group of world powers and the European Commission.

But a 100-million-euro funding gap for storing the spent nuclear fuel remains.

Even if that money comes through, it remains unclear who will foot the bill for the new dome's operations after it is installed.

With most of the main work now completed, the structure is being fitted out with high-tech equipment that, if everything goes according to plan, will be able to decontaminate the hazardous material inside.