The Big Question: 20 years after the Chernobyl disaster, what was its legacy?

What actually happened at Chernobyl?

Chernobyl is in the Ukraine, close to the border with Belarus, and is the site of the world's worst nuclear accident. The very name is synonymous with radioactive fallout, nuclear contamination and long-term damage to human health and the environment.

The disaster occurred in the early hours of 26 April 1986 at Chernobyl's reactor number 4. Prior to a routine shutdown, the reactor crew prepared for a test to determine how long turbines would spin and supply power following a loss of the main electrical power supply.

Operators deliberately disabled a safety mechanism designed to shut down the reactor automatically - a measure that contributed to the subsequent disaster. As the flow of coolant water fell, the power output increased. When the crew tried to shut down the reactor from its increasingly unstable condition, there was a dramatic power surge which caused the fuel elements to rupture.

A steam explosion lifted off the cover plate of the reactor, releasing radioactive material into the atmosphere - there was no strengthened containment shell built around the reactor as is the case with nuclear power stations in the West.

A second explosion blew out fragments of burning fuel and graphite from the reactor's core. A subsequent inrush of air caused the reactor's hot graphite core to burst into flames. Fire-fighters could not put it out for nine days. During this time huge quantities of radioactive material were released, amounting to somewhere between 20 and 400 times the fallout from the Hiroshima nuclear bomb.

How many people died?

This is the biggest and most contentious question. The radioactive plume spread across most of Europe - the world outside the former Soviet Union first heard of the disaster a couple of days later when a nuclear power station in Sweden raised the alarm as a radioactive plume passed overhead.

Three people died immediately as a result of the explosion and a further 20 died within a few weeks of the disaster after receiving very high doses of radiation. These were all Chernobyl staff and emergency personnel fighting the fire. A further 19 workers died between 1987 and 2004 from various causes thought to be related to acute radiation poisoning.

What is less clear is how many people beyond the facility died as a result of the fallout. Studies have clearly documented a rise in thyroid cancers, particularly among the young, who are particularly vulnerable. Radioactive iodine in contaminated milk is concentrated by the thyroid gland which can develop cancer as a result.

To date, more than 4,000 cases of thyroid cancer in Belarus, Ukraine and parts of Russia have been directly attributable to the release of radioactive iodine from Chernobyl. Thyroid cancer is easily treated, but it can be lethal, and 15 people have so far died of the disease in the area affected by Chernobyl.

A study by the Chernobyl Forum, a group of about 100 scientists drawn together by the United Nations, estimates that up to 4,000 people may ultimately die from Chernobyl fallout. But the group says there may be an additional 5,000 radiation-related deaths in the heavily contaminated regions.

Another study by Elizabeth Cardis of the International Agency for Research on Cancer in Lyon estimates that there may be as many as 16,000 radiation-related deaths in the whole of Europe resulting from Chernobyl fallout.

But this is far fewer than we were led to believe?

Quite. Some environmentalists have played up the scale of the disaster, saying that hundreds of thousands of people are dying or will die as a result. Some early predictions were based on all deaths in the affected region, whether or not they could have been caused by radiation.

One problem is trying to assess the health dangers of very low levels of radiation - the sort of levels that people can be exposed to from natural sources. The other is trying to disentangle the rise in health problems associated with the collapse of the Soviet Union, as well as the enforced evacuation of people from the "exclusion zone". The psychological impact of Chernobyl, and its corresponding effect on the physical health of those directly affected, have probably contributed to the general malaise of the families in the area.

Did Ronald Reagan say that Chernobyl was predicted in the Bible?

It is alleged he did, because the Ukrainian word chornobyl is the name of a plant that is a close botanical cousin of wormwood, which is a biblical symbol for sorrow and calamity. Wormwood is also a sign of the Apocalypse in the Book of Revelations. The Soviet authorities tried to debunk the story soon after it cropped up, but it would have had better success by pointing out that the chornobyl plant is Artemisia vulgaris, whereas wormwood proper is Artemisia absinthium.

Does anything live within the exclusion zone?

Yes, wildlife has flourished within the 30-kilometre zone where residents were evacuated in 1986. In simple terms this is because human activities such as farming are more damaging to wildlife than radioactive fallout.

The American-Ukrainian author Mary Mycio, whose book Wormwood Forest documents the return of wildlife to Chernobyl, said that she was shocked to discover on her first visit to the zone that it was teeming with rare animals and plants, including eagles, moose, deer, wild boar and 250 species of birds. "I've seen wolves in broad daylight and once even heard the bark of an endangered lynx, species which disappeared from the area long before the disaster," Ms Mycio said.

So what is the legacy of Chernobyl?

It is still the world's worst nuclear disaster. Tens of thousands of lives have been blighted by the events on 26 April 1986, and many thousands more will continue to suffer or die as a result of the long-term effects of radioactive contamination. Others have been affected by the "paralysing fatalism" of living near the world's most notorious nuclear accident.

Chernobyl marked a moment when the public were made to realise the inherent dangers of generating electricity from nuclear fission. Many countries stopped building new nuclear reactors as a result of the accident.

Some areas of Europe, such as hilltop sheep farms of North Wales, are still feeling the effects of radioactive contamination 20 years after it fell from the sky.

Chernobyl's reactor number 4 may have suffered uniquely from a flawed design, poor personnel training and bad crisis management, but it still represents a stark warning about the inherent dangers of generating power using nuclear energy.