By Stephen Mulvey
BBC News website
The world's worst nuclear accident, at Chernobyl in April 1986, was all the more alarming for taking place under a veil of secrecy, behind the Iron Curtain.
One of four reactors at the Chernobyl nuclear power plant, 70 miles (110km) north of Kiev, exploded at 0123 local time on Saturday 26 April.
The radioactive fallout was detected in Sweden the following Monday morning, but all day the Soviet authorities refused to admit anything out of the ordinary had occurred.
A sarcophagus was erected over the ruins of Chernobyl's fourth reactor
Only at 9pm, after Swedish diplomats gave notice they were about to file an official alert with the International Atomic Energy Authority, did Moscow finally issue a terse, five-sentence statement:
"An accident has occurred at Chernobyl nuclear power station. One of the atomic reactors has been damaged. Measures are being taken to eliminate the consequences of the accident. Aid is being given to the victims. A government commission has been set up."
The word "damaged" hardly reflected the truth of a reactor in meltdown, open to the sky, its graphite sections burning at 2,500C, sending a column of radionuclides thousands of feet into the atmosphere.
Few believed the reassuring Soviet reports which followed, and the fear that gripped many in the path of the fallout plume, was partly the fear of the unknown.
May Day parade
It was only two weeks after the explosion, when radiation releases had dramatically tailed off, that the first Soviet official gave a fully frank account.
"Until now the possibility of a catastrophe really did exist: A great quantity of fuel and graphite of the reactor was in an incandescent state," said nuclear physicist Yevgeny Velikhov.
Men were used to clear radioactive debris, when machines failed
No-one was left more in the dark than the Soviet citizens most closely affected. At first, life continued as normal in Pripyat, the model town built to house power station staff and their families, just two kilometres (one mile) from the Chernobyl plant.
Most people spent the Saturday outside, enjoying the unusually warm spring weather. Sixteen weddings took place.
The town was only evacuated 36 hours after the accident, while the evacuation of nearby villages took several more days.
Meanwhile in Kiev, citizens went ahead with their May Day parade, five days after the accident, completely unaware of the radiation bearing down on them.
The news vacuum also encouraged exaggeration and mistakes in the Western media.
The UPI agency quoted a source in Kiev saying that 2,000 people had died, and the figure appeared on many front pages the next day.
From Pripyat 36 hours after accident: 49,000
Total evacuated in 1986 (from 30km zone): 116,000
Others moved later: 220,000
Still living in contaminated areas of Belarus, Russia and Ukraine: 5 to 8 million
US officials, meanwhile, were led astray by satellite photographs, which, though confusing, were the main source of independent information.
A Pentagon source told the US television network NBC on 29 April that the 2,000 figure "seemed about right, since 4,000 worked at the plant", while the next day officials suggested that another reactor was in trouble.
CBS news anchor Dan Rather summed up events on 30th April citing "a much different, more dangerous view seen from western satellites above, enhanced eye-in-the-sky views that US intelligence says is a reactor-gone-wild accident still in progress and a second reactor possibly melting down."
In reality, the threat of fire spreading to the third reactor had been dealt with on day one.
But in the first days of May there was real alarm among the team battling the crisis on the ground.
Radiation releases had begun rising again, and the fear was that the molten reactor core would either burn its way through the base of the reactor, or that the base would collapse, bringing the molten nuclear fuel into explosive contact with a reservoir of water beneath.
Experts feared the second explosion would be bigger than the first, and that the core would continue sinking into the ground, possibly contaminating water supplies to Kiev, a city of 2.5 million.
"The reactor is damaged," Velikhov told Pravda on 13 May. "Its heart is the white hot core. It is as though in suspension... Down below, in a special reservoir, there might be water.
"How would the white-hot core of the reactor behave? Would we manage to keep it intact or would it go down into the earth? No-one in the world has ever been in such a complex position."
The heroes of the drama were those who battled the reactor, despite the intense radiation: People who put out the fires, who pumped water into the reactor or bathed it in liquid nitrogen, who dropped sand and lead from helicopters, dived into pools beneath the reactor to open sluice gates, or burrowed under the foundations to install a system of heat-exchanging pipes.
HOW MANY DIED?
Acute Radiation Sickness (ARS) deaths in 1986: 28
ARS patients who died later: 19 (some from other causes)
Others who died during explosion: 2
Child thyroid cancer deaths (1992-2002): 15 (UN figure)
Predicted extra cancer deaths: from 4,000 (UN) to 93,000 (Greenpeace)
Dozens killed in accidents building sarcophagus (according to an engineer)
And then the men who spent the summer erecting a vast concrete and steel sarcophagus above the reactor to seal it off from wind and rain.
There was also the US doctor, Robert Gale, who rushed to Moscow to carry out bone marrow transplants on patients suffering from radiation sickness.
The villains were the plant chiefs and senior operators, who were convicted of breaking safety rules, and jailed.
In the last 20 years a different story has emerged.
It now turns out that none of the measures taken to halt the meltdown had any major effect. Most of the materials dropped from the helicopters missed their target, the liquid nitrogen operation was called off almost as soon as it started, the water accumulated below and some was still there when part of the fuel fell into it.
Work is under way in 2006 to strengthen the sarcophagus
Fortunately that created a pumice-like rock instead of a huge explosion. The rest of the fuel, too, ran into chambers beneath the reactor, and solidified there of its own accord.
None of Dr Gale's bone marrow operations saved lives.
Questions have also been asked about whether it was right to evacuate so many people, as the uprooted communities have suffered severe social problems, and the health of people living on contaminated land has so far proved better than expected.
Most of the rules that the plant operators were accused of breaking, we now know, were only written after the accident. The chief problem, it is generally accepted, was the flawed design of the reactor.
This is the first instalment in a week of reports shedding light on aspects of the Chernobyl disaster.