Sasha Yuvchenko was working at the power plant on the night of the world's worst nuclear disaster. One of his workmates was vaporised and three others died within weeks. Vivienne Parry hears his terrible tale

On April 25 1986, 24-year-old Sasha Yuvchenko clocked on as usual for the night shift at the Chernobyl power plant in northern Ukraine. It was a beautiful evening, particularly warm and clear, and Yuvchenko, an engineer-mechanic, and his workmates were full of their plans for the upcoming May Day holidays. At home, his wife, Natasha, was still up with their fretful two-year-old, Kirill.

On that fateful night, the water pumps in the newly commissioned No 4 reactor were being safety-tested. As the clock ticked past midnight, an argument was raging about the right power level at which to start the test. But what no one knew, thanks to years of error and cover up, was that there was a fatal flaw in the reactor design that made it unstable at low power levels. As power levels were lowered in preparation for the test, they dropped too low and the reactor ground to a halt. Meanwhile, unseen, a dangerous hot spot was building deep in the reactor.

To raise the power, the boron control rods were removed. It was like cocking a gun, and when, at 1.20am on April 26 1986, the test began and the turbines were turned off, the reactor was turned into a volcanic steam pressure cooker. Emergency shutdown procedures were started, but when the control rods were reinserted, their graphite tips caused the power levels to rise so dramatically that a portion of the reactor was destroyed. There were two explosions and the 500-tonne safety cap was blown off the reactor. It was the worst nuclear accident in history.

Yuvchenko, now 42, recalls what happened that terrible night 18 years ago. He is a bear of a man, 6ft 5in tall, and a former Soviet champion rower. You can't help but notice his left arm, which is half the size of his right and shiny with scar tissue. His wife, Natasha, sits nervously on the edge of her seat.

"There was a heavy thud," he says. "A couple of seconds later, I felt a wave come through the room. The thick concrete walls were bent like rubber. I thought war had broken out. We started to look for Khodemchuk (his colleague) but he had been by the pumps and had been vaporised. Steam wrapped around everything; it was dark and there was a horrible hissing noise. There was no ceiling, only sky; a sky full of stars." A stream of ionising radiation was shooting starwards, like a laser beam. "I remember thinking how beautiful it was."

Yuvchenko went with a party of men to recce the damaged reactor hall. He stayed outside, propping the heavy reactor hall door open with his shoulder. The three men who went in all died within two weeks. "You don't feel anything at the time," he explains. "We had no idea there was so much radiation. We met a guy with a doseometer and the needle was just off the dial. But even then, we were still only thinking 'Rats, this means the end of our careers in the nuclear industry. We all thought, 'We've been exposed now, this has happened on our watch' and set about doing what we could. After about an hour, I started to vomit uncontrollably. My throat was very sore."

By 6am, he could no longer walk. He was taken to the local hospital. Still he had no idea of the huge hit of radiation he'd received. "We were thinking we might have had 20, perhaps 50rem. But there was a man there who'd been involved in a nuclear accident in the submarine fleet, he said it was more serious than that. 'You don't vomit at 50,' he said."

At the hospital, they worked out (through measuring the fall in his white blood-cell count) that he'd received 410rem - or as it's now since been styled, 4.1Sv (one sievert is equal to 100rem) - a measure of the absorbed dose of radiation per kilogram of body weight. Four sieverts is lethal for half of those affected.

In the EU, the maximum dose of radiation to which the population near a nuclear power station should be exposed to is one millisievert (mSv) a year and for nuclear workers, it is 20mSv annually. The average radiation dose from natural and medical radiation is 2.5mSv. The plant workers and firefighters at Chernobyl received 650 times their permitted yearly dose and more than 5,000 times the average annual dose.

Yuvchenko was seen once by a nurse during this time but was interviewed three times by the KGB. Startling film taken by the KGB of the devastated reactor is shown in a new documentary, Zero Hour: Disaster at Chernobyl. Whoever took the film is likely to have died.

Yuvchenko was then shipped off to Moscow. No one told Natasha where he was. A rule of thumb is that vomiting that starts within half an hour of irradiation indicates a fatal dose. Of those transported with him, five died. Those who died quickly were lucky. It is a truly horrible way to die, burnt from within and without.

Some 128 people were sent to the specialised treatment centre in Moscow. When Yuvchenko arrived, his head was shaved, but within days all his body hair fell out anyway. By now all were experiencing the effects of radiation to their lungs, nose, ears and throats. For those with severe exposures, rubber-like mucus caused breathing problems, and herpes-like rashes formed massive crusts on lips and facial skin. Those who had started vomiting early were given bone-marrow transplants. Yuvchenko received the first of many transfusions.

Nobody knows quite how radiation produces its early effects of nausea and diarrhoea. When the vomiting subsides, there is a period of calm. There is seemingly trivial reddening of the skin at first, but, again after a period of calm, the skin develops weeping ulcerations over layers of dead tissue. Yuvchenko recalls pulling back the sheets, and there being a cloud of black dust - his dead skin. A slime of gamma and beta-emitting nuclides had covered all surfaces in the plant after the explosion, and where his body had touched the door - his left shoulder, hip and calf - their deadly radiation had gnawed away at his flesh, causing the death of tissue deep in his arm. It became grossly swollen and his skin turned violet black.

He had the first of many, many operations and skin grafts. For a while, he thought that his arm would be amputated. Those champion muscles were his saviour. "Mine are small," he laughs, "you should see my little brother's." His arm was to be in bandages for the next seven years. He was plagued by - and occasionally still has - outbreaks of skin ulceration. Microsurgery in Berlin, in which blood vessels were transferred from his leg to his irradiated arm, finally saw him on the road to recovery. His colleagues were not all as lucky. One who worked in the turbine hall and took 10Sv survived a bone-marrow transplant and blindness only to die after a few months.

Death after acute radiation exposure usually comes from infection as the radiation destroys bone-marrow cells, causing a catastrophic drop in infection-fighting white blood cells. The body is overwhelmed, particularly where there is intense damage to skin and intestine. People assume that there is something that you can take to prevent radiation damage, as they do in Star Trek. Potassium iodate tablets, taken within hours of exposure, flood the thyroid, and make uptake of radioactive iodine less likely. But iodine is only one of the radionuclides. Supportive care and rigorous infection control is all the medical care there is.

Yuvchenko spent a year in hospital and a further two in rehab. He attributes his survival to his treatment in Moscow - and those muscles. He doesn't know whether he is infertile - although this is highly likely - but in any case, was advised not to have another child because of the risk that they, or their children, would develop leukaemia. Chromosomal damage over 4Sv is severe. What of his prospects now, particularly of cancer?

"The doctors told me that if you've survived this, you shouldn't worry about anything else." Every year, he has two weeks of check-ups. "I always think they might find something." But he has remained well, as has his wife and son, a tribute to the extraordinary repair properties of DNA.

"It's the nerves that get you," says Natasha. For many years, people literally ran away from them, terrified of contamination. The fear of radiation and what could happen, may yet prove to be a bigger killer than radiation itself. There have been nearly 2,000 cases of thyroid cancer, mostly in children, but the predicted surge in other types of cancer has, according to the Unscear 2000 report, not yet been seen. But there have been big increases in deaths from heart disease, alcoholism and suicide in Belarus and Ukraine.

Yuvchenko considers himself lucky, especially compared with those whose extraordinary heroism finally brought the incident under control. Yuvchenko has a generous pension; they had nothing. And asked what he now thinks of the future of civil nuclear power, his answer is emphatic, and perhaps surprising. "If one learns the lessons and keeps safety as the number-one priority in all developments, then it is safe."

· Zero Hour: Disaster at Chernobyl will be shown on the Discovery Channel at 10pm on August 27