Walking through the flat and endless Kazakh steppe, Nemytov Oleg suddenly stops, fumbles in his desert camouflage trousers and pulls out a Geiger counter. The device bleeps into life. He peers pensively at the reading. When we got out of the car it read 3. Now, within a couple of hundred yards, it has jumped to 10. He unwraps breathing masks and two pairs of disposable shoe coverings. "If we want to go any further we will have to wear these," he says.

Further along the dusty road he checks his device once more. "You see, the meter is now reading 21," he says. "If we were in a city far away from here it would read about 0.1. The radiation increases very quickly."

The reason Mr Oleg is keeping such a close eye on background radiation is because we are standing on the very spot where, 60 years ago, the Soviet Union launched the Cold War, with the detonation of its first nuclear bomb. Watched from a lead-lined bunker by Stalin's feared secret police chief Lavrenti Beria, First Lightning exploded at exactly 7am on 29 August 1949, throwing up an enormous mushroom cloud which billowed over the steppe and, unbeknownst to people nearby, dumping huge quantities of radioactive material on them, their houses and their fields.

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It is the names of Hiroshima, Nagasaki and Chernobyl that stand for the horrors of the new technology. The name of Semipalatinsk has no such resonance, and is all but forgotten. Yet nowhere else in the world was there such a large concentration of nuclear explosions in one place over such a long period. When Beria earmarked this far eastern corner of Kazakhstan to be the Soviet Union's top secret nuclear test facility, he described the place as "uninhabited" – conveniently forgetting the 700,000 people who lived in the surrounding villages, towns and cities. Overnight the region was deleted from the map and for the next 40 years Soviet scientists detonated 615 nuclear devices at their secret Semipalatinsk Polygon.

For the first 13 years, tests inside the 80,000 square kilometre Polygon site were conducted above ground, throwing huge amounts of nuclear waste into the atmosphere. The underground tests that followed polluted vast tracts of land with a toxic combination of radioactive chemicals which will continue to contaminate the soil for thousands of years. Kazakhstan shut down the test site almost as soon as the Central Asian republic gained its independence in 1991 (and also became the first country in the world to voluntarily give up nuclear weapons). But the deadly legacy of those tests lives on.

In a new hospital on the outskirts of Semei – the new Kazakh name for the otherwise unremarkable provincial capital which lies 150km east of the Polygon – Galina Bityukova, aged 54 and painfully thin, is midway through a second course of chemotherapy for ovarian cancer. "Sometimes I feel that my cancer is linked to the nuclear tests, you can't help but think so," she says. "It could just be cancer like anyone else gets but when you remember what happened here and how many people have cancer it makes you wonder."

On the bed opposite Svetlana, a woman in her late fifties who is recovering from a mastectomy, firmly agrees. "In my mind I know the nuclear tests had something to do with me getting ill," she says, flashing a strained smile which reveals a full set of gold teeth. Dr Baipeisov Muhametkalievich is the head of oncology at Semei's cancer ward, which treats up to 40,000 people every year. "It's difficult to know whether their cancer comes from the testing or not," he says. "But you only have to look at the data to know that this area of Kazakhstan has the highest rates of cancer of anywhere in the country." It is roughly one-third higher than the national average, he says, a clear indication that the Polygon continues to make people sick.

When Kazakhstan gained its independence following the Soviet Union's collapse, the country was left bankrupt and the damage caused by the nuclear tests was just one of the problems that Moscow consigned to the new government, dominated by the local Communist chief Nursultan Nazerbayev who is still Kazakhstan's President. As the Russian military convoys rolled back over the border they not only took away all the scientific data regarding the Polygon, but also most of the modern medical equipment from Semei's hospital.

For many years the victims of Semipalatinsk, unlike those of Chernobyl, were left to fend for themselves. But flush with new revenue from its enormous gas fields and mineral deposits, money is finally heading their way. The oncology department in Semei has just received state-of-the-art equipment from Japanese doctors in Nagasaki while a £40m radiology department is under construction. "When I first got here I was absolutely astonished at the level of poverty and neglect among the victims of nuclear testing," says Fiona Corcoran, an Irish charity worker who had seen the effects of nuclear fallout in Chernobyl and who now runs two orphanages in Semei. "Children with horrendous birth defects were just left to rot in institutions. But recently there have been some major improvements."

Ms Corcoran's charity, the Greater Chernobyl Cause, was one of many working in Chernobyl but when she arrived in Kazakhstan a decade ago outside aid was almost non-existent. "The Kazakhs would always say to me, 'People come here, they go and they forget'. There was none of the same sense of urgency that there was with Chernobyl. But what happened at Chernobyl was a single tragic accident. What happened here was the systematic and deliberate exposure of thousands of people to nuclear material."

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Most of those who worked on the test site have long since died, but the radiation levels continue to poison new generations of Kazakhs. In an anonymous-looking block of Soviet- era flats is Semei's only facility for disabled children. According to the centre's director Tylysova Toleakarovna, of the 346 children they regularly treat, 45 have illnesses which result directly from radiological contamination. Baurzhanaly Kuanysh is one of them. Now 16 years old, he was born in Abay district, one of the areas closest to the Polygon. He suffers from microcephaly, a common illness among radiation victims where the victim's head is abnormally small. "We can provide for some of the victims who live near the city but we need to get out to the villages," explains Mrs Toleakarovna. "That is my dream."

Some 160km west of Semei lies Kurchatov, a meticulously planned settlement that was once the most secretive town in the Soviet Union. Here scientists work to map and contain the nuclear contamination inside the Polygon.

What is already clear is that the three sites where the explosions were regularly conducted will be uninhabitable for thousands of years, and a river that flows through the site into the Irtysh is contaminated. Yet that has not deterred new arrivals: government and private investors are keen to open up some areas of the test site because it is littered with deposits of coal, copper and silver. There are already 400 miners digging for coal close to where some of the later and most powerful tests were carried out in the 1960s and 1970s.

But the rush to extract minerals from this poisoned land has set alarm bells ringing among medical experts. Boris Gallich, a specialist in the effects of radiation, said: "My biggest fear is that these people could become contaminated and pass it on to their children and families. That may be a matter of indifference for the company directors, but not for the people on the ground."

29 August 1949: The birth of the Cold War

* The Soviet Union's first successful test of a 22-kiloton nuclear weapon – called First Lightning – on 29 August 1949 was, in effect, the day that began the Cold War.

* Ever since the USA dropped two atom bombs on the Japanese cities of Hiroshima and Nagasaki in 1945, Stalin was desperate to obtain the same technology.

* Stalin placed Lavrenti Beria, the feared head of his NKVD secret police, in charge of the project and gave the country's top atomic scientist, Igor Kurchatov, virtually unlimited funds.

*The successful first detonation led to a massive nuclear arms race as the two foes frantically built up their arsenals, a contest which only ended with the collapse of the Soviet Union.