Mariana Kushnir was just a little girl when reactor four exploded. As with everyone else in Ukraine, it was days before her family had any idea what had happened. She remembers coming home after playing outside with her brother and being caught in the rain, and her mother insisting that they strip and wash all their clothes immediately. She also remembers that for weeks afterwards, not allowed to go outside, they looked longingly through the window at the spring sunshine.

Eighteen years later, Kushnir is PR manager for the Ukrainian tourist board. She has criss-crossed the country as part of her job, but until now has never made the trip to what is almost certainly Ukraine's most famous spot, and is becoming one of its hottest tourist destinations. For $250 (£139) per person, Kiev-based tour agencies have begun to offer all-inclusive day trips to the scene of the world's biggest nuclear power station disaster, Chernobyl. "Observe object sarcophagus - concrete-and-steel shelter covering radioactive masses and debris left after the explosion," enthuses one travel agent's website. The price includes transport inside the zone, the military permit required to enter, and they promise to return you safely to your hotel by 6pm, in plenty of time for dinner.

It was late into a spring night, April 26, 1986, when an explosion ripped the roof off Chernobyl's fourth reactor, causing the building's walls to bend like rubber and hurling tons of radioactive waste into the air. The red light could be seen from miles away: some said afterwards it looked like it was coming straight from hell. No one, though, knew what it was they were looking at, as the authorities did not tell them: the only thing on the government's mind was how to cover up the fact that the whole of Europe would shortly be sitting under a radioactive cloud. So effective was the political strategy that even today the death toll is not known: casualty figures range from 40 (official Soviet figure) to over 15,000 (the UN estimate).

Kushnir is taking this trip out of a mix of duty and curiosity. As we turn on to the main road out of the city and its outskirts of identikit slab-grey housing blocks, Chris Rea's Road to Hell comes on the radio. She leans forward and turns it up. "Good song, no?" she grins, nervously.

The road to Chernobyl, which lies around 70km north of Kiev, winds through a set of country scenes as pretty as they are unexpected. There are wide fields of ripening crops, dotted trees heavy with fruit, postcard perfect little farmer's houses and horses clopping home in the summer sun. But the nearer we get to the 30km exclusion zone that surrounds the site, the fewer the people and houses, until even the sunlit forests start seeming a little sinister in their emptiness.

At the entrance to the zone, there is a roadblock. It used to be easy to get in here: visitors a few years back reported that a $20 bill, a packet of cigarettes and a bit of chat would do the trick. These days, soldiers man the gate 24 hours a day, checking our government passes against our passports.

The first sign of human habitation is a set of houses; once the homes of villagers, they are now occupied by the hundreds of scientists and plant workers who still operate here, studying and monitoring the site. Our guide, a portly Ukrainian gentleman called Mykola Dmitruk, climbs into our van - travel within the zone is only allowed in closed vehicles, because there is still a lot of radioactivity in the site's dust. Some tours make you change into protective clothing but Dmitruk waves such suggestions away: the site isn't dangerous, he insists. "The dose of radiation you receive here is the same as the exposure on the flight over." All we take is a battered old Geiger counter. "Now," he says pleasantly. "We go to the reactor."

Our tour bus bumps through a post-apocalyptic landscape of rusting, skeletal pylons. "On your right," says Dmitruk, in the sing-song tone of tour guides the world over, "we see the remains of reactors five and six." These were being built at the time of the disaster, and haven't been touched since. The first three reactors are fairly intact - they actually carried on operating until 2000 when they were closed down under intense pressure from the EU. But it is reactor four that we have come to see.

It does not look like a power station now. All that can be seen, beyond a wire mesh fence, is the vast, concrete block that covers the devastated reactor. It is painted white but stained with rust. Birds swirl around it: they nest, says Dmitruk, in holes in the brickwork. Terrifyingly, underneath this crumbling hulk is around 90 tons of radioactive waste. Our Geiger counter is clicking, registering levels around 10 times those at the edge of the zone. We pose in front of the reactor, feeling for the first time a little uncomfortable about being here. There are plans for a new concrete cover, but the money is coming not from the penniless Ukrainian government, which still resents that it is stuck with this deadly, expensive mess, but from the EU. At present, it is all mired in paperwork, and while the bureaucrats bicker, the sarcophagus decays. This, says Dmitruk, is the real reason the Ukrainian government is letting visitors in: they want visitors to maintain pressure on Europe to help protect and monitor the site. "If we let people in, tell them the truth, they and their governments will not be able to forget."

Yet for all this waste, one of the oddest things about Chernobyl is that it is not entirely a wasteland. Most of it looks more like a nature sanctuary, with abundant forests, lush grass and herds of a rare species of wild horse. The lack of human activity has allowed wolves, foxes, wild boar and myriad other species to flourish. That does not mean, says Dmitruk, that they have not been affected: he cites a study involving fruit flies exposed to the blast in which problems of genetic mutation did not emerge until the 26th generation. But in the meantime, the flourishing ecosystems have prompted the UN to suggest that Chernobyl should be developed as, of all things, a nature reserve and ecotourism destination.

Our next stop is the abandoned town of Pripyat. Built in the 1970s for the workers at the site, Pripyat was home to 48,000 people and with its communal living blocks, cultural centre and sports stadium, was a model Soviet town. It wasn't evacuated until 36 hours after the disaster: for two days all 48,000 men, women and children went to school, did the washing and relaxed in the town square. Then 1,200 buses were brought up from Kiev and the army forced people to board them. No one was ever allowed back.

Today, the schoolrooms are a damp, rotting tangle of rusting children's chairs and desks. Outside some flats, tattered washing still flutters on the line; in the silent town square poplar trees have sprouted through the concrete. At the sports stadium, the track is barely discernable and the football pitch has become a small forest. Everywhere is broken glass, and inside the buildings feet crunch on fallen masonry and rotten ceiling insulation. The first tour groups here were so unnerved by the total silence that they asked to leave. It is a modern Pompeii, messy as the disaster that created it.

We wander down deserted streets and into the old cultural centre on the town square. On the second floor we find what must have been the town library: a room now open to the elements stacked high with rotting books, their pages flapping in the wind coming in through the broken wall. Trying to find our way out, we creak open a door leading to the back of the building and walk gingerly into what we soon realise is an old theatre. Faded scenery is stacked at the back of the stage, and out in the auditorium, stripped of its chairs, there are glimpses of gold on the ornate curls adorning the dress circle. There is no museum exhibit, no tour guide that could explain as eloquently as this the awfulness of such abandonment.

For foreigners, Chernobyl is easily added to a long list of tourist attractions whose fame turns on tragedy or disaster. Millions a year visit Auschwitz, and no trip to Cape Town is complete without a day on Robben Island. But for those in Kiev, who live daily with the knowledge that their surroundings and probably their bodies are poisoned, such a perspective is hard to explain. "This is not a right place for tourism," says Dmitruk. "It was a place of tragedy, and is a place of tragedy still." As we drive back to Kiev, Kushnir is silent. It has, she says, been a long day. She is glad she came, but is exhausted and can't see herself returning.

Back in Kiev, the Ukrainian tourist board's executive director Iryna Gagarina smiles wearily when asked about Chernobyl. Her frustration is understandable: Kiev, with its leafy streets, hills, curling river and cobbled streets is one of the prettiest cities in Eastern Europe, a place where golden onion domes mix with untouched Soviet architecture and the beer is ridiculously cheap. It could and probably will become Europe's new hot weekend-break destination before long, and yet all people seem to want to talk about is the site of a national disaster. "The name Chernobyl is better known than Kiev, or Ukraine itself," she says. For her, as for most in Kiev, the memories are too raw for exploitation. "Chernobyl is not a historical place," she says. "It is a sleeping lion. And when the lion is sleeping, you don't open the cage."