NOTHING scares you in Chernobyl's Zone of Alienation as much as leaving it. A snowbound police checkpoint guards the entrance to this contaminated zone, which stretches 30km (18.64 miles) from the stricken nuclear plant in all directions. If you enter this zone, you can't leave it again without taking the test.
A blue-uniformed officer shows you into a grey building, where a tall machine, looking like a battered telephone kiosk, sits in the middle of a bare floor. This is the radiation detector.
There are pads here for your feet, thighs, arms and hands. If a green light shows on the side of the machine, you are clean. But a red light will mean something very different. The machine takes an age to make up its mind, time enough to remember all the beeps and boops of the Geiger counter you had with you during your tour through the zone.
That morning these same police waved us through the checkpoint with hardly a care. Few sane people want to break into a zone that will stay contaminated for tens of thousands of years. Beyond the checkpoint, a potholed road leads the way through the contaminated forests.
Out here in the snow there is a surprisingly positive legacy of Chernobyl: a herd of wild Przewalski horses, around a dozen of them, eating from a haystack. Contrary to all expectations, wildlife has thrived in the Zone of Alienation, the irradiated soil evidently having had little effect on the wolves, deer, lynx, boar, bears - and these rare horses.
"The radiation does not hurt the horses like people hurt horses," says Mary Mycio, the Ukrainian-American author of Wormwood Forest, a book which charts the zone's flourishing wildlife. This is the Chernobyl paradox. By getting rid of the people, the accident has made the area safe for wildlife."
Possibly, the animals are sicker than they look and, possibly, the radiation cuts short their original lifespan. Yet the fact remains that the zone has become one of Europe's key wildlife refuges.
Mycio is calling for a study of the wildlife and, in particular, the strange instinct that stops them migrating, despite the crumbling boundary fences. "They are intelligent animals; they seem to realise that in the zone they are pretty much undisturbed."
Progressing through the abandoned villages along the road, however, it is clear that people as well as animals are still living in the shadow of Chernobyl. Several hundred of the 145,000 evacuees have returned to their homes, among them pensioners Maria and Mikhail, both 70.
"When it happened we were evacuated, but later that year we came back. This is our home," explains Maria, as she feeds chickens and a turkey in their farmyard.
Mikhail worked for a while on the nuclear clean-up as a barge captain, bringing material for the concrete sarcophagus built over the smashed reactor.
He remembers a taste, "like salty metal on my tongue", in the air near the plant, but neither he nor his wife has noticed any specific health problems.
"The climate here is healthy, lots of open air," Maria says. "Not many people to bother you."
Ten kilometres (6.2 miles) from Chernobyl is another police checkpoint, and here things start to get serious. We face instant arrest if we stray from the path.
"This zone is frozen in a Soviet time warp," says my guide, Max, a mathematician working with staff monitoring the plant.
Here villages are not just abandoned; they are bulldozed. Houses, barns and churches sit under hillocks dotted with yellow radiation-warning signs. Stunted trees with weird branches dot the Red Forest, so named because the trees turned red and died soon after the disaster.
And then comes Pripyat, the town purpose-built for the nuclear power plant. This town's 35,000 inhabitants were given four hours to evacuate and they were never allowed back - leaving in their wake an urban Mary Celeste.
Books lie abandoned on schoolroom desks; a diary's entries stop suddenly at 26 April. In the props department of the local theatre, a giant painting of the former General Secretary, Mikhail Gorbachev, sits ready for a May Day parade that was never held.
Walking around here is eerie: like finding yourself in of one of those science fiction films where the hero wakes to find his city mysteriously deserted.
Yet visitors do occasionally show up. A few years ago police arrested a man who had moved here to hide and avoid paying alimony.
Looters have stripped windows, door frames and marble steps from the crumbling buildings. All that material is presumably now outside the zone, slowly irradiating its new owners.
We move on to the power plant itself. Chernobyl is vast, not one reactor but four. Standing in the shadow of the great grey sarcophagus that now covers the smashed reactor hall, the Geiger counter that is mandatory on such trips starts to flip. Up to 300, then, simply by crossing the street, to 600. This, Max assures me, is the same dose of radiation you'd get from flying over the Atlantic for eight hours.
The first that the outside world knew of the accident here on 26 April 1986 was when a power worker arrived for the night shift at the Forsmark nuclear power plant in Sweden, and set off alarms. At first they thought it was a mistake - those alarms were for people leaving the plant, not arriving.
But a check showed that the plant, like much of southern Sweden and, later, much of northern Europe, was being bathed in an atomic cloud that eventually ranged as far as Scotland and the Welsh hills.
It happened as a result of a test to see what would happen when power to the water pumps was turned off. The result that most of us would expect duly occurred. The core - a mix of uranium and graphite rods - started to heat up without enough coolant flowing over it.
Control rods, long rods of boron to soak up neutrons and slow the reaction, were lowered into the core. But the core was by then so hot that the control rods warped and got stuck halfway in the core, and increased the temperature still further.
At 1:23 and 58 seconds the inevitable happened: the remaining water turned to steam and exploded through the roof, spewing graphite, smoke and 5 percent of the radioactive core out into the night sky.
This being the Soviet Union, where nuclear accidents "could not happen", the fire brigade was unprepared. They went into action wearing only the coats they wore for fire work. It is a decision that Ivan Gladish has lived with ever since: As the local minister for domestic affairs, he faced an agonising decision. "Of course I felt bad about doing it, but it had to be done," he tells me back in Kiev, at the museum he runs, which is dedicated to the memory of the disaster.
Within four months, 28 firemen were dead, out of a total of 52 deaths directly attributed to the Chernobyl accident, including nine of 4,000 children who contracted thyroid cancer.
Just how bad the overall damage has been is hard to assess. The Soviets played it down at the time, while some Chernobyl aid groups, keen to raise cash in the West, have claimed that numbers of up to 600,000 were affected.
The good news is that mothers do not appear to pass on radioactivity to their young. As with Hiroshima, studies indicate no significant genetic legacy. The level of cancers and deformities of those born afterwards appears no higher than normal. But it was only luck that stopped the toll being much worse.
Then there was the negligence of the authorities, for whom preserving a crumbling Soviet empire came above preserving the people.
Nobody told local residents about the dangers of radiation, so when the fire broke out, they spent hours watching it, unaware that their faces were being bombarded by gamma rays. Five days later the Soviet authorities went ahead with a May Day parade in nearby Kiev, despite warnings that children were at risk from the contamination cloud.
An hour after visiting the reactor, we stand in the green radiation machine, which takes an age to decide whether we can be readmitted to the world.
It is time enough to worry about those warnings we heard about the way radiation travels in dust: dust from the soil, dust from decaying buildings. Then, at last, the dim green light appears, a steel bar unlocks and we are declared clean.