As nuclear workers in Japan struggle to contain radiation from the Fukushima reactor, world attention is turning back to Chernobyl, Ukraine. There, people prepare to mark the 25th anniversary of the explosion that blew the roof off Reactor Number 4.

Soviet planners designed Chernobyl in the 1960s to become the largest nuclear power station in Europe.

Instead, Chernobyl is remembered today as the site of the largest nuclear disaster in the world.

Late on the night of April 25, 1985, Yuri Andreyev left his shift as an engineer at Nuclear Reactor No. 4. Ninety minutes later, a safety experiment went awry. The fuel rods melted down, an explosion blew the roof off, and a raspberry-colored light spewed into the night sky.

When Andreyev returned to work, he saw a scene of devastation. After stepping over the discarded boots, jacket and helmets of fire fighters, he stood in the ruined computer control room and looking up saw blue sky.

Twenty-five years later, Andreyev runs Chernobyl Forum, a political lobby for Ukraine’s 100,000 surviving "liquidators" or clean-up men and women. After weeks of heroic work, the liquidators had succeeded in sealing the plant in an improvised steel and cement "sarcophagus."

But that was not before Chernobyl leaked 10 times the radiation of the Hiroshima atom bomb into the environment.

Authorities mapped out the area of the highest contamination - and closed it to human habitation. About 350,000 were forcibly evacuated from a largely rural area slightly larger than the American state of Rhode Island. Still living in this area are sprinkled about 300 largely elderly holdouts, now called ‘forest people.’

After a quarter century, biologists call this zone "Europe’s largest wildlife refuge." With the presence of humans gone, the new colonists are thriving populations of gray wolves, brown bear, elk and wild boar.

In January, Ukraine opened the area to short, controlled visits by tourist buses.

Twenty five ago, a convoy of 1,000 buses evacuated the entire population of Pripyat. A bedroom community for nuclear power workers, it had once been a Soviet model city - home to 50,000 people.

On a recent afternoon, a lone tour bus made the reverse commute, moving slowly down a deserted Lenin Avenue. A recording of the original evacuation order played to a bus filled with Russian and Ukrainian tourists.

Dense forest covered what once were neatly tended playgrounds. Sturdy trees grew up between rusting swing sets. Bushes and trees made driving down side streets impossible. Through the branches, visitors could make out fading communist slogans - hailing the 50th anniversary of the founding of the Soviet Union and calling for ‘Atoms for Peace.’

Alexander Sirota lived in Pripyat, until he was 10. Now as a 35-year-old tour guide equipped with a walkie-talkie and digital Geiger counter, he shows tourists - some wearing face masks - his old apartment.

Surrounded by peeling paint, sagging strips of wall paper and light fixtures dissolving in rust, he said he is happy to visit his old home, a place where he spent "the happiest days of my childhood."

Boots crunching over broken glass, Sirota later takes tourists to the gutted cafeteria where he and his mother used to go for breakfast. Then, we go to his elementary school. There, 25 summers and 25 winters have taken their toll, causing a front wall to collapse, exposing old Soviet classroom murals.

For these tourists turned archeologists, the walk takes us below a rusting hammer and sickle sign atop the old administration building and then on to a frozen Ferris wheel - the centerpiece of an amusement park built for May Day festivities that never came.

Maxim, a young man from Donetsk, drops his face mask long enough to say Chernobyl tourism is ‘cool.' But he admits that none of his friends would join him. They said he was crazy to come here: "Insane. They are afraid. Afraid of radiation."

The tour bus rolls on to Chernobyl nuclear power station, stopping 200 yards from Reactor Number 4. Due to high levels of ambient radiation, we have only 20 minutes to pose for souvenir pictures in front of the old sarcophagus of decaying cement and rusting steel.

Laurin Dodd, an American engineer, has come to the site to talk to VOA. He is directing an American-led project to build a new, modern sarcophagus.

"The structure itself is almost a house of cards," says Dodd. "It was built with some robotics and under extreme conditions. And there are large gaping holes. If you go inside, you will see holes the size of picture windows with small mammals going in and out, birds flying in and out."

As scaffolding props up the old ventilation stack, Dodd races to keep the nuclear genie in the bottle.

"There is almost 200 tons of radioactive material still inside the old sarcophagus," said Dodd, who has worked here off and on since 1995. "And the existing sarcophagus was built in six months in 1986 under, I should say, fairly heroic conditions and it had a design life of 10 years  - that’s almost 25 years ago."

Built on rails and rising high enough to cover the Statue of Liberty, the new containment structure is to be the largest moveable structure in the world. On April 19, Ukraine officials will hold a donor conference in Kyiv to raise $1 billion to build a structure designed to contain Chernobyl’s nuclear mess for another century.

As authorities in Japan may soon discover, big nuclear accidents have a defined beginning. It is unclear when they ever end.

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