THE ruins of a city raise questions, and the high-rise ghost town where the Chernobyl nuclear power plant workers once lived raises a daunting number of them.

How many people have died, or may die in the future, because of the April 26, 1986, reactor explosion that spewed radioactive fallout across much of the Northern Hemisphere? The only clear answer is "too many" - the number is still hotly debated.

Will the effects of the world's worst nuclear accident ever go away? In time, maybe, but in this generation, the Chernobyl consequences can seem like an endless run of rapids as studies detect possible new problems.

Perhaps the only question with a possible concrete answer is how much it will cost to make Chernobyl reasonably safe. At least 1.6 billion euros (US$2.27 billion) - but 740 million euros of that has yet to be found.

If any of the troubled nuclear reactors in Japan go into full meltdown or explode like the one here did 25 years ago, one lesson from Chernobyl is that the consequences are likely to be breathtakingly expensive, unimaginably complicated and traumatic for decades to come.

Japan may not have fully learned Chernobyl's clearest lessons - that candor about a catastrophe is key. Authorities there are coming under increasing criticism from the international community as well as their own people for failing at full disclosure.

Still, the Japanese have been more forthcoming than former Soviet officials. There was no official acknowledgment of the blast until three days later; the first indications of trouble came from a Swedish nuclear plant where unusual levels of radiation were detected on workers' clothes.

"At that time, for a day and a half we did not know anything about what had happened," Mikhail Gorbachev, who was then Soviet leader, said last Tuesday.

Even in Pripyat, few knew what had happened when the plant's No. 4 reactor blew up around 1:30 in the morning.

Andrei Glukhov, a reactor operator, heard the explosion in his apartment, but didn't think much of it. The next morning he called that reactor's unit to find out and there was no answer. He called a colleague at another reactor's control room, who told him "everything went OK."

"When I asked him what was the condition of unit 4, he made a pause and said, 'Look out the window.' That was my first feeling that something serious had happened," he said recently during a trip to the town arranged for reporters.

Buses were still running and Pripyat's residents waited for them outside, unaware that fallout was sprinkling down on them. But within 36 hours, the city was dead, its 49,000 residents moved out in a mass evacuation. Eventually, some 120,000 people were taken out of a zone extending 30 kilometers around the plant.

Pripyat now is an eerie relic, its abandoned white apartment towers slowly disintegrating and half-hidden behind trees uncut in a quarter-century. Weeds and brush choke the town's main square, overlooked by a derelict hotel and a rusting sign bearing the emblem of the long-gone Soviet Republic of Ukraine. A Ferris wheel that never carried a customer looms on the horizon - it was to have opened the week after the explosion.

When Soviet authorities finally admitted publicly that something had gone wrong, they spoke in vague terms. The delay and opaqueness appear to have hindered protective measures Ukrainians could have taken.

It's difficult to assess whether the delay led to sicknesses. Scientists are even deeply divided on how many have died as a result of the Chernobyl explosion, which released about 400 times more radiation than the US atomic bomb dropped over Hiroshima.

Radioactive material stayed in soil and got into plants, and because livestock ate the vegetation, milk and meat were contaminated for many years. Thousands of children developed thyroid cancer from radiation exposure, and scientists are still working to document other possible health problems.

Even now, people who were children and teens at the time of the accident are still developing thyroid cancer, the US National Cancer Institute said last week in a new research study. That indicates these cancers - among the most curable when treated in a timely manner - can develop over a long time after exposure from drinking contaminated milk and no protection from potassium iodide pills.

In 2005, the Chernobyl Forum - a group comprising the International Atomic Energy Agency and several other UN groups - said fewer than 50 deaths could be confirmed as being connected to Chernobyl. It also said the number of radiation-related deaths among the 600,000 people who helped deal with the aftermath of the accident would ultimately be around 4,000.

The UN health agency, however, has said about 9,300 people are likely to die of cancers caused by radiation. Some groups, including Greenpeace, have put the numbers 10 times higher.

The ecological effects are also open to debate. Wildlife has returned to the region despite high radiation and even thrived - biologists even report seeing lynx and moose there.

Some researchers say that emptying the zone of people helped halt the destruction of habitat. Others say the animals appear to be suffering deformations and other ills.

Parts of the zone are apparently safe for short-term human habitation. The town of Chernobyl, about 15 kilometers from the plant, houses workers constructing a new shelter for the destroyed reactor's building - but they stay there only two weeks at a time.

They have years of work ahead, constructing a shelter resembling a gargantuan Quonset hut (Nissen hut) that is to be rolled on rails over the building housing the destroyed reactor. The structure is intended to block any radioactive emissions as the reactor is disassembled. The so-called "sarcophagus" that was hastily built to cover the reactor has already exceeded its life expectancy, and the shelter won't be completed until at least 2014.

But the project, directed by the European Bank for Reconstruction and Development, is still far short on money. The bank hopes a donors' conference coinciding with the explosion's 25th anniversary will bring pledges for the 740 million euros still needed to complete the shelter and a waste storage facility. International donors already have put up 864 million euros.

Most of the donors are highly developed nations whose budgets have been sapped by financial crises. One of the biggest donors is Japan, now facing its own costly nuclear crisis. There is quiet anxiety about where the rest of the money will come from.