PRIPYAT, Ukraine – It has been almost 25 years since the Chernobyl nuclear power plant exploded, but a daunting number of questions remain unanswered.

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 seems to be “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 not soon enough for a generation of the city’s residents to reclaim their homes.

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 — 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 openess and candor is the best policy because all will be revealed sooner or later. Authorities there, in fact, 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 Soviet officials, who were variously secretive, defensive and bewildered about the plant in what is now Ukraine. 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 week.

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

Buses were still running the next morning 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 19 miles around the plant.

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. Many first heard advice to take iodine to try to stave off thyroid cancer on Voice of America broadcasts they listened to clandestinely.

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 U.S. atomic bomb dropped over Hiroshima.

Radioactive material stayed in the 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.

Even now, people who were children and teenagers at the time are still developing thyroid cancer, the U.S. National Cancer Institute said Thursday 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.

The ecological effects are also open to debate. Wildlife has returned to the region despite high radiation and even thrived — biologists 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 deformation and other ills.

The trees in Pripyat have grown big enough to almost block the abandoned apartment towers from view, but they’re stunted in other parts of the zone. They’re growing, “but clearly they don’t feel comfortable here,” said Volodymyr Holosha, head of the Ukrainian agency that manages the so-called “exclusion zone.”

Parts of the zone are apparently safe for short-term human habitation. The town of Chernobyl, about 10 miles 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 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.

“I’m confident we will get the full amount. But you have to recognize that we are living in difficult financial circumstances,” said Jean-Paul Joulia of the European Commission’s Nuclear Safety Unit.

Even when the shelter is completed, there’s no consensus on whether the area around the plant will ever be habitable.

That’s a blow for Pripyat loyalists like Andrei Glukhov, a reactor operator at the time of the explosion, who remembers the town as a model of enlightened planning, with good services, a cinema and a stirring central plaza.

Glukhov eventually moved to the United States, but “despite the fact that I live in Washington (state), I would come back.”

A few hundred of the people who were sent out of the zone after the explosion have come back, despite warnings from the Ukrainian government to stay out. But they can’t be seen as harbingers of a better future.

“They just want to finish their days in the areas they were born in, close to the graves of their relatives,” Holosha said.