Chernobyl's coffin is cracking. Birds and rainwater have gotten inside the steel-and-concrete shelter hastily built over the reactor that blew up in 1986, and officials worry about what is getting out.

The "sarcophagus" over reactor No. 4 is reaching the end of its life span. A multinational $1.1 billion project to build a new shelter — a giant steel arch designed to last 100 years — is still on the drawing board.

"Twenty years have already passed since the accident, but the risks and the hazards posed by the reactor are still there," said Yulia Marusych, a spokeswoman for the power station.

The sarcophagus of nearly 700,000 tons of steel and 400,000 tons of concrete was hastily built to seal in an estimated 200-ton mix of radioactive fuel and materials like concrete and sand that fused when the explosion spiked temperatures to 1,800 degrees inside.

No one knows exactly how much radioactive fuel remains since only 25 percent of the reactor is accessible. Some estimate it all was discharged during the 10 days when the reactor spewed out its insides. Others counter that as much as 90 percent is still there. Sensors constantly check for signs of new reactions taking place.

"Could it begin again? It would need certain conditions and we can say that today those conditions do not exist," Marusych said. "But the chance that a chain reaction could be triggered is not zero. The danger remains."

Didier Louvat, a radiation waste expert with the International Atomic Energy Agency who studies Chernobyl closely, sees no reason for alarm — "The situation is stable ... at the moment the conditions are not a matter for concern."

Some accuse the Ukrainian government of playing up the dangers to get more international aid for the new shelter. But Yuriy Andreyev, head of the Chernobyl Union, an advocacy group, accused the government of not doing enough. He said water accumulating under the reactor is highly irradiated and could leak into the region's groundwater.

Authorities said the priority now is stabilizing the sarcophagus. The roof is not sealed properly. The water inside is weakening the concrete and metal. The shelter's original west wall is leaning precariously.

While a collapse would be unlikely to spark another explosion, it could release a huge burst of poisonous radioactive dust.

For now, while talks continue on who will build the new shelter, construction crews are working to shore up the aging sarcophagus. They have to work in 20-minute shifts to minimize exposure to radiation.

"About the danger? Well, everybody knows where he works and everybody realizes the real hazards, the real risks of working here," said Yuriy Tatarchuk, a Chernobyl official.