“It has always been important to resolve this spent-fuel issue and keep it in the hands of the power company,” Mr. Aikas said. Posiva, the company developing the repository, is a joint venture of the two utilities.

In the United States, spent fuel became the responsibility of the federal government, specifically the Energy Department, subjecting the issue to more political pressures.

At the Onkalo site, workers drill into the bedrock down near the 1,400-foot level, taking cores to study the characteristics of the granite. Above ground, near the curving entrance to the tunnel, construction has begun on a building where the spent fuel, currently cooling in pools at the Olkiluoto reactors, will be readied for burial, handled by remote-controlled machinery since radiation levels will be high. Spent fuel will also eventually be shipped here from Fortum’s reactors, on the country’s southeastern coast.

Kimmo Kemppainen, research manager for the project, said that in characterizing and mapping the rock, it was important to locate, and avoid, fractures where water could flow, since the disposal site was below the water table. But even if water gets near a canister, he said, the clay should form a barrier and keep corrosion of the copper — which could result in a radiation leak — to a minimum, even over tens of thousands of years.

Mr. Kemppainen has worked on the project for 14 years. “My personal opinion is that for this generation that has used nuclear power, at least we should do something about the waste,” he said. “It’s not safe to store it on the surface.”