A 35-man British commando team had been lost on a 1942 mission to sabotage the plant. Britain then enlisted the Norwegian volunteers under Mr. Ronneberg for Operation Gunnerside, endorsed by Prime Minister Winston Churchill. Air attacks were not ordered for fear of heavy casualties to Norwegian workers and a low probability of success because the heavy water had been distilled in a basement fortified against bombs.

After training in Britain, the group parachuted into Norway. A four-member advance team with a radio and supplies went first, in October 1942. Six others followed in February. They rendezvoused at a cabin 40 miles from the target, at Vemork, on the forested plateau called the Hardangervidda, a national park today.

Blizzards stalled them for a week. Finally, they moved out. One man was designated to break off from the team to maintain radio contact with London. Nine others set off with rations for five days, explosives, fuses, Tommy guns, grenades, compasses and a pair of metal shears that Mr. Ronneberg had picked up at a London hardware store. That final item would prove critical.

They skied by night, rested by day and reached the gorge late on the night of Feb. 27, 1943. Steep slopes plunged 1,000 feet to the Mana River. The power plant was perched on a ledge halfway up the far slope. A guarded suspension bridge over the river led to the front entrance. At the back was a railway line and the German troops’ barracks. Guards patrolled the tracks and a wire fence around the back entrance.

There was no easy way in. “There were so many things that were just luck and chance,” Mr. Ronneberg told The Times. “There was no plan. We were just hoping for the best.” They decided to try the back way.

They waited for hours after midnight, watching the changes of night-shift workers and guards. Then, 75 yards upstream, they climbed down into the windswept gorge, clinging to shrubs and branches to break falls. They crossed the river on an ice bridge, then slogged up the far slope, waist-deep in snowdrifts. They saw guards on the suspension bridge, but the rushing river and the plant’s hum masked their own noises.