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OPERATION GUNNERSIDE: SOE’s Sabotage of Germany’s Atomic Bomb Program

“We didn’t think about whether it was dangerous or not. . . . You concentrated on the job and not on the risks.”

—Lieutenant Joachim Rønneberg, leader of Operation Gunnerside

At the end of November 1943, high command at Britain’s Special Operations Executive (SOE) confronted a sobering situation. Operation Freshman had failed. Forty-one British commandos were either dead or soon to be executed under Adolf Hitler’s notorious Commando Order. Also, now the Nazis were alert to the fact that the Allies had targeted the Vemork hydroelectric power plant in Norway, the only source of heavy water for Germany’s atomic bomb program.

Vemork, located in the beautiful but rugged Hardanger Plateau in south-central Norway, was built to produce fertilizer. A byproduct of the process was deuterium oxide, better known as heavy water, one of two substances (graphite being the other) necessary for moderating neutron energy emissions in a nuclear chain reaction. The task of keeping heavy water out of the German nuclear scientists’ hands was assigned to SOE.

The Vemork hydroelectric power plant in 1947. Wikipedia

SOE now determined that the best chance of success lay in the insertion of a squad of operatives hand-picked from Kompani Linge. Originally named the Norwegian Independent Company, Kompani Linge was a group of SOE-trained Norwegian patriots led by Captain Martin Linge. The unit changed its name in honor of their commander following his death in an earlier mission.

Lieutenant Joachim Rønneberg, described by SOE as having “steadiness and inspiration in a high degree,” was selected to lead the mission, codenamed Operation Gunnerside, and instructed to handpick five men for it. Training for the raid, which included a scale model replica of the Valmork plant, was so thorough that Rønneberg later said, “none of us had been to the plant in our lives, but by the time we left Britain we knew the layout of it as well as anyone.”

The Gunnerside team parachuted into Norway on the night of February 16, 1943. Delayed by a sudden snowstorm, it took a week for the Gunnerside team to rendezvous with an SOE team of Norwegian scouts, codenamed Swallow, who had been conducting reconnaissance of power plant defenses and environs.

After Freshman, the Germans mined and booby-trapped the hill above Vemork and increased guards on the single-lane suspension bridge, the main route to the plant. The weak point in the defenses was the steep, 660-foot ravine the bridge spanned, which the Germans judged impassable. But Claus Helberg of Swallow had discovered a way to safely descend the ravine, cross the frozen river, ascend the other side and reach the plant by following a little-used and surprisingly unguarded railway line. Once at the facility, they would split into two teams, one to provide cover and the other to conduct the sabotage. Rønneberg would lead the sabotage team.

On the night of February 28, the teams successfully crossed the ravine just before midnight, when the guards were scheduled to change. At midnight, with the guards’ attention probably more focused on how to stay warm in the windy, below zero winter night than on a raid, Rønneberg and the sabotage team slipped unseen into the building.

Some details vary in accounts of what happened next. But all agree that Rønneberg’s team reached the electrolysis chamber where the heavy water was processed and stored, surprising a Norwegian night watchman there. With others guarding the watchman and keeping a lookout, Rønneberg and his fellow explosives expert Birger Strømsheim placed their charges. Possibly influenced by the presence of the night watchman and of a foreman captured as he was making his rounds, Rønneberg cut the fuses to 30 seconds. After everyone hustled out of the room, Rønneberg lit the fuse.

Though the noise of the blast was loud inside the building, outside the explosion attracted little notice. Guards initially thought the muffled sound was from the plant’s combustion equipment, which was known to periodically make similar-sounding explosions.

The raid was a success. Everyone in the team escaped. Though the plant itself was only slightly damaged, more than 1,000 pounds of heavy water had been destroyed, along with the equipment needed to make it. Two additional raids, a bombing raid in November that severely damaged the facility, and an SOE raid that sunk the ferry Hydrocarrying a shipment of heavy water, put an end to this source of supply.

Operation Gunnerside was regarded as SOE’s greatest mission, and all members of the team were decorated, with Rønneberg receiving one of three Distinguished Service Orders awarded. Media coverage of the Vemork missions (a total of six) was extensive. In addition to books and articles, in 1948 a Norwegian movie about the missions was released, starring some members of the team. In 1965, Columbia Pictures released the movie The Heroes of Telemarkstarring Kirk Douglass and Richard Harris, a highly fictionalized version of the action.

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