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Operation Gunnerside

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Friday, July 28, 2017
“They have three options: they can come down from the mountains above the plant, which is littered with minefields; they can cross the single-lane suspension bridge, which is closely guarded; or they can descend to the bottom of the valley across a half-frozen river and climb a 500-foot-high cliff. They decide to climb the cliff, in the middle of the night, in the middle of winter!” – Neal Bascomb, author of The Winter Fortress: The Epic Mission to Sabotage Hitler's Atomic Bomb, on Operation Gunnerside
Norsk Hydro's Vemork Plant Outside of Rjukan, Norway. Photo Credit: By Tor-Egil Farestveit (Own work) [GFDL ( or CC-BY-SA-3.0 (], via Wikimedia Commons

On the evening of February 27, 1943, nine Norwegian commandos infiltrated the German-held Vemork plant, a hydroelectric plant owned by Norsk Hydro just outside of Rjukan, Norway. Their mission was to destroy the water pipes in the basement of this plant. Although these commandos did not know the significance of their sabotage at the time, they later learned that their mission played a part in hindering Germany’s atomic bomb program.

German Atomic Bomb Project

Haigerloch Nuclear Pile

As early nuclear research began in the United States, Germany was moving forward with its own nuclear energy research and atomic bomb program. In April 1939, Germany began a secret program called the Uranverein or “Uranium Club.” Led by physicist Kurt Diebner, the German program recruited some of the top scientific minds in Germany, including recent Nobel Prize winner Werner Heisenberg.

Unlike their American counterparts, the Germans decided to use heavy water as a moderator instead of graphite. In a nuclear reactor, a moderator is used to slow down the bombardment of neutrons and control the fission process. In this way, the moderator helps to sustain a chain reaction

Deuterium oxide (D2O) or “heavy water” is a water molecule made with two deuterium ions rather than two hydrogen ions. Deuterium is an isotope of hydrogen whose nucleus includes a neutron in addition to a proton and electron. This additional neutron causes the deuterium to have a higher molecular weight and thus form “heavy water” when combined with oxygen.

The Germans hoped to use heavy water for this purpose; however, they were never able to achieve a successful chain reaction. This failure was partly due to heavy water’s limited effectiveness in comparison to graphite, but more generally resulted from the lack of coordination and support for the German atomic bomb program amongst scientists, the government, and military.


Norsk Hydro’s Heavy Water Plant

Following Germany’s invasion of Norway in April 1940, the Germans took control of Norsk Hydro’s Vemork plant just outside of Rjukan. Although the Vemork plant was originally designed to use mountain water for electrolysis to produce ammonia for nitrogen fertilizer, the plant had recently become the first industrial-scale production site of heavy water in the world.

In 1933, Leif Tronstad, a chemistry professor at the University of Trondheim, and Jomar Brun, the head of Norsk Hydro’s hydrogen electrolysis plant, collaborated to design Vemork’s heavy water production facilities. By January 1935, the plant had produced more than 100 grams of heavy water. The significance of this amount becomes clear after considering the fact that heavy water and regular water generally exist in about a one to 41 million molecule ratio.

Following the Germans’ seizure of the Vemork plant, they forced the plant workers to increase the production of heavy water. By the end of 1941, the output of heavy water exceeded previous production rates by approximately 100 kg more per month and totaled about four kilograms per day.


‘The Ministry of Ungentlemanly Warfare’ 

British intelligence had first received word of the Germans’ desire to increase the plant’s production of heavy water during the summer of 1941 in a message from the Norwegian underground resistance. With his connections to the plant and role in the Norwegian resistance, Leif Tronstad obtained this information and anonymously corresponded with the British. These connections provided the foundation for the later raids on the Vemork plant by the British-sponsored Norwegian commandos. The British designed these sabotage missions because they realized the Germans were probably using the heavy water for a nuclear reactor and an atomic bomb program.

King Haakon VII, Prof. Leif Tronstad (front, left), Capt. Harald Sandvik, Crown Prince Olav at SEO Training Facility in Scotland (1944). Photo Credit: By Henrikaamodt (Own work) [CC BY-SA 4.0 (], via Wikimedia

To avoid capture by the Gestapo, Tronstad fled to Sweden and then Britain, where he revealed himself to be the source of the intelligence on the Vemork plant. Given his valuable information about the Germans’ production of heavy water and his background in chemistry and Norwegian intelligence, Tronstad was prevented from joining the field teams and instead was appointed the leader for training commando units for sabotage operations in Norway. 

The British were rather familiar with wartime sabotage; the country had a secret unit called the Special Operations Executive (SOE). Neal Bascomb, the author of The Winter Fortress: The Epic Mission To Sabotage Hitler’s Atomic Bomb, explained that the SOE could be better known as ‘the ministry of ungentlemanly warfare.’ The SOE trained operatives for covert sabotage missions. The Norwegian branch of the SOE was known as Company Linge. The branch recruited Norwegians who fled to England following Germany’s invasion of Norway. Operatives completed grueling training in Scotland that featured night training exercises like climbing mountains, fording rivers, and camping outdoors for weeks.

In order to prepare for their future mission at Vemork, operatives needed to be put through this type of special forces training. In an interview with National Geographic, Bascomb explained that “Vemork was a natural fortress.” The plant was surrounded by the Hardangervidda, a high mountain plateau. As described by Bascomb, the Hardangervidda is where, “according to legend, it grows so cold, so fast, that it freezes flames in the fire.”


Operation Grouse and Operation Freshman

For their knowledge of the plant, land, and climate, the British turned to Tronstad and Company Linge to stop the Germans’ heavy water production at Vemork. In October 1942, the first attempt to infiltrate the Vemork plant began with Operation Grouse and Operation Freshman. Although bombing the plant was favored by the American military, the British opted to run an SOE-sponsored mission based on Tronstad’s guidance.

Airspeed Horsa ExCC - British Glider Plane-Type Used in Operation Freshman

Tronstad was strongly against bombing the plant for two reasons. First, if the bombs hit the liquid-ammonia storage tanks at the plant’s chemical complex, the civilian population in Rjukan was at risk. Second, the heavy water facilities were located in the basement of the plant under layers of concrete and metal, so bombing would not necessarily destroy them anyway. 

In the first stage of the mission, a small scout team of Norwegians codenamed Grouse parachuted from a plane and landed in the surrounding area of the plant to gather intelligence and prepare for the attack group. A month later in November 1942, Operation Freshman commenced with a group of thirty-nine British airtroops. Carried by two military gliders, these soldiers were supposed to land near the plant under the guidance of the Grouse group and then attack the plant.

A combination of bad weather and communication issues, however, prevented the mission from proceeding as planned. One of the gliders crashed into a mountain and the other crash-landed far away from the targeted landing site. All of the men in the glider that hit the mountain died from the crash. While some of the soldiers on the second glider died in the crash, the others were executed by the Gestapo, who were acting on Hitler’s recently issued Commando Order.

Although they found a map that identified Vemork as the British target, the Germans were not able to find and capture the Grouse group, who were still in the area. Remaining in the wilderness with their limited supplies, the Grouse group was forced to live off the land and hunt wild reindeer. The group, renamed Swallow, proved to be integral in the second mission to raid Vemork, Operation Gunnerside.


Operation Gunnerside

Rjukanbanen (Rjukan Train Line) from Rjukan towards the power plant at Vemork, Norway. Photo Credit: Anders Beer Wilse [Public domain], via Wikimedia Commons.Three months after the first attempt, the Swallow group received word from Britain that six more Norwegians would be sent to Rjukan for Operation Gunnerside. Unlike in Operation Freshman, the special forces group was a small group of Norwegian commandos from Company Linge. The group was supposed to parachute to their target zone rather than use a glider to land and meet up with the Swallow group before raiding the Vemork plant.

Led by Joachim Rønneberg, the group jumped from a plane under the cover of snowfall at around midnight on February 16, 1943. The commandos all dressed in British uniforms underneath their snowsuits; they reasoned that if the British were blamed for the sabotage rather than the Norwegian resistance, the local population would be less likely to face German repercussions. While the group survived the landing and avoided initial German detection, they landed miles from the planned target site. After traveling for about five days, the Gunnerside group connected with the Swallow group.

Late in the evening on February 27, 1943, the Gunnerside group began the raid on the Vemork plant. There were three ways to access the plant: 1) come down from the mountains above the plant, which was an area covered in minefields; 2) cross a heavily-guarded, single-lane suspension bridge; or 3) travel to the bottom of the gorge, cross a half-frozen river, and climb a 500-foot-high cliff. According to Rønneberg, the group voted to take the gorge, which led to a route alongside a railway line that a local contact said was relatively unguarded.

In order to get through the plant’s side gate fence, Knut Haukelid used a pair of heavy-duty metal cutters brought by Rønneberg. Fortuitously, Rønneberg had purchased the metal cutters in Cambridge, England after going to a movie on his day off. He told The New York Times that the handsaw provided by the British military “would have taken too much time, made too much noise and alerted Nazi guards.”

Vermork, Norsk Hydro. Heavy Water Production Cells. An example of the equipment destroyed in the WW2 raid. Photo Credit: Martin V. Morris via Martin_VMorris Flickr (

After getting through the fence, the group divided into their four-man explosives group and five-man cover squad. The explosives group planned to enter the plant through a side door. This door, however, was locked. Rønneberg was able to find an access tunnel and entered the plant with Fredirk Kayser.

He recalled to The Times, “Getting inside I was quite certain that the rest of the party would follow me, but only one chap came. The other ones hadn’t found the entrance to the tunnel. Therefore we decided we would have to do it ourselves and started laying out the charges.” After getting separated from their squad mates, the other two men on the explosives team, Kasper Idland and Birger Strømsheim, decided to break in through a window.

Once in the building, Rønneberg and his men placed two strings of explosive charges next to the heavy water production cells. In order to provide enough time to escape but still hear the explosion, he decided to shorten the fuses so that they lasted only thirty seconds rather than the designed two minutes. The explosion apparently was not as loud as expected, but nonetheless, Rønneberg heard it and knew the mission was a success.  

After fleeing the plant and reconnecting with their cover squad, Rønneberg and the rest of the Gunnerside group began to ski toward Rjukan. Once they reached the mountain plateau, the group split up. In uniform and fully armed, the explosives team traveled more than 200 miles to Sweden on skis. The cover group, on the other hand, spread out throughout the plateau. Despite the Germans’ search and pursuit of the group, none of the members were killed or captured. Upon inspecting the damage to the heavy water facilities, General Nikolaus von Falkenhorst, the head of German forces stationed in Norway, referred to Operation Gunnerside as “the most splendid coup” (Bascomb, p. 213).


Outcomes of Operation Gunnerside

D/F Hydro that operated between Rollag and Mæl as a railway ferry. Sunk by Norwegian resistance during World War II. Photo Credit: Anders Beer Wilse [Public domain], via Wikimedia Commons.

Operation Gunnerside successfully destroyed the Vemork heavy water production facility and supplies. The raid caused the Germans to lose about 500 kg of heavy water and decommissioned the plant for a few months. The mission’s success, however, was not a final blow to the Germans’ heavy water production. By May 1943, the heavy water production facilities were rebuilt and operating again. 

Even though bombing the plant was initially ruled out, the United States decided to bomb the Vemork plant following the Germans’ reconstruction of the heavy water facilities. On November 16, 1943, 140 American bombers flew over Rjukan and bombed the Vemork plant. According to Thomas Gallagher’s Assault in Norway: Sabotaging the Nazi Nuclear Program, the heavy water production facilities experienced minimal damage from the bombing (p. 168). Figuring the attacks would only continue, the Germans decided to stop producing heavy water at Vemork after the bombing. Unfortunately, there twenty-two Norwegian civilian casualties in the bombing raid, the tragedy Tronstad had hoped to avoid (Powers, p. 212).

Germany’s attempt to move their heavy water supplies from Norway to Germany also ended in failure at the hands of Norwegian saboteurs. Led by Knut Haukelid, a group of Norwegian saboteurs was ordered to sink a ferry carrying the Germans’ semi-finished heavy water products to research centers in Germany. On February 20, 1944, the “Hydro” ferry was sunk by an explosion in the boat’s bow, and the Germans lost their last supplies of heavy water from the Vemork plant. There were four German and fourteen Norwegian casualties from the explosion.


Historical Legacy of Operation Gunnerside

In discussing Operation Gunnerside, some authors and historians downplay the significance associated with the operation’s success. In his book, Operation Big: The Race to Stop Hitler’s A-Bomb, Colin Brown wrote: “It is easy to understand why such hyperbole should be lavished on Operation Gunnerside, but it was an exaggeration.” Considering that the mission only temporarily halted heavy water production, Per F. Dahl called the mission “a qualified success” (p. 205). 

From the left: Knut Haukelid, Joachim Rønneberg, Jens Anton Poulsson shaking hands with HM King Haakon VII of Norway (1948). Photo Credit: Leif Ørnelund [CC BY-SA 4.0 (], via Wikimedia Commons.

In a similar vein, historical evidence shows that the Germans had actually slowed investment in their atomic bomb program by the time of the operation. In his memoirs, Albert Speer recalled that “on the suggestion of the nuclear physicists we scuttled the project to develop an atom bomb by the autumn of 1942” (p. 227).  

Bascomb, however, provides an important reminder as to why Operation Gunnerside deserves the plaudits. He points to the fact that Allies did not know how far along the Germans were in their research and production of an atomic weapon. All they knew was the Germans were greatly interested in heavy water and that heavy water could be used in a nuclear reactor as a moderator. 

The uncertainty about German progress was also a major driving force for the United States’ huge investment in the Manhattan Project. Robert Furman, who was the Chief of Foreign Intelligence for the Manhattan Project and Alsos Mission member, stated that “the Manhattan Project was built on fear: fear that the enemy had the bomb, or would have it before we could develop it.”

At the very least, Operation Gunnerside should be recognized as one of the most successful SOE missions during World War II. For a mission that Rønneberg and his squad frequently imagined as a one-way trip, the operation experienced no casualties and succeeded in temporarily destroying the Germans’ single source of heavy water at the time. During wartime, time is of the essence and any kind of setback has disadvantages. Rønneberg later commented that London could have suffered a different fate and ended up ‘looking like Hiroshima’ if his team had failed. 


Operation Gunnerside in Popular Culture

Today, the contributions of Rønneberg, Tronstad and the other members of Operation Grouse and Gunnerside have been memorialized through museums, military accolades, and movies. Even with the film’s various historical inaccuracies, The Heroes of Telemark (1965) starring Kirk Douglas is probably the most well-known film representation of the operations.

Most recently, the Norwegian Broadcasting Company produced a six-episode miniseries, “Kampen Om Tungtvannet” (“The Heavy Water War”), about the Norwegian saboteurs. In its premiere, the series had 1,259,000 viewers, and in the following weeks, the number of viewers continued to increase with the series eventually breaking the record for the most highly viewed television drama in Norway. Later, the series was released in the United Kingdom, Denmark, Poland, France, and the United States. The series was availain the United States on Netflix until August 1, 2017. For images from the television show’s set, see Ian Brodie’s website here.


List of Operation Grouse and Gunnerside Members

Grouse/Swallow Team:

  • Jens-Anton Poulsson (Grouse Team Leader; Cover Squad)
  • Arne Kjelstrup (Cover Squad)
  • Knut Haugland (Radio Operator)
  • Claus Helberg (Cover Squad)
  • Einar Skinnarland (Radio Operator)


Gunnerside Team:

  • Joachim Rønneberg (Team Leader; Explosives Squad)
  • Knut Haukelid (Second in Command; Cover Squad Leader)
  • Fredrik Kayser (Explosives Squad)
  • Kasper Idland (Explosives Squad)
  • Hans Storhaug (Cover Squad)
  • Birger Strømsheim (Explosives Squad)


The three isotopes of hydrogen. Image by Lamiot for french version, from Dirk Hünniger [GFDL (, via Wikimedia Commons.

Norwegian volunteers in battle gear, somewhere in Northern Finland. (Jan. 1940). Operation Gunnerside members wore similar attire. Photo Credit: [Public domain], via Wikimedia Commons.

Bridge to Vemork Plant. Photo by martin_vmorris (Norsk Hydro power plant, Rjukin, Norway), via Wikimedia Commons.

Related Video:
Trailer for The Heavy Water War (2015) television miniseries: