The Vemork Heavy Water Sabotage: The Race to Stop the Nazi Bomb

In the autumn of 1941, Leif Tronstad, a key Norwegian scientist, escaped German-occupied Norway and fled to the UK. With him, he carried details of several top-secret Nazi projects, including, significantly, the V2 rocket programme. But news that the Germans were dramatically ramping up production at the world’s only heavy-water plant at Vemork alarmed Tronstad’s British interrogators.

It seemed to confirm their worst fears: the Germans had given the highest priority to developing an atomic weapon. The Allies were left with two choices: either acquire a nuclear bomb before the Germans, or destroy the Norwegian heavy-water plant. In the end, they chose both.

What followed is probably the most daring sabotage operation of the Second World War, which did much to thwart Nazi Germany’s efforts to produce a nuclear bomb — a story which starts at the very opening of the European theatre.

Nuclear Fission and the Elephant in Europe’s War Rooms

At the outbreak of war in September 1939, there was an elephant in the war rooms around Europe: nuclear fission. The mighty potential of the atom had been known about for some time, but it was the discovery earlier the same year that this energy could be harnessed with devastating consequences that was troubling those in power. It would allow a bomb with unimaginable destructive force to be built.

It would also be sure to guarantee the first to assemble such a weapon instant victory in the growing conflict. This was clearly an issue that could not be put aside for long. Indeed, the brisk scientific progress meant that by the end of 1939, the elephant was showing absolutely no signs of leaving.

By then, scientists in France, Germany, and the United States had all reached the same conclusion: it would certainly be possible to produce a nuclear bomb by transforming the radioactive metal uranium into something else. But to do this, they would need to construct an atomic pile — a simple nuclear reactor. And for this, they needed something known as a moderator.

A moderator allows the nuclear chemistry to take place in an atomic pile. It does this by slowing down fast neutrons resulting from the fission of uranium atoms, allowing a chain of similar fissions to occur. This nuclear chain reaction is fundamental both to atomic piles and to atomic bombs.

A better moderator means that less uranium will be required to construct a pile, and given the very limited supply of uranium at the time, finding the best possible moderator was the highest priority. The teams in the three countries all found that by far the best moderator was going to be water — not just any kind of water, but the heavy kind, a very costly and difficult to produce kind, composed of a rarer isotope of hydrogen. At the time, only one country produced it, and that country was Norway, at the remote Norsk Hydro fertilizer plant in Vemork.

All eyes turned towards Norway’s Norsk Hydro and its huge stockpile of 180 litres of heavy water. The race was on.

The French Gambit to Secure Norsk Hydro’s Heavy Water

By January 1940, the Germans had already placed an order with Norsk Hydro for 22 litres of heavy water. When Norwegian intelligence services informed the French, this unsettled them. Although the quantity of heavy water the Germans had ordered would not nearly be enough to construct an atomic pile, it was suspiciously large nonetheless.

But it was news that the Germans had also enquired about buying up Norsk Hydro’s entire stock and even increasing production which finally spurred the French into action: they dispatched three secret agents to Oslo to rendezvous with a banker who had close ties with the Norwegian firm. The banker would then arrange a meeting with the plant’s director, Axel Aubert. To the delight of the French agents, Aubert turned out to be fully on their side.

“You want heavy water?” he told them. “Well, help yourself to the whole lot, and you can settle the bill when the war’s over.”

The water was now theirs. The only problem that remained was transporting the valuable dense liquid across wartime Europe. The French agents reserved several flights from Oslo to Amsterdam, but they also reserved flights to Scotland and pretended to load the canisters on one of the Amsterdam-bound aircraft.

The decoy worked: German fighters forced the Dutch-bound flight to land in Hamburg, while to the chagrin of the Germans, the agents, along with the critical cargo, made it safely to Scotland. Almost the world’s entire stock of heavy water was now safely in Allied hands. But the real trouble started in April 1940, when Germany invaded Norway and took possession of the Norsk Hydro heavy water plant at Vemork.

While the Allies already possessed virtually all the existing supply of heavy water, the Germans now controlled the only means of producing more of it. Neither of them had the five or six tons required to construct an atomic pile. With Norsk Hydro’s feeble output of nine litres per month, this was going to take quite some time.

Operation Grouse: Four Men on the Hardanger Plateau

With the Second World War raging throughout Europe, the exiled Norwegian scientist Leif Tronstad’s news that the Germans were dramatically increasing production at Vemork became the first piece of information that spurred the British into action. The second was perhaps even more alarming. In March 1942, Otto Frisch and Rudolf Peierls, two German-Jewish physicists working at the University of Birmingham, wrote and circulated a memorandum which included, for the first time, calculations on the amount of fissile material needed to make an atomic bomb.

It turned out to be surprisingly little. In the case of uranium-235, a lighter and much rarer isotope of the radioactive metal, a sphere of only 20.8 millimetres’ radius would be enough to reach the critical mass needed to sustain a chain reaction. This meant that little more than 600 grammes of the material would be needed for a bomb equivalent to thousands of tons of conventional high explosive.

Plutonium, which could be readily produced in an atomic pile, would be similarly devastatingly effective. Following the Frisch-Peierls Memorandum, the British had decided by July 1942 to launch a carefully coordinated sabotage operation against the Norsk Hydro plant at Vemork. Leif Tronstad, who had helped design the plant’s heavy-water facility, proved invaluable.

He advised against an air raid, since the electrolytic cells used for production were well protected in the concrete-lined basement. Specialist commandos would be sent instead. Einar Skinnarland, a Norwegian engineer who had fled to Britain, was parachuted to Telemark to gather intelligence for the upcoming operation.

Skinnarland already had several contacts with local workers at Norsk Hydro, and these would prove invaluable to his intelligence-gathering. Meanwhile, four other Norwegians, led by Jens Anton Poulsson, went through intensive training in Wales to prepare for the great distances they would have to cover in mountainous terrain. These four men would take part in Operation Grouse, to lay the way for arguably the Second World War’s most daring sabotage raid.

The plan was that they would be parachuted into one of the most inhospitable terrains on earth: the Hardanger Plateau in the Telemark region of Norway, a high-altitude sub-Arctic wilderness that lies to the north of the Vemork plant. They would have to do this in the depths of winter, in temperatures as low as minus thirty degrees Celsius — the most effective way to escape detection. From their landing site they would work their way south and scout for the best possible routes into the complex.

Crucially, they carried a Eureka radio beacon — guidance equipment that would allow troop-carrying gliders to land nearby. When the Grouse team landed in the dead of night of 18 October 1942, they carried provisions for five days, the time needed to trek south and reach an abandoned refuge hut near the plant. Unfortunately, they encountered some of the most severe weather conditions on record.

They were unable to carry all their provisions and equipment in one go, and the fact that they had to shuttle back and forth in ten-kilometre stages only made a bad situation much worse. The team’s leader, Jens Anton Poulsson, fell through the ice twice while crossing frozen lakes. Instead of five days, it took them fifteen to reach their planned destination.

Despite their extensive survival experience and local knowledge, they arrived undernourished, on the brink of exhaustion and hypothermia. Without the help of an abandoned sledge, which amazingly one of the team recognized as his own and had thought lost for years, they may not have made it at all.

Operation Freshman: Disaster Over Norway

On 19 November 1942, thirty-four British volunteer sappers, military engineers trained in secrecy for the operation, took their places in two Horsa troop-carrying gliders, while the Halifax heavy bombers that would tow them started up their engines for takeoff. Just before sunset, the planes climbed into the clear winter skies. If all went well, the Grouse team’s radio beacon would guide them down to marshland, some twenty kilometres to the west of Rjukan, the nearest town to Vemork.

The men knew that Operation Freshman was of critical importance, although none of them knew the exact significance of their target. Their orders stated: “Whatever happens, someone must arrive at the objective to do the job. Detection is no excuse for halting the raid.”

Prisoners were not to be taken. If one of them was wounded, he was to be simply left behind with morphine. Unfortunately, the operation was doomed as soon as they hit the first heavy cloud banks over Norway.

The towline of the first glider began to ice up, and the additional weight caused both the glider and its Halifax tug to lose altitude. In low visibility, the Halifax hit a Norwegian mountainside, releasing the glider, which crash-landed nearby. All crew of the Halifax were killed instantly, while many in the glider were left severely injured, with several dead.

Meanwhile, the second tug-glider pair tried desperately to locate the landing site, unable to pick up signals from the Grouse team’s beacon. Lost in cloud and low on fuel, they were forced to turn back. Again, disaster struck when they encountered severe turbulence, which caused the towline to snap, forcing the second glider to crash land.

No one wanted to leave the badly injured alone in the wreckage; instead, small groups left to seek help. When one group came across a farmhouse, they called the nearest doctor by telephone, knowing the Germans would be eavesdropping. Assuming they would be treated as prisoners of war under the Geneva Convention, the men were resigned to being captured.

Unfortunately, they did not know that Hitler had secretly issued his infamous Kommandobefehl — the “Commando Order” — only a month beforehand: all enemy combatants involved in sabotage operations were to be executed. Although both Freshman teams burned all the plans and maps of the area they had in their possession, a single map was missing. When the Germans captured the British commandos, they also found the map nearby.

Now they knew the mission’s target. Having this information did not prevent the Germans from torturing their prisoners before shooting them and burying the bodies in shallow graves. While the Germans initially claimed all the troops had died in combat, details of their true fate only came to light after the war.

Operation Swift and the Long Winter of Survival

Back in London, news of the failed Operation Freshman meant that a new strategy had to be found. Only the four men of the original Grouse team had survived and remained within reasonable distance of the Vemork plant. They knew the Germans were now on the alert and were certain to reinforce security at the plant.

It was decided that another glider raid would be too risky; instead, they would parachute in a small unit of troops. Six more Norwegians who had fled to Britain were carefully selected to mount the new sabotage operation, codenamed Gunnerside. Meanwhile, London renamed Operation Grouse to Operation Swift.

The four men left on the Hardanger Plateau, out of provisions in the depths of winter, had little more to eat than pine shoots and reindeer moss, a bitter-tasting lichen that is nearly indigestible for humans. Starvation and death were never far away. London may have given them a new name, but that was as far as the British could go.

Constant bad weather made it impossible to parachute in supplies. The Gunnerside team were initially due to arrive on Christmas Eve, but continued bad weather repeatedly delayed the landing. The Swift team were obliged to survive in the wilderness for a further two months.

While they laid snares for ptarmigans, a kind of Arctic grouse, they knew fishing in the frozen lakes would be fruitless, the dormant trout being unlikely to bite. Their staple diet of reindeer moss left them severely undernourished, and its unpleasant taste did little to raise their morale. Severely weakened, they found physical effort difficult.

Moving from mountain refuge to mountain refuge to escape detection, they knew seeking assistance from fellow Norwegians was risky: unfortunately, there were many collaborators. When they came across a herd of migrating reindeer, their luck changed. For the first time in weeks, they were able to eat meat.

The team’s knowledge of the traditional survival techniques of the Caribou Inuit meant no part of the reindeer went to waste, including the stomach contents, which consisted mostly of partially digested reindeer moss. The fact that the reindeer moss was partially digested made it more palatable, as it lost much of its acrid taste. When mixed with blood and served as a stew with pieces of meat, the Caribou Inuit themselves even consider it a delicacy.

Towards the end of February, their time finally came. The weather had improved, and the Gunnerside team were able to parachute in. Under a full moon, but in poor visibility, the six specially trained Norwegian commandos ended up parachuting twenty-five kilometres to the north of their planned destination.

Although Norwegians by birth, they all wore British army uniforms under their winter clothing. It was hoped that should they be captured, the Germans would not blame Norwegian resistance and take revenge on the local population. When the Gunnerside team finally found the remote hunter’s lodge where the Swift men were hiding, their new hosts greeted them with a meal of cooked reindeer stomach contents.

Still used to more conventional British army rations, the new arrivals refused. The prospect of this new diet gave the Gunnerside team one more reason to get the job done as quickly as possible.

Operation Gunnerside: The Most Splendid Coup

The narrow road that led to the Norsk Hydro plant lay on one side of a deep gully; a railway freight line lay on the other. A single-track suspension bridge took road traffic across the ravine just before the plant. The suspension bridge was far too well guarded to consider crossing, and they would be sitting targets on the road, so it was decided to try to cross the valley downstream of the river.

This would allow the saboteurs to approach the plant by the railway line, hidden in the rocky shadows of the cliff face. Thankfully, the Germans had judged the gorge untraversable and had generally neglected to guard it. On the night of 28 February 1943, the nine Norwegians scrambled down the rugged slopes of the ravine.

Without climbing gear and laden with their weapons, one of them could easily have slipped: not only would it have resulted in certain death, it could have also alerted the German guards. Give or take a sprained ankle, the crossing was successful. In full moonlight, they crept along the single-track railway line.

At the perimeter fence, they departed from the planned strategy. Instead of sawing through the wire as they had been instructed, one of the men, who just so happened to have brought heavy-duty wire cutters with him, opened a hole in the fence in a matter of seconds. It immediately became apparent that without the wire cutters, their cover could easily have been blown.

Sawing would have alerted the guards nearby. The teams split into two groups, with the Swift team staying behind to cover the Gunnerside team’s passage into the heavy-water concentration facility. Contrary to their intelligence, the saboteurs found the outer door locked; instead, they would have to use a cable duct, which was just large enough for the men to climb through.

In the basement, they quickly located the electrolytic cells. They were about to deploy explosives to destroy Germany’s entire production capacity for the precious moderator when they surprised a Norwegian plant worker, who quickly recognized fellow countrymen and was happy to help. Laying their explosives, the Gunnerside team chose thirty-second fuses.

They had intended to use two-minute fuses, but they wanted to be sure of success before they left the building. Suddenly, the plant worker announced that he could not find his glasses. Glasses were extremely hard to come by in wartime Norway, and he was anxious that he would not be able to replace them.

Taking a slightly tragicomic turn, the saboteurs then spent the next few instants rummaging around, looking for the lost glasses. When one of them found a glasses case, everyone was relieved. But as they were preparing to light the fuses, the worker unexpectedly called to them: the case was empty; the spectacles were not there.

Eventually, the worker found that he had used his glasses as a bookmark. The mission to thwart the Nazi bomb could now continue. The plant’s concrete basement muffled the explosion, and the Germans did not realize what had happened until the men had scrambled back up the valley and were already heading north on their escape route to neutral Sweden.

They made it to safety despite the massive search operation that followed. The entire production capacity of heavy water at Vemork had now been destroyed without a single loss of life. As even the German high commander for Norway, General Nikolaus von Falkenhorst, later remarked, Operation Gunnerside was “the most splendid coup.”

The American Bombing and the Sinking of the SF Hydro

Although Operation Gunnerside had been a resounding success by any standard, the Germans were able to resume production of heavy water at Vemork within five months. With drastically increased security at the plant, it was clear that another commando raid would be impossible. The Americans decided they wanted a piece of the action.

Not sticklers for subtlety, they replied to a British request with 173 Liberator and Flying Fortress heavy bombers. The aircraft took off early on the morning of 16 November 1943. Crossing the North Sea in groups of twenty, they were due to assemble in formation above the Rjukan Valley about a quarter of an hour before midday.

To minimize civilian casualties, they were to strike the factory while the majority of the workers would be having lunch. The first wave of American bombers arrived fifteen minutes early, and at great risk to themselves and the mission, they doubled back and circled to make sure they arrived when no civilians were present. Compared to raids over Germany, this was little more than a “milk run” for the airmen; yet this delay allowed the enemy to muster their anti-aircraft defences and scramble Messerschmidt and Focke-Wulf fighters.

In the end, it cost the Americans a number of aircraft. Pockets of low cloud and poor visibility hampered the targeting. Not only did the bombs destroy much of the power station, its water supply, and many other industrial facilities, they also landed on the nearby town of Rjukan.

A direct hit on an air-raid shelter killed sixteen Norwegian civilians, mostly women and children. In total, twenty-one Norwegians died during the attack. Unfortunately, the heavy-water plant remained largely intact.

The Germans, however, got the message. It became clear to them that they could no longer continue production of heavy water at Vemork. They decided to relocate the facilities to Germany, along with all existing stocks of heavy water.

On 20 February 1944, under the tightest possible security, the Germans began the transport of the remaining stocks of heavy water by rail. The journey would involve crossing mountainous terrain, including Lake Tinnsjø, which lies in a deep valley. The freight cars containing the precious moderator would have to be loaded on a ferry, the SF Hydro.

Unfortunately for the Germans, the watertight security surrounding the docked ferry on the morning the crossing was due to take place was anything but watertight the night before. This was the weakest link that allowed the Norwegian resistance to strike. Two saboteurs managed to board the ferry and place timed explosives against the hull, setting the charges to detonate when the ferry was due to be at one of the deepest sections of the lake.

At 10:30 am on Sunday, 20 February 1944, the detonators fired and the SF Hydro, along with the canisters of heavy water, sank 430 metres to the bottom of the lake. Being a Sunday meant there were relatively few passengers; nevertheless, eighteen people lost their lives: four German guards, the rest Norwegian civilians. This final operation gave the coup de grâce to Germany’s aspirations to acquire an atomic weapon.

The Legacy of Vemork and the Shadow of Nuclear Warfare

While the German armaments minister, Albert Speer, had, in his words, “scuttled the project to develop an atom bomb by the autumn of 1942,” reasoning that the war would be decided long before Germany could acquire such a weapon, the Allies had no way of knowing this. Operation Gunnerside’s commander, Joachim Rønneberg, later commented that London could have ended up “looking like Hiroshima” if the mission had failed. The heavy water lying at the bottom of the Norwegian lake could have been sufficient to allow the German secret experimental nuclear reactor to reach criticality and sustain a chain reaction.

Yet, if this had been the case, they would have been many years away from producing a nuclear weapon. Their atomic programme was underfunded, and following Speer’s decision of 1942, priority had been given to research into nuclear power for submarines and even non-nuclear projects such as the V2 rocket, which seemed to give more immediate promises of success. Armed with only conventional explosives, the V2’s limited accuracy would make it little more than a weapon of propaganda and terror.

The final outcome of the operations to sabotage heavy-water production at the Norsk Hydro plant in Vemork is nevertheless a testimony to the courage, determination, and skill of the Norwegian and British personnel who participated in Operations Grouse, Freshman, Swift, and Gunnerside, as well as the American airmen. The subsequent threat of nuclear warfare coloured much of the Cold War that was to follow. The fear of immediate annihilation hung like the sword of Damocles over much of the world’s population.

Vocal anti-nuclear movements sprang up, and the doctrine of Mutually Assured Destruction — otherwise known as MAD — dominated the thinking of NATO and Warsaw Pact strategists alike. Without the profound psychological effect the American bombings of Hiroshima and Nagasaki had, the Cold War could have easily taken another, more deadly turn. These bombings proved that the Third World War would certainly be the last.

In these days of nanorobotics, bioengineering, and autonomous weapons driven by artificial intelligence, it is perhaps fitting to remember those who risked and lost their lives in and around Vemork in order to prevent an earlier force for mass destruction entering the wrong hands. Now is perhaps the time to seriously debate how these newer technologies — and their deadly potential — should be monitored and restricted, before it is too late.

Frequently Asked Questions

Why was heavy water at Vemork so strategically important?

Heavy water was the most effective available moderator for an atomic pile—a device needed to sustain the nuclear chain reaction at the heart of both atomic reactors and atomic bombs. The Norsk Hydro plant at Vemork was the only facility in the world producing it in significant quantities. When Germany invaded Norway in April 1940, it seized control of the plant, making Vemork the linchpin of any German effort to build a nuclear weapon.

Why did Operation Freshman fail so catastrophically?

On 19 November 1942, both Horsa gliders carrying thirty-four British sappers crash-landed in Norway after icing and severe weather caused their towlines to snap. The survivors did not know that Hitler had secretly issued his Kommandobefehl just weeks earlier, ordering all enemy saboteurs to be executed rather than treated as prisoners of war. The Germans captured the injured men, discovered a single surviving map that revealed the mission’s target, tortured the prisoners, and then shot them.

How did the nine commandos of Operation Gunnerside succeed where a much larger force had failed?

The Gunnerside team crossed a gorge the Germans had judged impassable, approached the plant along the shadowed railway line, cut through the perimeter fence with wire cutters brought on personal initiative, and squeezed through a cable duct to reach the basement. Working in the basement, they located the electrolytic cells producing heavy water, laid explosives with thirty-second fuses, and escaped back up the valley before the Germans realized what had happened. The entire production capacity was destroyed without a single loss of life.

What happened after the Germans resumed production at Vemork?

Within five months of Operation Gunnerside, the Germans repaired the plant and resumed production under drastically tightened security. The Americans responded with 173 Liberator and Flying Fortress heavy bombers on 16 November 1943. The raid destroyed much of the power station but left the heavy-water plant largely intact and killed twenty-one Norwegian civilians. The bombing persuaded the Germans they could no longer safely produce heavy water at Vemork, so they decided to relocate all equipment and existing stocks to Germany.

How was Germany’s remaining heavy-water supply finally destroyed?

On 20 February 1944, the Germans began transporting the remaining heavy-water stocks by rail toward Germany. The route crossed Lake Tinnsjø, requiring the freight cars to be loaded onto the ferry SF Hydro. Two Norwegian saboteurs boarded the ferry the night before and placed timed explosives against the hull, set to detonate when the ferry reached the lake’s deepest point. At 10:30 am the charges fired, and the SF Hydro—along with all its heavy-water cargo—sank 430 metres to the bottom of the lake, ending Germany’s atomic ambitions.

Sources

1. https://www.bbc.com/news/uk-22298739
2. https://media.nationalarchives.gov.uk/index.php/audacious-raids-operations-grouse-freshman-swallow-and-gunnerside/
3. https://www.atomicheritage.org/key-documents/einstein-szilard-letter
4. https://www.atomicarchive.com/resources/documents/beginnings/frisch-peierls-2.html
5. https://www.atomicheritage.org/history/german-atomic-bomb-project
6. https://www.youtube.com/watch?v=aUfiMoY30ac
7. https://fhs.brage.unit.no/fhs-xmlui/bitstream/handle/11250/99533/INF0495.pdf
8. https://www.atomicheritage.org/history/operation-gunnerside
9. https://weaponsandwarfare.com/2017/02/04/bombing-of-vemork