Operation Gunnerside

During the night of 27–28 February 1943 nine Norwegian soldiers in British uniforms climbed down into the rocky gorge of the Måna river, crossed the ice to the far bank, scaled the slope and broke into the Vemork plant near Rjukan in southern Norway. In the basement of the hydrogen factory belonging to the Norsk Hydro company they placed charges under the electrolysis cells in which heavy water was produced – the only industrial production in occupied Europe of a substance that German physicists needed to build a nuclear reactor. The explosion destroyed the installation and the stock of finished product. Not a single shot was fired, no one was killed, and all the saboteurs evaded a manhunt by several thousand men.

The operation bore the code name Gunnerside. The assault team proper consisted of six soldiers of the British Special Operations Executive (SOE) under Lieutenant Joachim Rønneberg. It is therefore often described as the work of six Norwegians, but nine men went to the factory: the six from Gunnerside were guided by three soldiers of the Swallow team, who had been hiding in the mountains for four months and had known the terrain since childhood. The whole undertaking was the work of two Norwegian teams, not one.

Heavy water and the German uranium project

Ordinary water contains a small admixture of molecules in which the place of hydrogen is taken by its heavier isotope, deuterium. Such heavy water is an excellent moderator: it slows down the neutrons released by the fission of uranium while hardly absorbing any of them itself. With it, a reactor can run on natural uranium, without costly enrichment. At Vemork, where cheap power from the waterfalls made it possible to carry out the electrolysis of water on a huge scale, heavy water was recovered as a by-product of producing hydrogen for nitrogen fertilisers. The installation was designed in 1933 by the chemist Leif Tronstad of the Norwegian Institute of Technology in Trondheim and Jomar Brun, the manager of the hydrogen plant; production began in 1935 and throughout the war it was the only industrial-scale production in the world.

The German physicists of the so-called Uranverein (Uranium Club), among them Werner Heisenberg, concluded in 1940 that the graphite available was too impure to serve as a moderator, and opted for heavy water. Even before the German invasion, in January 1940, the IG Farben concern, a co-owner of Norsk Hydro, tried to order far more heavy water than Vemork could produce. The French, who were working on the same problem, forestalled the Germans: in March 1940 agents of French intelligence took out of Norway the entire stock then available, some 185 kg, lent by Norsk Hydro’s management. After the fall of France, Frédéric Joliot-Curie’s team handed it over to the British; the canisters reached England in June 1940 aboard the SS Broompark.

After the German invasion of Norway in April 1940 the plant fell into the occupiers’ hands. The Germans ordered production to be increased many times over and the cascade of electrolysers to be expanded. News of this reached London from Norwegian engineers, above all from Tronstad, who escaped to Britain in 1941 and became an adviser to SOE and the Norwegian High Command. The Allies did not know how far the German scientists had got, but they did know that they themselves were already close: in December 1942 the first atomic pile went critical in Chicago. The increase in heavy water shipments from Vemork was read in London as a sign that the Reich was building a reactor and, in the longer term, a bomb.

Grouse and Freshman

The first attempt to destroy the plant was made in the autumn of 1942. In the spring of that year SOE had infiltrated the engineer Einar Skinnarland, a local man, into Rjukan to reconnoitre the area and maintain communications. On 18 October 1942 four soldiers of the Norwegian Independent Company 1 – known as Kompani Linge after its fallen commander, the same company whose men had fought a few months earlier in the Vågsøy raid, in which Captain Martin Linge was killed – parachuted onto the Hardangervidda plateau. The Grouse team – Jens-Anton Poulsson, Arne Kjelstrup, Knut Haugland and Claus Helberg – came from the Rjukan area. Dropped in the wrong place, after many days of skiing across the snowbound wilderness they prepared a landing ground on the frozen lake Møsvatn.

There, two Airspeed Horsa gliders carrying British sappers of the 1st Airborne Division were to land and blow up the factory (Operation Freshman). During the night of 19–20 November 1942 the expedition ended in disaster. The Halifax bombers towing the gliders failed to find the landing ground because the radio beacon link failed, and over Rogaland the aircraft began to ice up. One of the tugs crashed into the mountains, and both gliders came down in south-western Norway, far from the target. The Germans murdered the survivors of the crashed gliders under the Commando Order issued a month earlier: fourteen were shot the very next day near Egersund, four were killed after torture in the prison in Stavanger, and five more were shot in January 1943. According to Norwegian figures, a total of 41 men died in the operation.

The failure of Freshman had two consequences. The Germans realised that Vemork had been the target and strengthened the plant’s defences: they mined the surrounding slopes, installed searchlights and doubled their vigilance on the only access road – the suspension bridge over the gorge. The British, for their part, decided to give up on a large airborne landing and entrust the task to the Norwegians. The Grouse team, renamed Swallow for security reasons, was to hold out in the mountains until reinforcements arrived. The winter of 1942–1943 was a hard ordeal for them: their supplies ran out, and the men lived on lichen and moss until, just before Christmas, Poulsson shot a reindeer.

The drop and the march on Vemork

Six volunteers from Kompani Linge were chosen as reinforcements. They were commanded by 23-year-old Joachim Rønneberg from Ålesund, an instructor in the company; his second-in-command was Knut Haukelid, and the team also included Fredrik Kayser, Kasper Idland, Hans Storhaug and Birger Strømsheim. The plan was drawn up by Tronstad, and in Britain the soldiers rehearsed on a mock-up of the electrolysis hall until each knew the layout of the cells and where the charges were to be placed. They also knew about the Commando Order and the fate of the British soldiers of Freshman; each was given a cyanide capsule.

The first flight, in January 1943, failed – the pilot could not find the drop zone. Only during the night of 16–17 February did a Halifax of No. 138 Squadron RAF drop the team onto a frozen lake on the Hardangervidda, several dozen kilometres from the Swallow hideout. Gunnerside sat out a blizzard lasting several days in an abandoned hunting hut and found Poulsson’s team only after a few more days. The combined teams set up a base in huts above Vemork. Helberg went down to reconnoitre and established that in winter the water level in the Måna was low, and that on the far side of the gorge a railway track led into the factory grounds, which the Germans had neither mined nor guarded. Rønneberg therefore abandoned the idea of attacking across the guarded bridge.

The night of 27–28 February

On the evening of 27 February, at around 8 p.m., nine men set out from the base; Haugland and Skinnarland stayed behind with the radio to maintain contact with London in case of failure. The team climbed down the steep, icy slope to the bottom of the gorge, crossed the river on the ice and scaled the cliff opposite. By around 11.30 p.m. they were at the factory. Rønneberg’s men wore British uniforms – this was meant to protect the local population from reprisals, although in view of the Commando Order it gave no guarantee for their own lives. The same purpose was served by a Thompson submachine gun, which they left behind as proof that the attack had been carried out by British forces and not the local resistance.

At around 12.30 a.m. they cut the chain on the gate. A covering party under Haukelid took up positions opposite the hut in which the German guard of a dozen or so men was resting, ready to fight if the alarm was raised. The demolition party – Rønneberg, Kayser, Strømsheim and Idland – was to enter the electrolysis building. The doors were locked, so Rønneberg and Kayser crawled into the basement through a cable duct, while Strømsheim joined them through a smashed window; Idland guarded the entrance. In the hall they found only a Norwegian watchman, who offered no resistance. The commandos placed explosive charges on the eighteen cells in which the final stage of heavy water concentration took place. To make sure the charges would go off, they shortened the fuses from two minutes to about half a minute. Before setting them off, they let the watchman find his spectacles, which would have been impossible to replace in wartime.

The explosion came when the saboteurs were already outside. Within the thick walls it sounded muffled; the German guards looked around but did not raise the alarm, reportedly assuming that the snow had set off one of the mines. The whole action at the factory took about half an hour. The team went back through the gorge and climbed back up onto the plateau before the Germans realised what had happened. The hard, frozen snow left no ski tracks.

Estimates of the damage vary from source to source. Most often it is said that eighteen cells and some 500 kg of heavy water were destroyed – in practice the entire stock produced during the occupation and not yet shipped to Germany. Rønneberg and some authors add some 400 kg of lost future production, hence the figure of about 900 kg in Norwegian literature; in some accounts the figure is as high as a ton and a half. Production was halted for several months: according to most sources the installation was rebuilt in April, and it regained full capacity in the summer of 1943.

The escape and the manhunt

Five of the Gunnerside soldiers – Rønneberg, Kayser, Idland, Storhaug and Strømsheim – set off on skis for Sweden. They crossed the mountains in uniform and armed, avoiding inhabited valleys, covering some 400 km in two weeks. On the evening of 18 March 1943 they crossed the border into the neutral country and from there returned to Britain. Haukelid and Kjelstrup stayed on the Hardangervidda to organise the resistance in the region, Haugland and Skinnarland continued transmitting to London, and the others dispersed across the country. News of the operation’s success was sent by radio to SOE and Tronstad.

The Germans committed between 2,800 and 3,000 soldiers, according to various sources, to the manhunt in the Rjukan and Hardangervidda area. They caught none of the saboteurs. Helberg came closest: pursued by a German soldier on skis, he wounded him in an exchange of fire, fell from a rock after dark and broke his arm, and then managed to pass himself off as a Norwegian helping with the search and to get treatment from a German doctor. When all the guests at a hotel in Dalen were detained on the orders of the Reich Commissioner, Josef Terboven, Helberg jumped from the bus carrying them and got away.

Terboven demanded harsh reprisals against the people of Rjukan and ordered hostages to be arrested. General Nikolaus von Falkenhorst, commander of the German forces in Norway, who inspected the damage, nevertheless judged the action to be a military operation carried out by uniformed soldiers; according to post-war accounts it was he who secured the release of those detained, and he called the action itself “the finest coup” he had seen in this war. The ruse with the uniforms and the abandoned submachine gun had thus served its purpose. The guard around Vemork was strengthened still further.

What came next

Operation Gunnerside did not end the struggle for Vemork. The Germans rebuilt the installation faster than London had expected, and by the summer of 1943 production was again in full swing. SOE’s leadership concluded that a second commando attack against the reinforced guard had no chance, so the task passed to the air forces. On 16 November 1943 more than 140 American bombers dropped hundreds of bombs on Vemork and the neighbouring factory in Rjukan. Most missed, and the basement with the electrolysers, protected by several concrete floors, survived, but some twenty civilians were killed (sources give between 20 and 22 victims). Before the raid Tronstad had warned that this would happen; the Norwegian government-in-exile had not been informed of the attack in advance and protested sharply.

After the raid the Germans decided to move production to the Reich and ship out the remaining stocks. On orders from London, Haukelid, with the help of local resisters, planted a charge in the hull of the railway ferry Hydro, which was to carry the tank wagons across Lake Tinnsjø. The ferry sank during the night of 19–20 February 1944; fourteen Norwegians, passengers among them, and four Germans were killed. According to the Store norske leksikon, the whole series of these actions, known as the heavy water battle, cost a total of 91 lives.

Significance

Historians disagree about the importance of the operation. After the war it emerged that the German nuclear programme had been far from a bomb. As early as June 1942 the Armaments Minister, Albert Speer, had concluded that an atomic weapon could not be built during the war, and the programme was relegated to the background; the scientists competed for meagre resources in several centres, and their experimental reactor at Haigerloch had not gone critical by the end of the war. Even had Vemork operated without interruption, the Germans, by most assessments, lacked the industrial and organisational capacity to produce enough plutonium for a bomb by 1945. The Norwegian historian Olav Njølstad therefore doubts whether the heavy water from Rjukan could have had any military significance during the war. Other authors point out that in February 1943 the Allies did not know this: they acted in the belief that the threat was real, and the destruction of the stocks and the subsequent interruptions in supply did indeed mean that the German physicists never obtained enough moderator.

Whatever the outcome of this debate, Gunnerside has gone down in history as one of the most successful sabotage operations of the Second World War: a small team, without losses of its own and without civilian casualties, achieved what a glider expedition had failed to do before it and a hundred-odd bombers would fail to do later. It was also the most convincing argument for cooperation between SOE and the Norwegian resistance, which in the years that followed moved from individual actions to systematic sabotage, carried out together with the Milorg organisation. Rønneberg was awarded the War Cross with Sword and the British Distinguished Service Order. He later admitted that he had known little about the purpose of the operation, and that only the news of the atomic bombs dropped on Hiroshima and Nagasaki made him realise what had been at stake. The story was popularised by the 1965 film The Heroes of Telemark, which its participants, Rønneberg among them, regarded as largely fictional.