Saboteur whose raid on Nazi hydro plant crippled atomic bomb project
In January 1942 an informant from German-occupied Norway reported a significant increase in the output of heavy water, necessary for the production of plutonium, at the Norsk Hydro plant at Vemork, 80 kilometres west of Oslo.
Concluding that this was proof that Hitler was attempting to build what would later be described as an atomic bomb, the allies attempted to disrupt the work in October of that year. They sent in by glider British engineer commandos, who were to be led to the Vemork site by a Norwegian four-man Special Operations Executive (SOE) team, led by Captain (later colonel) Jens-Anton Poulsson, who had been dropped by parachute in advance. After problems caused the gliders to crash wide of their landing zone, the survivors were captured and, on Hitler’s orders, executed. Poulsson and his team took to the mountains.
As soon as failure of the first mission became known, a sabotage team of six men was selected from Norwegian SOE commandos at the Aviemore training base in Scotland. Joachim Ronneberg, a 23-year-old sabotage and demolitions instructor noted for his complete calmness under stress, was appointed leader and Professor Leif Tronstad, who knew the Vemork plant intimately having worked there, made a scale model on which to plan the attack.
The team was to be delivered by parachute, but the first attempt on the night of January 23-24, 1943 was frustrated by fog over the drop zone, enforcing a delay until February 16. The drop zone was on the Hardangervidda plateau. Ronneberg and his companions parachuted on to the plateau and skied across country before finding a trapper’s hut in which to shelter from a storm that raged for three days.
On the day the storm subsided a lone skier approached the hut. Questioned, the man protested he was a ‘‘Quisling’’, a collaborationist, but Ronneberg could not be sure whether this was the case or a reaction from the man’s fear that they, being in uniform, were German. Reluctant to kill in cold blood, Ronneberg ordered him to be tied on a toboggan found in the hut and dragged to the rendezvous with Poulsson.
Next morning, two of Poulsson’s team were sighted during the trek to the rendezvous and by late afternoon the teams were united. The lone skier was then released after signing an admission that he was illegally hunting reindeer for the black market, as a form of guarantee of compliance, and he was told not to return home for three days.
Ronneberg with four men and Poulsson with two began to ski down to Vemork, carrying the explosives Ronneberg had brought, leaving only the two radio operators at a planned post-operation rendezvous on the mountain. Vemork was reached two days later at 8pm on February 27. The plant was a formidable target. It comprised seven storeys built into the mountainside, with water gushing down from a reservoir and a 600ft drop to the River Maan in front. Access for the workers was by a 75ft suspension bridge.
A reconnaissance of the valley floor revealed an ice bridge by which the party could cross the Maan and then climb up to the track of a narrow-gauge supply railway. The attack plan was simple: Ronneberg’s team would carry out the sabotage while Poulsson’s would be prepared to give covering fire and fight off any German guards who appeared. To avoid delay in withdrawal, each man undertook to bite his cyanide pill if wounded. ‘‘We very often thought that this was a oneway trip,’’ Ronneberg recalled.
Leaving their skis at a forward rendezvous, the teams climbed down to the river, crossed it and followed the rail track to the point where the electric cables serving the plant entered the tunnel that gave entry to the building. Once inside, Ronneberg and his team placed the charges on the machinery producing the heavy water, set them with a 30-second fuse, and left by the way they had come. The dull thud of the explosions sounded as they walked along the railway.
The alarm at the plant did not sound until they had recrossed the river. By dawn they had collected their skis and returned to the Hardangervidda. Afterwards, Ronneberg wrote: ‘‘It was a lovely morning and we were sitting there knowing that the job was done and nobody had been hurt on either side.’’ Shortly before darkness fell, they returned in a snowstorm to their main base where the two radio operators were waiting. His radio message to London was a model of clarity and brevity: ‘‘Attacked 0045 on 28.2.43. High concentration plant totally destroyed. All present. No fighting.’’
Leaving one of his team behind with two of Poulsson’s to train and distribute arms and equipment to the resistance, Ronneberg and the rest of his team made a 400km ski trek eastwards, and in mid-March crossed the frontier into neutral Sweden. Ronneberg and Poulsson were awarded the DSO.
The story was dramatised in the 1965 feature film The Heroes of Telemark, starring Kirk Douglas, but Ronneberg branded the retelling ‘‘hopeless’’. ‘‘They took a true story and spun their own idea around it,’’ he said.
For many years he refused to speak about his wartime experiences, but age weakened his resolve. ‘‘I realised a few years ago that I am part of history,’’ he said in 2010, ‘‘and people must realise that peace and freedom have to be fought for every day.’’
Joachim Holmboe Ronneberg was born in Alesund, on the west coast of Norway, the oldest of three sons. His parents helped to run the family fish export business. He graduated in 1939 and married Liv Muriel Foldal, a craft teacher, in 1949. She survives him along with a son, Jostein, the chief executive at Space Norway, and daughters, Asa, an athletic therapist, and Birte, an executive.
After the war he worked for the Norwegian broadcasting corporation NRK. When asked about his daring mission in later life, he casually described it as ‘‘a gang of friends doing a job together’’. It was, he added with a smile, ‘‘the best skiing weekend I have ever had’’.
‘‘We very often thought that this was a one-way trip.’’