Freshman was a hard blow both for the British and the advance party, codename "Grouse", on the Hardangervidda-plateau. The rations were up, and they nearly starved to death during the extreme winter conditions, surviving by hunting reindeers.
A second attempt was decided, this time under SOE command and with Norwegian saboteurs from the Linge-company. 6 excellent skiers and mountainers were picked, underwent intense training in Scotland, the commander sub-lieutenant Rönneberg planning the operation in great detail.
The target was now only the high concentration plant. The operation was code-named "Gunnerside".
The parachute drop took place at last on February 17 1943. The party landed way off planned drop zone, and they were immediately beset by a five days of gale-force winds. When the weather cleared the joined up with the advance party. Reconnaissance revealed the immense difficulties of the attack, and security at the plant had been tightened. Three routes were discussed:
1. Down the pen stock, but they would have to pass a Flak-battery at the top and guards, the area was mined, so this was ruled out.
2. The direct approach across he suspension bridge. They would to have to engage the German guards in battle, the killing of the guards would alarm the rest of the guards (a force of 15), trap the saboteurs and subject Rjukan to reprisals. The majority found this too risky.
3. Down the road on the north side and crossing the gorge making it through the railway-gate. Rönneberg, who was described by Lt.Col. Charles Hampton as "very, very intelligent and tremendously thorough," favoured the suspension bridge both ways, but didn't insist. His way of carrying out military leadership was democratic. In the discussion of routes, he consulted every member of the group privately, summing up the views and suggesting the majority view: Decent down the gorge, retreat the same way, climbing the mountains from a point nearer Rjukan.
The attack took place in the night of February 28. The men cut the railroad-gate, the Covering party of 5, took their positions around the guards' hut. The Demolition party of 4 went down to the high concentration plant. Having problems finding an entry, they came apart. Eventually one team found a cable conduit leading to the room with the heavy water cells.
One from the other team entered by breaking a window, helping with the demolition charges. Plastic charges were wrapped around the base of each electrolysis cell and a two minute fuse, reduced to 15 sec. just to be sure, was lit. Two Norwegian worker was told to run for their lives. As the saboteurs ran out of the building they heard a heavy low concussion.
"It sounded like a dull thud," writes Rönneberg. It struck them as a paltry event, almost like an anti-climax.
They joined with the covering party, made the retreat along the railroad, crossed the gorge, picked up the equipment, went along the power line towards Krosso, began the long climb up the hill on foot, watching nervously for movements of the cable car, reaching the mountain plateau in the morning, making it to a hut, going to sleep for 12 hours. Next day they reached base camp.
From March 4, 5 of the men made their withdrawal by ski over a distance of almost 500 km to Sweden. The rest of the men joined the Norwegian underground.
Before the operation they were told they had a fifty-fifty chance of carrying out the sabotage, less than that of getting out alive. In fact they departed with the knowledge that they might not see each other again.
The operation was a success, London received this signal: "High concentration installation at Vemork completely destroyed on night of 27-28th. "Gunnerside" has gone to Sweden. Greetings". The reasons for the success were luck, good planning, the use of mountaineers and knowledge of the area, intelligence, reconnaissance and mastering of Norwegian terrain.
The sabotage destroyed the 18 heavy water cells completely with all the content of destilled water and heavy water, about 500 kg. In addition there was a loss of almost 3 months production, altogether the action caused a loss of about 900 kg, without any loss of life.
Nevertheless, the plant was rebuilt, the equipment replaced and nearly pure heavy water shipped back to Rjukan so that all stages in the process could be resumed at once. Production of heavy water was resumed in mid-April, to the great disappointment of the Norwegian High Command and MI6 in London.