Foiling the Nazis’ Nuclear Plans
The Secret War in Scandinavia
On the night of February 28th, 1943, an explosion threw the Nazis’ plans to build a nuclear bomb into disarray. The explosion took place in a small factory named Vemork, perched at the top of a hundred-meter–tall waterfall in Norway, and it went off so quietly that it was several minutes before anyone realized that disaster had struck.
This disaster was no accident: it was the product of months of careful planning by Allied forces, and was just one scene in a secret war which was being fought across Scandinavia to keep nuclear weapons out of the hands of the Third Reich. That particular scene was led by Joachim Rønneberg, a second lieutenant in the Norwegian special forces, Knut Haukelid, his second-in-command, and four more men who had parachuted into enemy territory to perform this one specific act of sabotage.
Today, Rønneberg is 96 years old, Norway’s most decorated war hero, and a retired sub-editor at the Norwegian Broadcasting Corporation. Haukelid died in 1994, after a distinguished military career. Both of them have shared their stories, and thanks to them, we have a fairly complete picture of just how this secret war was fought: with the courage of people both ordinary and extraordinary, with far more humanity and care for life than you would expect, and with the horrible knowledge of what might happen if they failed perpetually in the background.
This is the story of the Heavy Water War.
A nuclear bomb of the sort which exploded over Hiroshima works on a fairly simple principle. There are two forces acting on the nuclei of atoms: electrical repulsion and strong attraction. The first is the rule that “like charges repel,” and the nuclei are full of protons that all have the same charge and really don’t want to be that close to one another. The second is a force that binds protons and neutrons together with tremendous strength – so long as the distance between them is less than about a femtometer, 10⁻¹⁵ of a meter. Beyond that distance, the strong force suddenly switches off, and all that’s left is repulsion: the atom breaks into small pieces, flying off with great energy.
Some larger atoms have nuclei which are so big that their left side is just on the edge of being too far from their right side to be held together by the strong force. If they’re struck by a neutron, the force of the impact can be enough to shatter them – releasing scrap bits of atom, lots of energy, and more neutrons, which in turn can break up more atoms. If neutrons start breaking up atoms and producing more neutrons faster than neutrons can escape from the lump of material, a chain reaction begins, and within a few billionths of a second, the entire lump of material is transformed into a spectacular explosion. The rate at which neutrons escape or strike other atoms depends on things like the surface area and volume of the chunk of material; the general rule is, any big enough lump will go off (“go critical”) more or less instantly.
So the challenge of building a bomb is twofold: First, find materials which will go off when assembled into the right configuration, and more importantly, won’t go off until you assemble them; and second, build a machine that can assemble them into the right configuration quickly enough that the explosion doesn’t really start until everything is in place. (If it starts too early, it will “spontaneously disassemble” itself and only a bit of the nuclear material will have a chance to go off. This is referred to as a “fizzle.” Nuclear weapons design has great euphemisms.)
As it happens, there are only two types of fuel that are at all useful for this: Uranium-235 and Plutonium-239. (The number refers to the total number of protons plus neutrons in the nuclei; U-235 has 92 protons and 143 neutrons, and Pu-239 has 94 protons and 145 neutrons) Anything else is either too small to break up easily, or so big that it falls apart on its own. But they have different pros and cons.
U-235 is the less reactive of the two, which paradoxically makes it easier to use: all you have to do is take two suitably-sized and -shaped lumps of it, and fire them at each other with a sawed-off bit of cannon. (There’s a bit more to it, but not too much more; any reasonably competent undergraduate could design a working “single-gun” U-235 bomb) The problem is that it’s very hard to make U-235. When you dig Uranium out of the ground, it’s a mixture of several different kinds of Uranium, with the useful “235” isotope only being 0.7% of the total. To make a bomb, you need to get well above 90% purity, and have strict control over the types of impurity. (U-233, for example, is far too reactive, and can easily cause fizzles) But separating different kinds of Uranium is extremely complicated; it requires tremendous factories full of centrifuges, or other very expensive technologies. (It’s often underappreciated how key the enormous refinement factory in Oak Ridge, Tennessee, was to the American bomb effort)
Pu-239, on the other hand, is relatively easy to get: it’s produced as a by-product of the operation of certain kinds of nuclear reactor (called a “breeder reactor”), and can easily be separated from the fuel afterwards. But it’s too sensitive to use in a gun design; instead, you have to assemble it into a precise sphere, and surround it by explosive lenses which crush it to a small fraction of its size almost instantaneously.
(The Americans, incidentally, pursued both methods in parallel; the Hiroshima bomb, Little Boy, was a Uranium-based gun design, while the Nagasaki bomb, Fat Man, was a Plutonium-based implosion design. The Trinity test was of the implosion design, which was so complex that people weren’t very confident it would work. There was enough confidence in the gun design that its first test was over Japan)
Because building giant enrichment factories was out of the question, the Germans were pursuing Plutonium. They hadn’t gotten as far as designing the implosion architecture, because there was an even more basic first step: to characterize the critical nuclear parameters of these materials. After all, you couldn’t just look them up on the Internet in those days: you had to test it yourself. And that meant that they desperately needed a nuclear reactor.
While nuclear reactors gain their energy from the same source as bombs, their designs differ as much as their purposes. Nuclear reactors are based on maintaining a slow, steady rate of atoms splitting; this produces heat, which ultimately boils water which spins a turbine and produces electricity. In breeder reactors, spare neutrons are used to convert U-238 into Pu-239.
A basic reactor design takes a low-intensity fuel (traditionally, 5% U-235) and mixes it in with a moderator, a substance whose job is to absorb neutrons or slow them down. It’s controlled by having additional rods of strong neutron absorbers which can be moved in and out of the reactor. (There’s an entire art to designing reactors so that if anything goes wrong, the immediate mechanical consequence is that they shut themselves down – and the subtleties of that could fill many articles by themselves.)
Moderators are challenging, though, because they need to slow just enough neutrons so that the reaction keeps running without either overheating or sputtering out. Graphite can work for this, but it has to be extremely pure; the most common impurity in graphite is Boron, which absorbs neutrons so effectively that it would completely shut down the reactor. And since the Germans didn’t have access to excellent graphite sources (they tried), they used the next best thing: Heavy water.
Ordinary water consists of a single Oxygen atom, with two Hydrogen atoms on either side. In heavy water, the Hydrogen atoms have been replaced with Deuterium, also known as “Hydrogen-2:” an atom where the nucleus has one proton and one neutron, as opposed to the single proton of ordinary Hydrogen. Heavy water is an excellent moderator, and occurs naturally in seawater, at a concentration of about one part in 3,200. All you have to do is extract it, and that’s what the Vemork factory did.
Vemork sits at the top of the Rjukanfossen waterfall in southern Norway. When it was opened in 1911, it was as a factory which used hydroelectric power from the waterfall to manufacture fertilizer. In 1934, it became the first place in the world to manufacture heavy water, using a newly-invented electrolytic process to manufacture twelve tons each year. Here it was a tool for the nascent fields of nuclear medicine (being used to trace the motion of fluids through the body), and for nuclear research – until it was captured by the Germans in 1940.
At this time, the joint US/UK atomic bomb project was in its first days, but the importance of denying the Germans access to critical nuclear materials was already clear. Early in 1940, the “Paris Group” – a band of scientists at the Collège de France in Paris which was one of the centers of nuclear research – realized that heavy water would be the best possible moderator for a reactor, and asked the Minister of Armaments to obtain as much as possible. When he went in search of it, he discovered to his alarm that the Germans had already made an offer to buy Vemork’s entire stock. Realizing the military implications, he entered into secret negotiations with the Norwegian government, convincing them to sell to them instead. They put their entire stock (about 50 gallons’ worth) into the hands of a French secret service agent, who smuggled it into Paris only days before Nazi troops rolled into Norway and seized the factory. Two months later, on June 14th, the Nazis marched in to Paris: five days after that, the heavy water was secreted aboard the steamer Broompark, bound for Cambridge, together with the entire Paris Group.
Having lost the heavy water itself, the Nazis had to content themselves with second prize: control of the only factory in the world that could make it. There was just one problem: the electrolytic process used at Vemork needed heavy water to make more heavy water. Without it, the process could only move at a crawl. It would end up taking them a crucial year until the plant was, once again, manufacturing its key ingredient.
In 1942, Vemork was back on the minds of the Allied High Command. In April, intelligence sources reported that heavy water production had resumed. General Leslie Groves, newly-minted military commander of the Manhattan Project, asked that its destruction be made a top priority. Churchill concurred, and on October 18th, four Norwegian commandos parachuted into Rjukan to reconnoitre in Operation Grouse. They returned with a clear layout of the area, and on November 19th, two Halifax bombers towed gliders carrying 34 British soldiers – sappers from the 1st Airborne Division – into enemy territory.
The gods did not smile on Operation Freshman. The first airplane’s radio receiver failed over Norway, leaving them blind to the beacons left by the reconnaissance party. Navigating on maps through heavy weather and thick clouds, ice began to form on the aircraft, the glider, and most fatally, the tow rope. When it snapped, the airplane, now desperately low on fuel, was forced to return home, only barely managing to land. The glider crashed near a farm, killing eight and severely wounding four of the seventeen aboard. They were rescued by farmers who treated the injured and attempted to keep them hidden, but it was no use; the next day, German soldiers arrived and captured all nine survivors, leading them on a forced march through the cold to the Slettebø prison camp.
The second airplane fared even worse: in harsh weather, it crashed into a mountain only moments after releasing its glider. The glider, in turn, went out of control and crashed as well. The entire airplane crew, and seven of the men aboard the glider, were killed on impact, and the other ten seriously injured. Two went to seek help, and like their compatriots from the first glider, encountered friendly Norwegians, who helped them destroy all their maps and documents; but as with the other group, the German Army soon showed up and captured all ten survivors.
All nineteen of the captured soldiers were executed by the Nazis.
The decision about what to do next fell to R. V. Jones, director of intelligence for the British Air Staff. Should they send a second demolition party in after the first? In what he later recounted as “one of the most painful decisions that [he] had to make,” he concluded that if the raid was worth doing the first time, it was still worth doing now.
This time, though, it would be done differently. Instead of the thirty-four Englishmen of Operation Freshman, Operation Gunnerside sent a team of only six: Norwegian special forces, each native of the area, under the command of Joachim Rønnenberg. Gone were the gliders, as well; on the night of February 16th, 1943, under a full moon, the six men parachuted onto the surface of a frozen lake, thirty miles north of Vemork, wearing white jumpsuits, carrying skis and cyanide capsules, and equipped with eighteen sets of plastic explosives: one for each of the eighteen stainless-steel cells of the electrolysis plant.
The weather was no friendlier this time than it had been for the Freshman team; Knut Haukelid, another member of the team, described it as one of the worst storms he had ever experienced. It took them several days to rendezvous with the Norwegian advance party, which had been trapped high in the mountains and run out of food. As the team recovered from their malnourishment, an advance party skied on to Rjukan for reconnaissance. There they found a fortress: minefields surrounding the approaches, guards on the suspension bridge crossing the gorge, searchlights, machine guns – but only fifteen soldiers on duty. The Germans were counting on the factory’s perch on the edge of a tremendous ravine to provide natural defenses.
On the evening of February 27th, the team set out under the dark of a new moon. The searchlights were off, and a high wind drowned out all sound. Half an hour before midnight, they came to a snow-covered shack five hundred yards from Vemork, where they stopped to eat some chocolate and wait for the changing of the guard.
Haukelid led the covering party, and Rønnenberg the demolition party. Haukelid’s team spread into prearranged positions surrounding barracks and factory. The demolitions team avoided the locked doors to the plant, instead sending two men crawling in through a cable intake. (They knew about this because Leif Tronstad – a physical chemist and the designer of Vemork’s electrolysis plant – was now part of Norwegian Intelligence’s office in London!)
And after a few minutes, there was the sound of a small explosion. As Haukelid put it, it was “an astonishingly small, insignificant one. Was this what we had come over a thousand miles to do?” In fact, the noise was so small over the roar, that only one German soldier appeared to check what happened; he looked around, checked that the doors were still locked, looked to see if snow had triggered a land mine, and walked right back inside.
The Norwegians were on the other side of the river before the sirens began to sound.
This did not, however, mean the end of the Vemork story. Operation Gunnerside had successfully destroyed the electrolysis cells and poured their contents down the drain, but the plant was intact, as was the already-established German supply of heavy water. And by April, intelligence reports showed that Vemork was already repaired and running again, having shipped precious heavy water from Germany to restart it without another year’s loss. But in October, far worse news arrived.
Niels Bohr, the great Danish physicist, was admired and respected by all in the field; as a result, Werner Heisenberg, perhaps the greatest physicist to stay in Germany, came to him for advice in 1941, dropping hints about the Nazi nuclear program. (To this day, there is much question as to Heisenberg’s real role as scientific leader of that program: was he, as he later claimed, trying to sabotage the program the entire time? The open question there, and the famous meeting between the two scientists, has prompted not only endless analysis, but an excellent play) But Bohr had enemies, as well: he had been helping refugees escape the Nazis since the 1930’s, and in 1943, he received warning that he was to be arrested and shot. With the help of the Danish Resistance, he escaped by sea to Sweden.
The next day, September 30th, Bohr met with King Gustav V of Sweden, where he convinced him to publicly commit to protecting Jewish refugees. On October 1st, Hitler ordered all Danish Jews to be rounded up and deported; on the 2nd, King Gustav announced his intention to help them on the radio. Over the next few days, the Danish Resistance managed to smuggle 7,200 of Denmark’s 7,800 Jews, along with nearly 700 of their non-Jewish spouses, over the sea into Sweden.
But for Bohr, this was only a stop: on October 6th, he reached Scotland, and in his pocket he had Heisenberg’s designs for an experimental heavy-water nuclear reactor. Meeting regularly over the next few months with Sir John Anderson (Britain’s “Home Front Prime Minister”), and comparing notes with their American counterparts, they concluded that the Nazis were indeed on their way to a working nuclear design.
On November 16th, a flight of B-17’s made their way to Vemork, timing their arrival to be during the lunch period, to minimize worker casualties. (As Tronstad had warned them, Vemork’s original purpose as a fertilizer factory was still very much a going concern, and when bombs hit the ammonia storage tanks, the resulting explosions would kill large numbers of workers. The Allies had, by now, largely abandoned the notion of precision bombing: a few months earlier, the U.S. Bomber Command’s Operation Gomorrah had had the brutally simple objective “To destroy HAMBURG.” But still, they took effort, and spent extra time circling over enemy airspace, to protect who they could.) 140 aircraft made it through enemy flak, dropping over 700 bombs; while none hit the aiming point directly, the damage to the factory was such that it never again would function during the war.
But the heavy water itself remained undestroyed in the plant, and could be used again if the Germans built another factory – or could simply be used to power reactors directly. On February 9th, 1944, Allied agents in Norway reported over the radio that this remaining water was to be transported back to Germany under heavy guard within a week or two. Only one person was on the ground and in a position to respond: Knut Haukelid.
Haukelid had been living behind enemy lines for the entire past year, organizing military operations against the Nazis. He snuck into Rjukan that night to meet with Vemork’s new chief engineer, Alf Larsen. Larsen agreed to help, recruiting the transport engineer as well, and gave Haukelid the details of the planned transport: 39 drums labelled “potash-lye,” going by train from Rjukan to Lake Tinnsjå, then crossing the lake by ferry, then by train to the port, and by ship to Germany.
Vemork itself was no longer a good target; after the bombing raid, the Germans took security there very seriously, and a one-man raid was out of the question. Haukelid similarly ruled out the trains, being difficult to destroy and certain to create tremendous civilian casualties. So the target had to be the ferry – but that, too, would have civilians aboard. Worse, the shipment’s German guards would certainly have to be killed in the raid, and their deaths would bring heavy reprisals onto the local population.
Haukelid and the engineers discussed their options at length. By this point, they were openly discussing the possibility of nuclear weapons; the engineers doubted that the Germans were anywhere close to a solution, or whether one was even possible, and felt that the risks to the local population were not worth it. Haukelid finally radioed London for permission, emphasizing the engineers’ concerns. London, knowing the state of both the Manhattan Project and having seen the designs Bohr brought, disagreed:
“Matter has been considered. It is thought very important that the heavy water shall be destroyed. Hope it can be done without too disastrous results. Send our best wishes for success in the work. Greetings.”
And so the plan began. The transport engineer coordinated the shipment so that it would be aboard the ferry on a Sunday morning, when the ship would be emptiest. Haukelid determined that the ferry which would be on that route was the aptly named SF Hydro; disguising himself as a worker, with his Sten gun hidden in a violin case, he rode it, timing its passage over the deepest part of the lake. He found a twenty-minute window in which the explosion had to happen.
For this precision, the time fuses Haukelid had wouldn’t be enough; he needed detonators and a clock. The detonators came from a local hardware store; Haukelid visited the owner at night and was turned away suspiciously, but one of his local friends succeeded. Alarm clocks were provided by Larsen and by a retired Vemork handyman, and modified to be the most traditional B-movie detonators you can imagine: the bells were removed, and the hammers would instead strike contact plates, completing a circuit. To test that mechanism, Haukelid set an alarm to trigger a few spare detonators outside his cabin in the mountains, and then fell asleep. A few hours later, the efficacy of explosives as a wake-up call was vividly proven, sending him leaping in surprise from bed, grabbing the nearest gun, and covering the door before he realized what had happened. He later noted: “the timing apparatus seemed to be working perfectly.”
Similarly, the explosives needed to be designed. How big a hole would be needed to sink the ferry quickly? “As the Tinnsjå is narrow, the ferry must sink in less than five minutes, or else it would be possible to beach her. I… spent many hours sitting and calculating how large the hole must be for the ferry to sink quickly enough.” He settled on a ring of plastic explosives, some twelve feet around.
On Saturday, February 19th, Haukelid slipped into Rjukan together with Rolf Sörlie, another of the locals helping him. The city was packed with soldiers and SS; from a bridge over the river Måna, they saw the train parked near the lake, under heavy guard. Next they met their driver, in a car which Haukelid had arranged to “steal in the name of the King” and return in the morning. Once they got the car started – it took over an hour, as the car had been modified to run on methane – they picked up Larsen, who knew he would have to flee the country after this. He had come from a dinner party, where he had met a visiting violinist who was planning to leave on the morning ferry. Larsen had tried, unsuccessfully, to convince him to spend another day in the area and enjoy its skiing. A fifth man, another of Haukelid’s agents (unnamed to this day), joined them as well. Together, they approached the ferry. In Haukelid’s words:
Armed with Sten guns, pistols, and hand-grenades, we crept… down toward the ferry. The bitterly cold night set everything creaking and crackling; the ice on the road snapped sharply as we went over it. When we came out on the bridge by the ferry station, there was as much noise as if a whole company was on the march.
Rolf and the other Rjukan man were told to cover me while I went on board to reconnoitre. All was quiet there. Was it possible that the Germans had omitted to place a guard at the weakest point in the whole route to the transport?
Hearing voices in the crew’s quarters, forward, I stole to the companion[way] and listened. There must be a party going on down there, and a game of poker. The other two followed me onto the deck of the ferry. We went down to the third-class accommodation and found a hatchway leading to the bilges. But before we had got the hatch open we heard steps, and took cover behind the nearest table or chair. The ferry watchman was standing in the doorway.
Haukelid told the watchman that they were fleeing the Gestapo and needed to hide; he immediately helped them get the hatch open, telling them that they often smuggled goods there. The Rjukan man then kept talking with the watchman, keeping him busy as Haukelid and Sörlie went below. Standing in foot-deep water, they laid the ring of explosives along the bottom of the hull, towards the forward end of the ferry so that when water came on, the rudder and propeller would be pulled above the surface, and the boat would be unable to navigate to safety. The alarm clocks were taped to the hull; detonators attached to the timers; fuses connected the detonators to the explosive; the alarms were set for 10:45; and finally, the batteries were connected to the detonators, and the detonators to the fuses. The bomb was now set to go off, with only a third of an inch gap between hammer and contact plate preventing it from detonating under their feet.
They now convinced the watchman that they needed to return to Rjukan to collect some possessions. Haukelid considered warning him, but decided that it was too likely to endanger the mission; he could only thank him, and shake his hand. Ten minutes later, they abandoned the car; Haukelid and Larsen to ski forty miles to Kongsberg, from which they would take a train to the Swedish border; Sörlie to radio a report back to London; and the driver and the local man to simply stroll home, nonchalantly. The transport engineer had an even better alibi: he had asked doctors at the local hospital to operate on him for appendicitis, and spent the weekend having surgery. No questions were asked.
At 10:45 the next morning, with the ferry over the deepest part of the lake, the explosives went off. As passengers and crew frantically worked to release the lifeboats, the freight cars, with their heavy water aboard, broke loose from their chains, rolled over the edge, and sank like stones. The last hope of the Nazi atomic bomb project would spend the rest of the war at the bottom of Lake Tinnsjå.
Of the fifty-three people aboard the ferry, twenty-six drowned. The concert violinist was among the survivors: he made it into a lifeboat, and when he spotted his violin case floating by, someone fished it out for him.
With seventy years’ worth of hindsight, it’s easy to underestimate the significance of the Vemork raid. Many scholars have raised the same question that Alf Larsen and the transport engineer did: Would it even have been possible for the Germans to build such a bomb?
The Germans certainly thought so. The project was continuing at full speed, fueled by Hitler’s love for super-weapons. (The V-2 project, started a few months before the sinking of the Hydro, would end up killing substantially more people in its construction than in its use) Kurt Diebner, who headed the Nazi bomb project, said in a postwar interview that “the elimination of German heavy-water production in Norway… was the main factor in our failure to achieve a self-sustaining atomic reactor before the war ended.”
The Allies had reason to think so as well: they had Heisenberg’s reactor plans in hand, and had even better reason than the Germans to believe that such a bomb was actually possible. Even without a bomb, the threat of a Nazi reactor was terrifyingly real: imagine the spent fuel of a reactor, at its most radioactive stage, powdered and spread over city or farmland by bombers.
Beyond its action-movie structure, there is one thing I find particularly fascinating in this story: the persistent humanity of its protagonists. All the players in the field worked hard and took great risks to protect civilians: bombers took longer routes to hit the factory at lunch, Haukelid spent considerable effort on the alibis and deniability of the people helping him, Bohr stopped on his escape to help orchestrate the rescue of over 7,000 civilians. And mirroring this, we see over and over how ordinary people stepped up to help fight the Nazis: from the villagers who tried to shelter and aid the survivors of Operation Freshman, to the Rjukan man who arranged for his car to be “stolen,” to the watchman who didn’t hesitate to hide two strangers from the Gestapo. None of this could have happened without both ordinary people taking great risks for the mission, and the mission taking great risks for ordinary people. And had it not been for their collective courage, the war might have taken a far darker turn.
To all the men and women who made these raids possible: I salute you.
For those who wish to know more of this story, the best place to start is Richard Rhodes’ The Making of the Atomic Bomb. This book covers the entire history of the bomb, from the people, to the science, to the politics, to the warfare, and is generally one of the best-written history books I’ve ever encountered. You should read it.
Those who want to go straight to the primary sources should begin with Knut Haukelid’s own book, Skis Against the Atom. There have also been two film treatments, a docudrama titled “Operation Swallow” and a heavily fictionalized action movie, “The Heroes of Telemark,” but your mileage may vary. Rønneberg has given many interviews and spoken extensively on the subject; his interview with The Telegraph is another good place to start.
