Heavy water became one of the most fiercely contested resources of World War II, linking directly to the nuclear ambitions of the Axis powers. This rare substance was a high-stakes component sought for its unique properties in developing atomic technology. The struggle to control its supply led to a hidden war of espionage and sabotage in Nazi-occupied Europe. Allied forces recognized that denying the German “Uranium Club” access to this material was crucial to crippling their efforts to build a nuclear weapon.
The Scientific Definition of Heavy Water
Heavy water, chemically known as deuterium oxide (\(\text{D}_2\text{O}\)), is distinct from ordinary water (\(\text{H}_2\text{O}\)). This difference stems from the composition of their hydrogen atoms, which exist as different isotopes. In regular water, the hydrogen atom (protium) consists only of a single proton. Conversely, the hydrogen in heavy water (deuterium) possesses one proton and one neutron, making it approximately twice as heavy as protium.
The resulting \(\text{D}_2\text{O}\) molecule is roughly 10% denser than \(\text{H}_2\text{O}\), which is why it is termed “heavy water.” In nuclear physics, this difference is significant because heavy water functions as an effective neutron moderator. When uranium atoms undergo fission, they release fast-moving neutrons that are too energetic to reliably trigger further fission. The moderator’s job is to slow these neutrons down to “thermal” speeds, enabling a sustained nuclear chain reaction.
Heavy water is valuable as a moderator because its deuterium atoms absorb far fewer neutrons than the protium atoms in ordinary water. This low absorption rate allows a reactor using heavy water to sustain a chain reaction using natural, unenriched uranium as fuel. Reactors using ordinary water, by contrast, must use uranium that has been “enriched” to increase the concentration of the fissile isotope uranium-235. This quality made heavy water essential for any nation attempting to build a reactor without the industrial capacity for large-scale uranium enrichment.
Strategic Importance in Wartime Nuclear Programs
The German wartime nuclear effort, known as the “Uranium Club,” centered its strategy on heavy water, making its acquisition a primary military objective. German scientists, including Werner Heisenberg, aimed to construct a self-sustaining nuclear reactor, or “pile.” This reactor was necessary for two potential paths: achieving a chain reaction to study fission physics, and breeding the new element, plutonium-239.
Plutonium is a highly fissile material used to make atomic bombs. Producing it requires bombarding uranium-238 with neutrons inside a reactor, transforming it into plutonium. Since the German program lacked the infrastructure to enrich uranium on a massive scale, heavy water was the only practical moderator for them to build a reactor using their natural uranium stock. This strategic dependence was cemented partly by an erroneous early measurement by physicist Walther Bothe, which incorrectly suggested that graphite, the moderator successfully used by the Allies, would not work.
The Allies, specifically the American Manhattan Project, developed parallel paths, using both a heavy water approach (the P-9 Project) and a graphite-moderated approach. However, the German commitment meant their success hinged entirely on securing a constant, large-scale supply of the material. This singular focus created a vulnerability, as the world’s only major source of heavy water production was a single plant in occupied Norway. Denying Germany this supply became a strategic imperative for the Allied war effort.
The Sabotage Operations in Norway
The heavy water conflict centered on the Vemork Hydroelectric Plant, an isolated industrial complex in the Telemark region of Norway. The plant, owned by Norsk Hydro, originally produced fertilizer but created heavy water as a byproduct of its electrolysis process. Following the German occupation of Norway in 1940, the Nazis took control and significantly ramped up heavy water production capacity.
The Allies quickly recognized the threat posed by the Vemork supply and launched a series of operations to destroy the plant. The first attempt, Operation Freshman in November 1942, was a failure; both gliders carrying British airborne troops crashed. Surviving commandos were captured and executed by the Germans, putting the facility on high alert.
Despite the increased German security, Operation Gunnerside was launched in February 1943. Six highly-trained Norwegian commandos parachuted onto the Hardanger Plateau and linked up with an advance team. The team infiltrated the plant by descending into a steep ravine and climbing the cliff face, avoiding the guarded bridge and minefields.
Once inside, the commandos located the heavy water concentration cells and planted explosive charges. They destroyed the equipment and the existing stock, estimated at about 500 kilograms of heavy water, before escaping. The Germans decided to move the remaining semi-finished heavy water stores out of Norway after an Allied bombing raid damaged the plant later that year.
The final blow came on February 20, 1944, when the Germans attempted to transport the remaining heavy water stocks by rail and ferry toward Germany. Norwegian saboteurs, led by Knut Haukelid, planted a time-delay explosive charge on the railway ferry, the \(\text{D/F}\) Hydro, while it crossed Lake Tinn. The explosion sank the ferry, sending the drums of heavy water to the bottom of the lake. This action effectively ended Germany’s practical ability to secure the sustained supply of heavy water necessary for their nuclear project.