Heavy water, scientifically known as deuterium oxide (D₂O), is a form of water where hydrogen atoms are replaced by deuterium. Deuterium is an isotope of hydrogen containing one proton and one neutron, making it approximately twice as heavy as common hydrogen. This atomic difference gives heavy water distinct physical properties, such as a slightly higher density, melting point, and boiling point. During World War II, these properties made heavy water strategically important in the global race to develop nuclear technology.
Understanding Heavy Water’s Role
Heavy water’s value in nuclear fission stems from its ability to act as a neutron moderator. In a nuclear reactor, fission releases high-energy neutrons. These fast neutrons are too energetic to efficiently sustain a chain reaction. A moderator slows these neutrons to “thermal” speeds, increasing their likelihood of causing further fission without being absorbed.
Deuterium in heavy water has a significantly lower tendency to absorb neutrons compared to protium in ordinary water. This makes heavy water a highly effective moderator, allowing for a sustained nuclear chain reaction even when using natural, unenriched uranium. Reactors using light water, conversely, require uranium enriched in uranium-235 to compensate for higher neutron absorption. This capability made heavy water appealing for producing plutonium, a fissile material for atomic weapons.
Germany’s Nuclear Ambitions
Germany’s Uranverein (Uranium Club) initiated its nuclear research program in 1939. German scientists, including Werner Heisenberg, understood that a nuclear reactor would require a moderator to control the chain reaction. Their chosen design heavily relied on heavy water, as they concluded that the available graphite was not pure enough.
The acquisition of heavy water became paramount for the German program. The world’s only industrial-scale heavy water production facility at the time was the Vemork hydroelectric plant in Rjukan, Norway. After occupying Norway in April 1940, Germany took control of the Vemork plant, which had begun mass-producing heavy water in 1934 as a byproduct of fertilizer production. They immediately ordered a fivefold increase in its heavy water output, making the Vemork facility a critical asset for their atomic technology.
The Sabotage of Vemork
The Allies became aware of Germany’s reliance on Vemork and its heavy water production, leading to a series of operations to disrupt their nuclear ambitions. The British Special Operations Executive (SOE) launched “Operation Grouse” in October 1942, parachuting a four-man Norwegian advance team into the Hardanger Plateau to scout the area and prepare for a larger assault.
“Operation Freshman” was attempted in November 1942, involving British engineers transported by gliders to destroy the plant. However, both gliders and their towing aircraft crashed due to severe weather and navigational difficulties, resulting in casualties and the capture and execution of many survivors. Despite this setback, the “Grouse” team, renamed “Swallow,” remained hidden and continued their mission.
The most successful raid, “Operation Gunnerside,” took place in February 1943. Six additional Norwegian commandos, led by Joachim Rønneberg, parachuted in and linked up with the “Swallow” team. On the night of February 27, the eleven-man team infiltrated the Vemork plant. They successfully planted explosives, destroying the heavy water production cells and approximately 500 kilograms of heavy water, effectively halting production for several months.
Even after the Germans rebuilt parts of the plant, subsequent Allied bombing raids, including a significant USAAF B-17 attack in November 1943, caused further damage and convinced the Germans to abandon the facility. In a final act of sabotage in February 1944, Norwegian resistance fighters sank the ferry Hydro, which was transporting the remaining heavy water stocks across Lake Tinn, ensuring the Germans lost their last significant supply.
The War’s Nuclear Outcome
The Allied efforts to disrupt heavy water production at Vemork had a significant impact on Germany’s atomic program. The repeated sabotage operations and bombing raids significantly hampered Germany’s ability to acquire sufficient heavy water, which was essential for their chosen reactor design. This continuous impediment contributed to their failure to develop a functional nuclear weapon before the war’s end.
In contrast, the Allied Manhattan Project, which successfully developed the atomic bomb, primarily utilized graphite as a neutron moderator in its plutonium production reactors. This strategic difference meant the Allied program was not bottlenecked by the limited global supply of heavy water. While the German scientists faced a shortage of this specific material, the Allies pursued an alternative path that allowed them to achieve nuclear fission capability.