Heritage

Water that made nuclear fission possible

A Tale of Scandinavian Inventiveness and Bravery

By Peter Berlin

During WW II, a liquid known as Heavy Water was “brewed” near the town of Rjukan in southern Norway. No, heavy water is not a brand of Norwegian aquavit but was once an important element in the making of nuclear bombs.

It all began in 1905 when a Norwegian hydro-electric power company decided to exploit the 104-metre Rjukan water fall with the aim of generating large-scale electric energy. The result was the world’s biggest hydro-electric power plant at the time, and its primary purpose was to supply electric power to a local factory that made agricultural fertilizer.

Thirty years later, a factory for producing hydrogen was built next door to the power plant. Hydrogen is obtained by subjecting ordinary water to power-hungry electrolysis (see Company File on page 10). What the factory operators did not realize at first was that they were also producing small quantities of heavy water as a by-product. It took a future Norwegian Nobel Prize winner to point that out to them.

Recall that ordinary water (H2O) is made up of two hydrogen atoms (H2) bound to a single oxygen

The Rjukan Waterfall lay the foundation for the Vemork Power Plant, which became the largest of its kind in the world when completed in 1911. The Vemork Power Plant now houses the Norwegian Industrial Workers Museum and the World Heritage Centre. Photo: Norsk industriarbeidermuseum

atom (O). Heavy water has the formula D2O where the “D” stands for deuterium. The deuterium atom is a hydrogen isotope where a neutron is attached to the lone hydrogen proton inside the atom. The addition of the neutron makes a deuterium atom roughly twice as heavy as a regular hydrogen atom; hence the term “heavy water.”

The idea behind a nuclear device is to bombard uranium with neutrons. The bombardment splits the uranium atoms and precipitates a hail of secondary neutrons in the process. The secondary neutrons are meant to dislodge more secondary neutrons in adjacent uranium atoms, and so forth – in other words, a chain reaction called fission which causes a nuclear explosion. Paradoxically, a substance is needed which slows down the secondary neutrons, so that they can be captured by other uranium nuclei. Heavy water is one such medium.

More happily, heavy water is also used as a moderator in some of the nuclear reactors that supply a substantial portion of the electricity used in everyday life – although many ecologists nowadays question whether nuclear power is such a “happy” resource, after all.

The story about heavy water doesn’t end here. In September 1940 the Germans occupied Norway. Having heard about the heavy water production and its potential use for making nuclear weapons, the German army took control of the plant at Rjukan.

Following failed efforts by the Allies to put the Rjukan plant out of action, the Norwegian resistance movement managed to disable the plant once and for all. On a cold night in February of 1943, a group of commandos entered the plant unseen as the Germans were changing the guards. The saboteurs planted dynamite charges under the heavy-water-making equipment. By the time the Germans heard the explosions and sounded the alarm, the Norwegian commandos were back on their skis heading for the Swedish border.

The sabotage of the Rjukan plant was the subject of a 1948 FrancoNorwegian docudrama, Operation Swallow: The Battle for Heavy Water (“Kampen om Tungtvannet” in Norwegian). In 1965 an American movie on the same theme called The Heroes of Telemark starred Kirk Douglas and Richard Harris. A Norwegian TV mini-series, The Heavy Water War, aired in 2015. Nowadays the name Rjukan is firmly embedded in WW II history.