Finland Iceland Norway The Norwegian heavy water sabotage (Bokmål: Tungtvannsaksjonen; Nynorsk: Tungtvassaksjonen) was a series of Allied-led efforts to halt German heavy water (deuterium) production via hydroelectric plants in Nazi Germany-occupied Norway during World War II, involving both Norwegian commandos and Allied bombing raids.

During the war, the Allies sought to inhibit the German development of nuclear weapons with the removal of heavy water and the destruction of heavy-water production plants.

Between 1940 and 1944, a series of sabotage actions by the Norwegian resistance movement and Allied bombing ensured the destruction of the plant and the loss of its heavy water.

News of the discovery spread quickly among physicists and it was realized that if chain reactions could be controlled, fission might be a new source of great power.

Although historical records provide limited detail on the German decision to pursue the heavy water approach, it became clear after the war that they had explored that option.

Plutonium-239 (239Pu) makes effective weapons material, although it requires an implosion-type mechanism rather than the simpler gun-type trigger used in the Thin Man uranium bomb.

[8] Norwegian Institute of Technology lecturer Leif Tronstad and Jomar Brun, head of the hydrogen plant, proposed a project in 1933 (the year heavy water was first isolated).

Crucial to the success of the mission was the role played by Charles Howard, 20th Earl of Suffolk, the British liaison to the French scientific establishment.

The Norsk Hydro management's collaboration with the Germans was examined during investigations of collaborationism begun by Norwegian authorities after the war, but Aubert's cooperation with the French aided the company's case.

The Norwegians were unable to reach the crash sites in time; the survivors were captured by the Gestapo, who tortured and later had them executed under Adolf Hitler's Commando Order.

[12] The surviving Norwegian Grouse team had a long wait in their mountain hideaway, subsisting on moss and lichen until they captured a reindeer just before Christmas.

One container was buried in the snow by a Norwegian patriot to hide it from the Germans; he later recovered it, and gave it to an officer of the British Army Air Corps (which was conducting exercises in the area) in August 1976.

[12] The saboteurs then placed explosive charges on the heavy-water electrolysis chambers, and attached a fuse which allowed sufficient time for their escape.

In an attempt to prevent reprisals, a Thompson submachine gun[17][18][19] was purposely left behind to indicate that this was the work of British forces and not the local resistance.

The entire inventory of heavy water produced during the German occupation, over 500 kg (1,102 lb), was destroyed with equipment critical to the operation of the electrolysis chambers.

The plant was repaired by April; SOE concluded that a repeat commando raid would be extremely difficult, since German security was considerably improved.

[22] There was less need for ground assaults than a year earlier, since night bombing (previously unrealistic due to German air cover) was now an alternative.

The Germans, convinced that air raids would result in further serious damage, decided to abandon the plant and move its remaining stocks and critical components to Germany in 1944.

[23] Knut Haukelid (the only trained commando in the immediate area) was informed of the German plan to remove the heavy water, and was advised to muster support and destroy the shipment.

Eight-and-a-half kilograms of plastic explosives (with two alarm-clock fuses) were fixed to the keel of the SF Hydro, which would transport railway cars with drums of the heavy water across Lake Tinn.

The concentration of heavy water in a number of the barrels was too small to be of value to a weapons program, however, which might explain the lack of tight security around the shipment and why the ferry was not searched for bombs.

[26] In the film The Heroes of Telemark, the locomotive and train is shown covered with German soldiers; according to Ray Mears' BBC Television coverage, the general in command had ordered this disposition of troops.

However, the unsuccessful Operation Freshman and the efforts of the saboteurs in Swallow, Grouse and Gunnerside made the secret war on heavy-water production internationally known.

[30] The New York Times reported that at 95, Rønneberg was "still mentally sharp ... and possessed of the unflappable calm that so impressed British military commanders more than 70 years ago.

The Real Heroes of Telemark: The True Story of the Secret Mission to Stop Hitler's Atomic Bomb, a 2003 book by Ray Mears (ISBN 0-340-83016-6), emphasises the Norwegian commandos' unique survival skills.

Jens-Anton Poulsson (Swallow and Grouse) wrote The Heavy Water Raid: The Race for the Atom Bomb 1942–1944 (ISBN 9788245808698), a 2009 book.

The raid is the subject of Assault in Norway: Sabotaging the Nazi Nuclear Program (ISBN 9781585747504), a 2002 book by Thomas Gallagher based on interviews with many of the commandos.

An account of Operation Gunnerside is part of Neal Bascomb's 2016 The Winter Fortress: The Epic Mission to Sabotage Hitler's Atomic Bomb (ISBN 9780544368057).

Damien Lewis's book Hunting Hitler's Nukes: The Secret Race to Stop the Nazi Bomb (ISBN 9781786482082), also published that year, details the raid and the sinking of the SF Hydro.

The 2018 book, Heroes of Telemark; Sabotaging Hitler's atomic bomb, Norway 1942–44 by David Greentree (ISBN 9781472827678), describes the operation's planning, execution and aftermath.