Oppenheimer and the path of heavy water

by Jeremy Whitlock

August 2023

There's a scene in the 2023 epic Oppenheimer where the eminent Danish scientist Niels Bohr arrives at Los Alamos in late 1943, and reports that German scientists were trying to build a nuclear reactor based on heavy water. The Manhattan Project scientists react with obvious relief at this news, noting that this meant the Germans were on the wrong path to a bomb.

It's a very short scene and heavy water is never mentioned again; many viewers will probably not even recall it.

Director Christopher Nolan should be commended for including it however, since it's a seminal moment in the film's narrative – essentially guaranteeing that the Germans would not master nuclear fission fast enough to impact the course of the war, and thus foreshadowing the moral dilemma around the bomb's continuing development.

Nolan's brilliance is in the way he references the heavy-water issue, but wastes no more time on it since the film is largely Robert Oppenheimer's point of view: he does the same for the first sustained chain reaction in Chicago, the manufacture of plutonium at Hanford, and the enrichment of uranium at Oak Ridge – all Herculean efforts worthy of movies of their own.

However, if you're curious why heavy water would elicit the reaction it did in the Los Alamos scientists (or what heavy water is anyway), read on.

In 1939, four years before that scene from Oppenheimer takes place, and just before the outbreak of World War II, fission was discovered: the splitting of atoms to release unprecedented amounts of energy. This was not just the culmination of 20th-century physics but an all-time pinnacle of human achievement. To say it's unfortunate that its first application was a bomb is an understatement.

Regardless of one's intentions however, the first step after the discovery of fission is to build a machine that maintains a self-sustaining fission reaction: a nuclear reactor. To do this you need two basic ingredients: nuclear fuel (uranium) and something called a moderator. The uranium does the fissioning; the moderator is the catalyst.

A good moderator is any material that slows down the key subatomic particles (neutrons) without absorbing too many of them – while at the same time, importantly, being available in large quantities since you'll need tonnes of it. Shortly after the discovery of fission, French scientists confirmed that the best moderator is technically heavy water – an extremely rare but natural component (less than a gram per litre) of regular water.

The problem with heavy water at the time was that it lacked the third important characteristic: it wasn't readily available. Discovered only a decade earlier, heavy water until then had almost no practical use as a stand-alone material (separated from regular water). Virtually the world's entire supply, less than 200 litres, was quickly moved from Norway to France under the noses of the Nazis.

From France the heavy water then moved to Britain, accompanied by some of the French scientists who continued to use it in their nuclear fission research. Shortly afterwards the precious inventory, and the French scientists, moved to the relative safety of Canada – thereby seeding an entire post-war nuclear program based on heavy-water reactors.

Meanwhile back in WWII, the Allies, aware of the Germans' continuing interest, made the Norwegian heavy-water production plant a special target. It was eventually destroyed enough to forever hamper the Nazi efforts to harness nuclear fission.

And all of this wouldn't have mattered if the Germans had pursued graphite, at least as Plan B.

Graphite, a common form of carbon, was without a doubt the best candidate moderator at the time – all things considered, including (crucially) availability. The Germans had investigated graphite but incorrectly concluded otherwise – that it absorbed too many neutrons – and so they stuck with the elusive heavy water.

Elsewhere, the US and other allied scientists had already confirmed through their own top-secret experiments that graphite's apparent drawbacks were entirely due to easily-removed impurities, and in fact that pure graphite was an excellent moderator.

Unlike the traditional openness of science, they kept this fact confidential, and hence everyone's obvious satisfaction when Niels Bohr confirmed that the Germans were still doggedly pursuing heavy water: to build a bomb you needed first to build a reactor, and to build a reactor any time soon you needed graphite (the Manhattan Project had secretly passed this groundbreaking milestone a year earlier in Chicago).

Not that heavy water was ignored on this side of the front lines: while the Manhattan Project did build a series of massive graphite-moderated nuclear reactors in Washington state to support its immediate wartime needs, it also initiated a number of heavy-water production projects (one in Canada and three in the US) – producing enough of the precious liquid by May 1944 to supply the world's first heavy-water reactor, CP-3, in Chicago.

This would only have been a few months after the conversation with Niels Bohr in the scene from Oppenheimer: the Germans were barely out of the starting blocks, oblivious to the fact that the Allies were already choosing new finishing lines to cross.

In short order heavy water gradually shed its albatross of "unavailability", and became the elegant moderator of nuclear fission that it was destined to be. By the 1950s the US was building massive heavy-water reactors at Savannah River to support its ongoing weapons program, while in Canada the technology soared: the first reactor outside the US (ZEEP), the most powerful research reactors in the world through the 1950s and 1960s (NRX and NRU), and a fleet of power reactors (CANDU) that run without the need for enriched uranium – all based on heavy water.

In France those displaced scientists went home after the war and finally got to build their own heavy-water reactor (ZOE), which seeded that country's massive nuclear program.

And that's the story behind a ten-second scene in Oppenheimer.


Discussion welcome.

©2024 Jeremy Whitlock

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