D for danger

Chemistry & Industry, 21 February 2000

Heavy water and the wartime race for nuclear energy
Per F Dahl
Bristol: Institute of Physics Publishing
Ppxvi+399, 35/$60, ISBN 0 7503 0633

When the Nazis invaded Norway in April 1940, the Norsk Hydro electrolysis plant at Vemork in the Telemark district featured high on their list of core objectives to be secured. Not only did the plant occupy a prime strategic site in the Vestfjord valley that cuts the massif dividing the east and west of the country, but it was the world's leading producer of a simple but much-misunderstood substance that the Germans believed would help them win the war.

Heavy water - an isotopic form of good old H2O, with the hydrogen atoms replaced by deuterium - was central to German research into the mysteries of nuclear energy.

Early attempts to create a chain reaction of nuclear fission in uranium failed because too many neutrons escaped. The answer was to envelop the uranium in some substance which would moderate the reaction, slowing down escaping neutrons so that the chain reaction could continue. In trials using water, graphite and paraffin wax as moderators, the chain reaction was not sustained. Calculations suggested that only deuterated water would suffice.

Germany was therefore eager to secure the Norsk Hydro site, which included the largest electrolytic hydrogen plant in the world. Ordinary water is only one part in 4500 'heavy', but electrolysis to produce hydrogen for fertiliser manufacture removes only hydrogen and leaves deuterium behind, thereby increasing the proportion of deuterium. Repeatedly feeding deuterium-enhanced water through a succession of electrolytic cells could thus produce heavy water of over 99% purity.

As early as 1939, Norsk Hydro managers had grown suspicious of large German orders for heavy water. The company alerted authorities in France and the UK who, ignorant of the latest developments in nuclear physics, suspected some use in biological warfare.

In the weeks before the German invasion of Norway, Hydro's stock of heavy water was smuggled out of the country to the cellars of the College de France in Paris. It was later transferred to a bank vault in Southern France, and was eventually moved to the relative security of England.

Norsk Hydro's new masters made replenishing the stocks a priority, so that the nuclear research effort could continue. Under German orders, the company installed a series of 15 electrolysis cells to produce high-purity heavy water, later refining production with the Harteck-Suess catalytic exchange process. Production was, however, slowed by essential maintenance work, wartime scarcities, and mild sabotage by technicians unsympathetic to their new Nazi leaders. At least three employees added castor oil or cod-liver oil to the electrolyte, causing heavy foaming in the system.

It fell to the UK's controversial Special Operations Executive (SOE), with the help of Norwegian partisans, to put a more significant spanner in the works. However, a commando mission in July 1942 - the first Allied glider operation of the war - was an unqualified disaster thanks to poor preparation and worse conditions, and served only to alert the Germans to Allied interest in the plant.

A second assault in January 1943 was more successful, despite heightened security at Vermork. But although production was set back by around five months, this was much less than the one or two years anticipated by the SOE.

The assaults, followed by heavy and rather indiscriminate bombing by the US, persuaded the Nazis by late 1943 to remove the entire stock of heavy water to Germany. The Allies were again determined to stop this transfer when it was most vulnerable, and successfully did so by bombing the ferry carrying the shipment across Norway's Lake Tinn.

The Germans were by this time developing an alternative deuterium source at IG Farben's ammonia works near Leipzig. But the pilot plant was destroyed by a US raid in mid-1944, and the crumbling Nazi state abandoned the programme.

By mid-1945, many of the leading German researchers were in Allied hands at Farm Hall, near Cambridge. There, on 6 August, they learned of the deadly fruits of US nuclear research, which had concentrated on graphite as a moderator. Germany's abandonment of this potentially superior moderator stemmed from a simple but serious error by the pioneering researcher Walther Bothe, who significantly underestimated the amount of impurities in test samples.

After the bombing of Hiroshima, one German physicist, Carl Friedrich von Weizsacker, mused that he and his fellows should 'probably have concentrated more on the separation of isotopes and less on heavy water'. With hindsight, Germany's obsession with heavy water played a large part in the failure of its nuclear programme, but was by no means the only factor. And, of course, that failure can hardly be said to be regrettable.

The story of heavy water during World War II may be a familiar one, if only from the 1962 movie The heroes of Telemark. But Dahl's book fills many of the gaps left in previous accounts by concentrating on the underlying technology and scientific arguments.

The military adventure is cleverly interwoven with the scientific. The accounts of the commando raid on the Norsk Hydro plant and the bombing of the ferry are particularly gripping. Dahl also has a talent for sketching characters, and portrays a much more interesting and diverse cast than the 1962 movie's composite presentation of Kirk Douglas as the 'playboy scientist' and Richard Harris as the hotheaded Norwegian partisan.

The book does, however, suffer from poor editing. It contains several howlers, including one character who appears 'swinging a loaded hunting crop'. We're also told very early on that measurements of water's density 'varied by as much as 8x10(7)g/cm(3)' - heavy water indeed.

Dahl, a retired staff scientist at Lawrence Berkeley National Laboratories in California, closes his account with a note on the use of heavy water today. Although it is still considered a strategic material subject to international control, one of its primary applications is very far removed from the horrific potential of its wartime application. The loosely bound neutron in the deuterium nucleus makes it ideal for detecting neutrinos from distant supernovae. A nickel mine in Canada currently hosts a neutrino observatory containing 1000 tons of 99.92% enriched heavy water - over 300 times as much as the total wartime German production.