weather):
I remember we were writing our memorandum… together in my room in the Physics Lab on the ground floor; it was a fine day and the window was open… and while we were discussing the wording a face suddenly appeared in the open window. And we were a little worried! It turned out that just underneath the window (which was facing south) people were growing some tomato plants, and somebody had been there bending down inspecting what these plants were doing.
The first of the two parts they titled “On the construction of a ‘superbomb'; based on a nuclear chain reaction in uranium.” It was intended, they wrote, “to point out and discuss a possibility which seems to have been overlooked in… earlier discussions.” They proceeded to cover the same ground they had previously covered together in private, noting that “the energy liberated by a 5 kg bomb would be equivalent to that of several thousand tons of dynamite.” They described a simple mechanism for arming the weapon: making the uranium sphere in two parts “which are brought together first when the explosion is wanted. Once assembled, the bomb would explode within a second or less.” Springs, they thought, might pull the two small hemispheres together. Assembly would have to be rapid or the chain reaction would begin prematurely, destroying the bomb but not much else. A byproduct of the explosion — about 20 percent of its energy, they thought — would be radiation, the equivalent of “a hundred tons of radium” that would be “fatal to living beings even a long time after the explosion.” Effective protection from the weapon would be “hardly possible.”
The second report, “Memorandum on the properties of a radioactive ‘super-bomb,’” a less technical document, was apparently intended as an alternative presentation for nonscientists. This study explored beyond the technical questions of design and production to the strategic issues of possession and use; it managed at the same time both seemly innocence and extraordinary prescience:
1. As a weapon, the super-bomb would be practically irresistible. There is no material or structure that could be expected to resist the force of the explosion…
2. Owing to the spreading of radioactive substances with the wind, the bomb could probably not be used without killing large numbers of civilians, and this may make it unsuitable as a weapon for use by this country…
3…. It is quite conceivable that Germany is, in fact, developing this weapon…
4. If one works on the assumption that Germany is, or will be, in the possession of this weapon, it must be realised that no shelters are available that would be effective and could be used on a large scale. The most effective reply would be a counter-threat with a similar weapon.
Thus in the first months of 1940 it was already clear to two intelligent observers that nuclear weapons would be weapons of mass destruction against which the only apparent defense would be the deterrent effect of mutual possession.
Frisch and Peierls finished their two reports and took them to Oliphant. He quizzed the men thoroughly, added a cover letter to their memoranda (“I have considered these suggestions in some detail and have had considerable discussion with the authors, with the result that I am convinced that the whole thing must be taken rather seriously, if only to make sure that the other side are not occupied in the production of such a bomb at the present time”) and sent letter and documents off to Henry Thomas Tizard, an Oxford man, a chemist by training, the driving force behind British radar development, the civilian chairman of the Committee on the Scientific Survey of Air Defense — better known as the Tizard Committee — which was the most important British committee at the time concerned with the application of science to war.
“I have often been asked,” Otto Frisch wrote many years afterward of the moment when he understood that a bomb might be possible after all, before he and Peierls carried the news to Mark Oliphant, “why I didn't abandon the project there and then, saying nothing to anybody. Why start on a project which, if it was successful, would end with the production of a weapon of unparalleled violence, a weapon of mass destruction such as the world had never seen? The answer was very simple. We were at war, and the idea was reasonably obvious; very probably some German scientists had had the same idea and were working on it.”
Whatever scientists of one warring nation could conceive, the scientists of another warring nation might also conceive — and keep secret. That early in 1939 and early 1940, the nuclear arms race began. Responsible men who properly and understandably feared a dangerous enemy saw their own ideas reflected back to them malevolently distorted. Ideas that appeared defensive in friendly hands seen the other way around appeared aggressive. But they were the same ideas.
Werner Heisenberg sent his considered conclusions to the German War Office on December 6, 1939, while Fermi and Szilard waited for the $6,000 the Briggs Uranium Committee had allocated to them for graphite studies and Frisch prepared his pessimistic Chemical Society review. Heisenberg thought fission could lead to energy production even with ordinary uranium if a suitable moderator could be found. Water would not do, but “heavy water [or] very pure graphite would, on the other hand, suffice on present evidence.” The surest method for building a reactor, Heisenberg wrote, “will be to enrich the uranium-235 isotope. The greater the degree of enrichment, the smaller the reactor can be made.” Enrichment — increasing the proportion of U235 to U238 — was also “the only method of producing explosives several orders of magnitude more powerful than the strongest explosives yet known.” (The phrase indicates Heisenberg understood the possibility of fast-neutron fission even before Frisch and Peierls did.)
During the same period Paul Harteck in Hamburg was building a Clusius separation tube; in December he tested it by successfully separating isotopes of the heavy gas xenon. He traveled to Munich at Christmastime to discuss design improvements with Clusius, who was professor of physical chemistry at the university there. Auer, the thorium specialists, purveyors of gas mantles and radioactive toothpaste, delivered the first ton of pure uranium oxide processed from Joachimsthal ores to the War Office in January 1940. German uranium research was thriving.
Acquiring a suitable moderator looked more difficult. The German scientists favored heavy water, but Germany had no extraction plant of its own. Harteck calculated at the beginning of the year that a coal-fired installation would require 100,000 tons of coal for each ton of heavy water produced, an impossibility in wartime. The only source of heavy water in quantity in the world was an electrochemical plant built into a sheer 1,500-foot granite bluff beside a powerful waterfall at Vemork, near Rjukan, ninety miles west of Oslo in southern Norway. Norsk Hydro-Elektrisk Kvaelstofaktieselskab produced the rare liquid as a byproduct of hydrogen electrolysis for synthetic ammonia production.
I.G. Farben, the German chemical cartel assembled by Bayer's Carl Duisberg in the 1920s, owned stock in Norsk Hydro; learning of the War Office's need it approached the Norwegians with an offer to buy all the heavy water on hand, about fifty gallons worth some $120,000, and to order more at the rate of at least thirty gallons a month. Norsk Hydro was then producing less than three gallons a month, enough in the prewar years to glut the small physics-laboratory market. It wanted to know why Germany needed so vast a quantity. I.G. Farben chose not to say. In February the Norwegian firm refused either to sell its existing stock or to increase production.
Heavy water also impressed the French team, a fact Joliot pased on to the French Minister of Armament, Raoul Dautry. When Dautry heard about the German bid for Norsk Hydro's supply he decided to win the water for France. A French bank, the Banque de Paris et des Pays Bas, controlled a majority interest in the Norwegian company and a former bank officer, Jacques Allier, was now a lieutenant in Dautry's ministry. Dautry briefed the balding, bespectacled Allier with Joliot on hand on February 20: the minister wanted the lieutenant to lead a team of French secret-service agents to Norway to acquire the heavy water.
Allier slipped into Oslo under an assumed name and met with the general manager of Norsk Hydro at the beginning of March. The French officer was prepared to pay up to 1.5 million kroner for the water and even to leave half for the Germans, but once the Norwegian heard what military purpose the substance might serve he volunteered his entire stock and refused payment. The water, divided among twenty-six cans, left Vemork by car soon afterward on a dark midnight. From Oslo Allier's team flew it to Edinburgh in two loads — German fighters forced down for inspection a decoy plane Allier had pretended to board at the time of the first loading — and then transported it by rail and Channel ferry to Paris, where Joliot prepared through the winter and spring of the phony
