Diana Preston - Before the Fallout

By then most of Mark Oliphant’s work at Birmingham was concerned with radar development. Security regulations did not allow aliens born in enemy countries like Otto Frisch and recently naturalized British citizens like Rudolf Peierls to be employed on sensitive war work, and thus both men were excluded from taking part. Indeed, neither was supposed to know anything about the project. However, the secrecy was “a bit of a charade.” As Frisch recalled, Oliphant would sidle up to Peierls and pose him a “hypothetical” question, to which Peierls would furnish an answer, knowing full well what it would be used for. “Oliphant knew that Peierls knew, and I think Peierls knew that Oliphant knew that he knew. But neither of them let on.” However, their formal exclusion from the radar work freed the two men to think about uranium fission.

While Frisch had been at Niels Bohr’s institute, there had been little belief in a “superweapon” as a practical possibility. Frisch assumed that to be correct until in early 1940 in Birmingham he was invited to contribute an article on fission to the annual report of the British Chemical Society. He was then living in a freezing apartment, where in winter, even with the gas fire on, the daytime temperature did not rise above 43 degrees Fahrenheit and where at night “the water froze in the tumbler at my bedside.” Huddled in his overcoat and with his typewriter balanced on his knees, he typed out his article, writing in unfamiliar English, “There are now a number of strong arguments to the effect that the construction of a super bomb would be, if not impossible, prohibitively expensive and that furthermore the bomb would not be so effective as was thought at first.” He sent off his article, but writing it had raised nagging doubts about whether he was right.

Frisch also brooded about the possibilities suggested by some studies he had recently begun into a method for separating isotopes known as “thermal diffusion.” Invented by the German scientist Klaus Clusius, it consisted of filling a tube with a gas mixture. If this mixture was heated at one end and cooled at the other, experiments had shown that the lighter isotopes would migrate to the hotter end and the heavier ones to the cooler region, thus suggesting the possibility of separating the lighter, fissionable U-235 from the heavier U-238.

Frisch sought out Peierls and startled him with the question “Suppose someone gave you a quantity of pure 235 isotope of uranium—what would happen?” They began to calculate the consequences, using a formula worked out by Peierls for calculating the “critical mass”—the amount of fissionable material needed to be brought together to release sufficient neutrons to start a self-sustaining chain reaction. As Peierls recalled, “The work of Bohr and Wheeler seemed to suggest that every neutron that hit a 235 nucleus should produce fission. Since the number of secondary neutrons per fission had been measured approximately, we had all the data to insert in my formula for the critical size.” The result amazed them. Others who had tried to calculate the critical mass “had tended to come out with tons”; the Joliot-Curie team, for example, had estimated it at around forty tons. Frisch and Peierls’s first estimate was “about a pound,” which as Frisch observed was not, after all, “such a lot.” Frisch calculated that using the Clusius thermal diffusion method of isotopic separation, he could produce a pound of reasonably pure U-235 in a matter of weeks.

The two men also calculated whether the chain reaction would last long enough to cause a catastrophic explosion. Scribbling literally on the back of an envelope, they worked out that a substantial amount of the uranium would fissure, releasing energy equivalent to “thousands of tons of ordinary explosive.” As Peierls remembered, “We were quite staggered by these results: an atomic bomb was possible at least in principle! As a weapon it would be so devastating that, from a military point of view, it would be worth setting up a plant to separate the isotopes. In a classic understatement, we said to ourselves, ‘Even if this plant costs as much as a battleship, it would be worth having.’” With further understatement, Frisch said thoughtfully to Peierls, “Look, shouldn’t somebody know about that?”

Together they composed the famous Frisch-Peierls memorandum titled “On the Construction of a ‘Superbomb’; Based on a Nuclear Chain Reaction in Uranium.” The compelling three-page, two-part document dealt with scientific, strategic, and ethical issues. It suggested that “one might think of about 1 kg [of uranium] as a suitable size for the bomb.” Their estimates of the critical mass were, in fact, an underestimate. They were unaware that some of the neutrons colliding with U-235 would simply be absorbed, or “captured,” rather than causing fission, but, as Peierls later wrote, “the order of magnitude was right.” They also described how to explode a bomb with a mechanism that would force two pieces of uranium together at tremendous speed to constitute the critical mass.

The memorandum addressed the human consequences not only of the blast, which could probably destroy “the centre of a big city,” but of the subsequent effect of radiation, “fatal to living beings even a long time after the explosion.” “Most of it,” the note predicted, “will probably be blown into the air and carried away by the wind. This cloud of radioactive material will kill everybody within a strip estimated to be several miles long. If it rained the danger would be even worse because active material would be carried down to the ground and stick to it.” Frisch and Peierls suggested that the probably very high number of civilian casualties “may make it unsuitable as a weapon for use by this country,” but pointed out that as there was no effective defense, other than the threat of retaliation with the same weapon, it would be worth developing as a deterrent, “even if it is not intended to use the bomb as a means of attack.” They also warned that although “we have no information that the same idea has also occurred to other scientists… all the theoretical data bearing on this problem are published, [so] it is quite conceivable that Germany is, in fact, developing this weapon.”

The Frisch-Peierls memorandum—with its origins in a cold room in Birmingham where a refugee muffled in an overcoat tapped with chilly fingers on a typewriter balanced on his knees—was the first document to demonstrate scientifically the real possibility of creating an atomic weapon and the first to describe its shocking effects. For security reasons the two scientists typed the note themselves, making only one carbon copy, and gave it to Oliphant, who in March 1940 sent it to a startled Sir Henry Tizard. The depiction of a weapon that would be “practically irresistible” was about to kick-start the faltering British atomic program.

ON THE MORNING OF 21 June 1940, the British collier Broompark docked at Falmouth after a nerve-racking thirty-six-hour crossing from Bordeaux during which an accompanying vessel had been sunk by a German mine. On board was a motley cargo of twenty-six drums of heavy water, 4 million pounds’ worth of industrial diamonds, and piles of machine tools. The passengers included a bedraggled group of French scientists and their families. This was the conclusion of a mission entrusted to the eccentric, thirty-three-year-old earl of Suffolk—Charles Henry George Howard but known to all as “Jack.” Barred by his limp from the armed forces, he had been appointed scientific liaison officer at the British Embassy in Paris, where he lived at the Ritz and, according to his contemporaries, “spent a lot of time drinking kirsch” and carousing with pretty women. With France about to fall, his bosses ordered him to gather some fifty eminent French scientists and engineers they had identified as useful and whisk them to safety in Britain. In the time available Suffolk had been able to find only about half of them, and the disappointed reaction of the government official meeting the train bringing the party to London was “Oh, is that the lot?” However, the little group included Frederic Joliot-Curie’s two right-hand men, Hans von Halban and Lew Kowarski, though Joliot-Curie himself had opted to remain in occupied France.