by Close, Frank
In the interim, an important change had occurred: Halban had quit as director and stepped down to head the physics department. The new director of the Anglo-Canadian project was John Cockcroft. Cockcroft was famous for developing the particle accelerator at Cambridge that first split the atomic nucleus in 1932, and by 1944 he had become an effective administrator of scientific research on radar. As the Soviet army pushed the Germans back, however, research on radar was deemed to be less urgent than the development of nuclear physics, which was seen as vital.26 Cockcroft’s arrival would transform the Anglo-Canadian project, as well as the fate of Bruno Pontecorvo.
SPYING FOR BRITAIN
Cockcroft soon gave the project a new impetus. When he arrived, the reactor was still on the drawing board, and the possibility of a serious accident occurring when it was completed convinced him that it should be constructed at least 100 miles from any major city. He found a suitable location in the forests about 130 miles west of Ottawa: the village of Chalk River. The nearby Ottawa River would provide water to cool the reactor.
One of Cockcroft’s first actions was to encourage his team to coax information from the Americans. One result of the political constraints imposed by the nations’ governments and militaries was that the Americans would give aid to the Canadian team in their pursuit of a heavy-water reactor, but not in relation to other technologies, such as the graphite reactor. Britain, with an eye to the future, saw graphite reactors as key to its postwar energy needs (as indeed was the case). Consequently, the British, who regarded the political restrictions as unreasonable, did their best to extract information about graphite reactors from the Americans. Thus was born the officially sanctioned strategy in which the governing authorities were bypassed, and the community of scientists shared data and ideas among themselves.
One example of such sharing occurred in March 1944, when Nunn May and a Canadian colleague, Ted Hincks, shipped equipment to Chicago for use in an experiment. In Chicago, the head of experiments at the pile was a man named Herbert Anderson. Nunn May talked with him.27 Anderson agreed that General Groves’s instructions were too restrictive, and were being applied overzealously. The Canadian team’s engineers and physicists, frustrated by the difficulty of obtaining information from the Americans, had given Nunn May a “shopping list” of urgent questions, for which they wanted answers.28 Nunn May was successful, as Anderson allowed him to read several reports that had not yet been released to Montreal. This open sharing of data became a regular occurrence during a series of visits.
Later in the war, after the atomic bomb tests, several leading scientists from Los Alamos passed through Canada en route to Europe. They were “debriefed” at Cockcroft’s insistence. When Aage Bohr, the son of Niels and a future Nobel laureate, visited Canada, Cockcroft ordered Bruno Pontecorvo and Nunn May to “pump him dry.”29
Since Nunn May was spying for the USSR during this time, and it has been alleged that Pontecorvo was also, it is ironic that Cockcroft in effect ordered them to spy on behalf of the UK. During 1944 and 1945, Nunn May was given access to Fermi’s laboratory to perform several experiments. The possibility that Bruno Pontecorvo was passing information to the Soviets at this stage has been suggested, but never established. In any event, his intimate involvement with Nunn May would leave him compromised when the latter was exposed as a spy in 1945.
Years later, Pavel Sudoplatov, who was the deputy director of atomic espionage for the USSR during World War II, claimed that the Soviets obtained information from Pontecorvo, along with Bohr, Fermi, and others.30 Many have taken this claim to mean that these famous scientists were active spies, which in the case of Bohr and Fermi is so bizarre that some have dismissed Sudoplatov as a fantasist. Although some of Sudoplatov’s claims have inconsistencies, the accusation of “fantasist” might be due to a misunderstanding on the part of his critics: it is now clear that the Soviets did receive information, via Nunn May if not Pontecorvo, which originated with Fermi and other leading members of the American side of the Manhattan Project. It is also clear, as we saw earlier, that Fermi indiscreetly shared secret data with Pontecorvo in April 1942; while there is no evidence that this data would be of any interest to the Soviet Union, or even that Pontecorvo passed this information to anyone other than his colleagues at Well Surveys, the fact that Fermi was occasionally casual with information cannot be discounted.31
Of course, Pontecorvo had never tried very hard to hide his commitment to communism. His brother Gillo later said that Bruno had made no attempt to hide the fact that he was a communist in France or the United States, but that when he was invited to join the atomic project in Canada, he immediately became “nonpolitical.” Gillo was convinced that this was a cover because whenever they met after the war, Bruno “was too well informed on communist literature and events.”32 However, even after the move to Canada, there were clues as to Pontecorvo’s political affiliations. When Bruno and Marianne’s second son was born in March 1944, they named him Tito Nils—Nils after Bruno’s scientific hero, Niels Bohr, and Tito, as Bruno explained later, “in honor of the communist who had led the war of liberation in Yugoslavia.”33 Two years after Marianne had claimed to the FBI that she didn’t know what a communist was, here was a public display of the family’s sympathies.
1944
Bruno was part of the small group that visited Fermi’s team in Chicago regularly during 1944 and 1945. To design a working reactor, one needs precise data on how neutrons interact with a variety of materials. In Montreal, there was no source of neutrons powerful enough to make good measurements. The Chicago reactor, however, produced more intense beams of neutrons, which could give clear answers. Obtaining these answers involved several collaborative visits and exchanges of information. Upon returning to Canada, the experts would report the new data to their colleagues, evaluate them, and plan new lines of attack. Bruno was the author of several of these reports.
A successful nuclear reactor needs to maximize the number of neutrons available as fission-creating bullets in the reactor’s core. Therefore, one must avoid materials that absorb them. Neutrons can travel anywhere. They might induce radioactivity in unexpected ways and poison the reactor. In extremis, it is possible that this could cripple it fatally. In designing the reactor, it would be necessary to know the chance of neutrons being captured by all manner of elements—not only how they interacted with the fuel (the uranium and the heavy water or graphite) but also how they interacted with the concrete blocks and the potpourri of other materials in the assembly and surroundings. This was one of the reasons Fermi had been so interested in Pontecorvo’s work with neutrons in 1942. The team was working in a new field, where hands-on, practical experience trumped any amount of theory. Thus Bruno’s expertise in this area would now be invaluable in Canada.34
The first of Bruno Pontecorvo’s visits to Chicago occurred in January 1944, when he made the journey with Nunn May, Auger, and other senior members of the team.35 In line with the usual practice of compartmentalization, physicists such as Pontecorvo were only present for discussions of the physics program; chemistry and engineering were discussed in separate groups.36 This visit to Chicago kick-started the Canadian work on the nuclear reactor. Just nine people were involved in the discussions at the first two meetings, which evaluated detailed questions about the materials for the reactor and their optimal use.
The Canadian team visited Chicago on several occasions, and Bruno generally wrote the reports.37 These reports confirm the significance of this work, and justify the British government’s description of his role in both planning and research to have been “indeed of the greatest importance.”38 Bruno was in Chicago during the first week of March, and returned home in time for the birth of Tito Nils on March 20. Then he left for Chicago once again during the first week of April.
The physical principles of a chain reaction might be relatively straightforward, but the construction of a nuclear reactor is not. As the Canadian team forged ahead on the project, one probl
em after another arose.
In a heavy-water reactor, for example, is it better to mix some uranium compound with tons of heavy water to form a homogeneous sludge, or instead to immerse solid rods throughout the liquid? If the former, what is the optimal mixture? If the latter, what is the best matrix arrangement?
Some problems were relatively mundane but potentially serious nonetheless. One example is the fact that a working reactor produces energy, and gets hot. Solid uranium rods might expand or even buckle; how dangerous was this for a working reactor? Was it better to cool the rods by bubbling air or other gases through the heavy water, or by running ordinary water through pipes? Ordinary water could keep the uranium rods cool, but it might absorb some neutrons, slowing the reactor or stalling it completely. At their next meeting in May, the two teams discussed this issue. The knowledge gained in solving such problems took time to achieve, but was invaluable. Igor Kurchatov, the scientific head of the Soviet atomic bomb project, knew that if he could obtain solutions through agents in North America, a Soviet reactor program could save months or even years of effort. Even so, having access to all the data and rules of best practice is no substitute for hands-on experience.39
One of many uncertainties the reactor physicists faced in 1944 was what happened when neutrons hit heavy water. Experiments by the team in Chalk River and the Americans in Chicago gave contradictory results. So Nunn May went to Chicago in the spring of 1944 to make use of the American team’s intense beam of neutrons and repeat the experiment there.40
AT THE TIME, SCIENTISTS HAD ALREADY ESTABLISHED THAT FISSION IS most likely to happen in isotopes that contain an odd number of neutrons. Uranium-235 and plutonium-239 are well-known examples, but there was a possibility that another odd-numbered isotope of uranium, U-233, might be a suitable fuel too. Consequently, a third line of collaborative research focused on creating uranium-233 in the Chicago pile, one nucleus at a time, until there was enough to determine its properties.
Uranium-233 had two apparent advantages over its competitors. First, the chemists thought it would be easier to refine than plutonium. This was the scientific reason for pursuing this strategy, as understood by both the Anglo-Canadian and American teams. However, there was a covert political reason too. To breed U-233, neutrons are fired at atoms of the element thorium. This excited the British, as thorium was plentiful in India, which at the time was under British rule. The strategy was especially seductive because natural uranium, the seed for breeding plutonium, was in short supply. Thus, the British saw a potential niche market for thorium, which they’d be able to dominate, though they withheld this motivation from the Americans; on paper, the potential ease of refining U-233 was reason enough to be interested. With this scientific goal in mind, the Americans used their reactor to irradiate thorium and make a few milligrams of U-233, enough to compare it against U-235.
Nunn May went to Chicago to carry out these comparisons in September 1944. This too was in collaboration with Herbert Anderson, as part of the program agreed to in January.41 Thanks to Anderson’s collegial indiscretion, Nunn May received a small sample of U-233—and he was fortunate to receive it. At the time, Chicago was the only source of U-233 in the world.
Cockcroft’s diary records pithily that “Alan Nunn May returned to Montreal with very useful information.” He returned with more than that, of course. Nunn May kept his sample of U-233 initially, but then passed it to a Soviet contact.
BRUNO THE URANIUM PROSPECTOR
Using electronic instruments to detect radiation and identify its source was Bruno’s professional forte. This expertise now bore fruit, as his techniques were applied to the search for uranium, and other scarce minerals essential to the atomic project. Specifically, the technique was used to map areas of northern Canada.
In January 1944, Bruno once again traveled to the United States, this time to New York to see Gilbert LaBine, the Canadian mining entrepreneur we met in Chapter 6. LaBine’s Eldorado Mine had already supplied the US with uranium for Fermi’s pile, but to fill the needs of the Chalk River reactor and the overall Manhattan Project would require new sources of the element. As an outcome of the New York meeting, LaBine commissioned some of Bruno’s former colleagues from Tulsa, including Serge Scherbatskoy, to determine the optimal means of locating further deposits by performing tests in areas of Canada known to have uranium. This occupied the group during the summer and fall of 1944. For five days in September, Bruno himself joined them at Port Radium, near Great Bear Lake in the Northwest Territories.
The team compared different ways of detecting uranium in pitchblende, a naturally occurring ore. They found that although ionization chambers and Geiger counters were equally likely to locate the mineral when the rocks were in outcrops, the ionization-chamber technique was better when the rocks were a foot or more beneath the surface. The group performed surveys using lightweight instruments in helicopters, as well as ground surveys in trucks, which carried more cumbersome apparatuses. Bruno’s favored technology, ionization chambers, worked best for identifying the radioactivity of ordinary rocks and pebbles. The challenge in this case was to identify the signal of uranium amid the morass of other radioactivity.42
Bruno presented Cockcroft with an extensive summary of the group’s findings, and then announced the conclusions in October, at a special meeting in New York. Senior members of the US military were present. This marriage of nuclear physics and geology proved so successful as a means of finding uranium that the Manhattan Project adopted it. This was another area where Pontecorvo’s expertise would later have potentially huge importance for the USSR.
ZEEP AND THE END OF THE WAR
The uranium quest was Pontecorvo’s primary endeavor during the summer and fall of 1944. Then, for much of the first half of 1945, he investigated whether the fission products of radium-226 and uranium-233 might contaminate the reactor.43 Also, starting in June 1944, he was involved in a new project, which was the brainchild of Alan Nunn May: the Zero Energy Experimental Pile, or ZEEP.
Halban left Canada in 1944, and Nunn May took over as head of the physics division. He proposed that a small reactor—ZEEP—be built in order to test the soundness of the team’s theoretical calculations on the optimal distribution of the uranium rods in the tank of heavy water for the NRX.44 Big enough to be a reactor, but small enough that it produced negligible power and thus didn’t need concrete shielding or a cooling plant, ZEEP could be used to test different configurations of rods.
In Nunn May’s conception, ZEEP had a secondary benefit, not stated in the official documents. Although he only discussed the idea verbally, Nunn May felt it was imperative that, when the war ended, the Anglo-Canadian team be able to demonstrate a working reactor to politicians and journalists.45 This was a problem because it was clear from the outset that it would take a long time to complete the NRX.
Lew Kowarski had remained in Cambridge since 1940, but with his bête noire, Halban, no longer in Canada, Kowarski now joined the project. He was given special responsibility for constructing ZEEP. Kowarski later recalled his first sight of a working reactor, when he visited Chicago in September 1944. To outward appearances the machine was unimpressive, merely a cube of painted concrete. Herbert Anderson took Kowarski over to the pile and said, “Touch it. It’s warm.”
This visit helped Kowarski decide on his plan for ZEEP. Pontecorvo briefed him on the basics of constructing a pile, knowledge that had emerged from the American team’s work since December 1942. Kowarski recalled that Bruno did so “in a sort of lecture in a single afternoon, which was quite enough.”46
Although ZEEP was important for the Anglo-Canadian project, back in the United States General Groves was skeptical. His goal was to complete the atomic bomb and win the war. On a visit to Chalk River, Groves met Kowarski and bluntly asked, “Are you the man who is building this damn fool unnecessary experimental reactor?” Kowarski confirmed that he was, to which Groves replied, “America gives most of the heavy water for it, and it’s v
ery very costly stuff. Make sure that you don’t squander it.”47 Although stated brusquely, the point was well made: heavy water was precious.
ZEEP went critical on September 5, 1945, three days after Japan surrendered and brought an end to World War II. The team held a party to celebrate their success with ZEEP, and Nunn May made a speech. He declared that Kowarski should receive an accolade and, because Kowarski came from France, Nunn May proceeded to kiss him on both cheeks. The timing was ironic. Nunn May, who had been passing information to the Soviets, was about to be exposed. At the very moment when the celebrations began, Igor Gouzenko, a cipher clerk at the Soviet consulate in Ottawa, was feverishly filling his briefcase with a wealth of documents.48 These papers contained information about a ring of spies working for the Soviet Union. Gouzenko intended them to be the down payment on his application for asylum in Canada. The information in Gouzenko’s briefcase would lead to the arrest and conviction of Alan Nunn May six months later, in March 1946.49
By the end of the war, Pontecorvo was a much sought-after prize, the recipient of several job offers at universities in North America and Europe. To the surprise of several colleagues, in February 1946 he chose to join the Atomic Energy Research Establishment, the infant nuclear laboratory in Harwell, England. Then, having made this unexpected choice, that same month he prevaricated and decided to stay in Canada. His stated reason was that he wanted to work on the NRX reactor, which was the focus of the project. From 1943 to 1945 he had devoted a huge amount of effort to its design, writing some twenty-five reactor-related reports. In 1945 and 1946 he developed sensitive neutron monitors for the initial start-up of the NRX, capable of confirming the transition from zero flux to the first feeble reactions. Because of this responsibility, Bruno was one of only four physicists allowed in the NRX control room at the start-up.50
