C. Fundamentals of Nuclear Fission C.1. Energy From the Atom
Alpha particles and beta particles are ejected from the nuclei of radioactive atoms with enormous energy, like subatomic shrapnel from some miniature explosion. It is natural to ask how much energy is stored in the nucleus in this way, and how it can be tapped.
In 1903, at McGill University, Ernest Rutherford and Frederick Soddy estimated the total energy given off over thousands of years by the radioactive decay of radium. The answer was staggering. The energy released must be, they wrote, "twenty thousand times, and may be a million times as great" as that associated with even the most powerful chemical reactions.
Later that same year Rutherford playfully quipped "that, could a proper detonator be found, it was just conceivable that a wave of atomic disintegration might be started through matter, which would indeed make this old world vanish in smoke."
To his dying day in 1937, however, Rutherford never believed that such an "atomic explosion" was possible. As a scientist he knew that the rate at which an unstable nucleus releases its energy by radioactive decay is unalterable. Nothing can be done to speed it up or slow it down. Changes in temperature, pressure, or chemical composition have absolutely no effect on the rate of radioactive decay. Thus there was no potential detonator in sight. Until 1938.
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C.2. The Fission Process
In late December, 1938, scientists in Berlin discovered that neutron bombardment can cause a uranium atom to fission into two or more fragments (see A.21). This news spread like wildfire through the international community of atomic scientists. Nuclear fission was an entirely new phenomenon, bringing with it new possibilities.
The energy released by each fissioning atom is hundreds of times greater than that released by radioactive decay. Moreover the rate of fissioning, unlike the rate of radioactive decay, can be influenced by an external agency -- namely, the number of neutrons used to bombard the uranium sample. For the first time the rate of release of nuclear energy could be influenced by human intervention.
Within weeks, a group of scientists in Paris -- Fréderic Joliot, Hans van Halban, Lew Kowarski and Francis Perrin -- had shown that when a uranium atom fissions, two or three extra neutrons are also given off, travelling at very great speed. This important observation suggested that a self-sustaining chain reaction might be possible.
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C.3. The Concept of a Chain Reaction
Suppose a single neutron causes a single uranium atom to fission, thereby freeing two or three more neutrons. Suppose further that these extra neutrons cause two or three more atoms to fission, thus liberating from four to nine additional neutrons. If this cascading process were to continue, then at each stage the number of neutrons would grow larger, leading to the fissioning of more and more atoms. Such a self-accelerating process is called a "chain reaction".
If a chain reaction is possible, just a small burst of neutrons would be enough to get it started. Accordingly, a few scientists believed that the neutron could serve as the atomic "detonator" referred to by Rutherford. Most scientists, however, were profoundly skeptical. A chain reaction is easy to imagine but difficult to achieve. How can one ensure that almost every neutron causes another atom to fission?
The question was more than academic. An uncontrolled chain reaction, if it occurred fast enough, could produce an incredibly powerful explosion, dwarfing even the most spectacular chemical explosions. As the world was then poised on the brink of war with Germany and nuclear fission had been discovered in Berlin, the implications were terrifying. At the outset, however, it was not clear whether such a bomb was a real possibility or just an unfounded fear.
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C.4. The Power of a Chain Reaction
The explosive power of an uncontrolled nuclear chain reaction can be illustrated by means of a simple mathematical analogy. Imagine a job that pays two cents the first day, four cents the second day, eight cents the third day, and so on, doubling each day. How much money will such a job earn over a sixty-day period?
A bit of arithmetic shows that this Alice-in-Wonderland job will pay more than forty quadrillion dollars ($40, 000, 000, 000, 000, 000. 00) in two months. That is forty millions of billions of dollars. Such a sum can be envisioned as a stack of thousand dollar bills reaching from the Earth to the Moon. If the United States of America continued to operate at its current level of economic activity for eight thousand years, the dollar value of all the goods and services produced in that vast period of time would barely equal this amount.
Something similar happens in an uncontrolled chain reaction. When one atom fissions, a certain amount of energy is released, along with (let us say) two neutrons. If those extra neutrons cause two more atoms to fission, then twice as much energy is released along with four more neutrons. The new neutrons may cause another four atoms to fission, adding four times as much energy and eight more neutrons. And so the total energy grows, doubling with each new generation of neutrons. The result is a stupendous release of energy.
Neutrons move so fast that, unless they are slowed down, sixty generations of neutrons -- sixty doublings -- can occur in less than a millionth of a second. So in a chain reaction, the energy from each fissioning atom is magnified by a factor of forty quadrillions in less than a microsecond. Such an enormous amplification, happening so quickly, accounts for the awesome power of the atomic bombs that devastated the cities of Hiroshima and Nagasaki in August, 1945.
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C.5. Neutron Efficiency
In the spring of 1939 the Paris group laboured to achieve the world's first self-sustaining chain reaction. They knew that, in principle, enough extra neutrons are given off to trigger a chain reaction in a core of natural uranium. They also knew that a very high degree of efficiency is needed to ensure that those neutrons aren't wasted.
If too many neutrons escape from the uranium core without causing additional fissions, the chain reaction has no chance of getting started. This problem can be offset by increasing the size of the uranium core. Francis Perrin defined a "critical mass" of uranium to be the smallest amount needed to sustain a chain reaction. [40]
Similarly, if too many neutrons are absorbed by chemical impurities, they become unavailable for the fission process and the reaction fizzles out. Any substance that absorbs neutrons is called a "neutron poison"; too much poisoning will prevent a chain reaction.
But neutron poisons are essential for regulating a chain reaction. Without some neutron-absorbing materials, the chain reaction would quickly get out of hand. Such materials are needed to limit, control and (if desired) shut off the chain reaction. They can be inserted or withdrawn at will from the core, using long-handled rods for instance.
The Paris group developed all of these important concepts by May 1939. They are fundamental to the design of every nuclear reactor.
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C.6. The Need for a Moderator
The Paris group also recognized that slow neutrons are better than fast ones for causing fission in natural uranium. Some years earlier, in Rome, Fermi had put a block of paraffin between a neutron source and a uranium target (see A.20) The parrafin slowed down the neutrons or "moderated" them, thereby increasing the yield of radioactive materials (fission products) and, by inference, the number of fissions.
In fact, it is impossible to get a chain reaction using natural uranium unless the neutrons are moderated. Natural uranium is a blend of two main isotopes. The heavier variety, uranium-238 (representing 99.3 percent of the total mass) will usually capture one or more neutrons without fissioning, while the lighter variety, uranium-235 (only 0.7 percent of the total) always fissions following a neutron capture. By slowing the neutrons down, one can minimize the number of neutrons captured by uranium-238, thereby maximizing the fission of uranium-235. Unless the neutrons are moderated, the poisoning effect of uranium-238 will prevail, stopping the reaction.
To achieve a chain reaction using natural uranium, one must make the greatest possible use of slow neutrons. Not only the incoming neutrons, but all subsequent neutrons released by fission have to be slowed down. The uranium core must be immersed in a "moderator": a material that speeds up the reaction by slowing down the neutrons.
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C.7. The Search for a Moderator
The Paris group first tried surrounding the uranium with paraffin as a moderator, but they discovered that parrafin absorbs too many neutrons to sustain a chain reaction. Next they tried immersing the uranium in a pool of water. Water was a better moderator, but still not good enough, having a slight but significant poisoning effect.
Each water molecule contains two hydrogen atoms whose nuclei are protons -- exactly the same size as neutrons. The Paris researchers anticipated that fast neutrons would ricochet off these protons like billiard balls on a pool table, losing momentum with each impact (see A.13). They were right; the neutrons did slow down, but many of them were also captured by protons. The chain reaction then came to a halt due to an insufficient number of bombarding neutrons.
When a proton captures a neutron, the newly-formed pair becomes the nucleus of a hydrogen atom twice as heavy as before. This non-radioactive isotope of hydrogen, first discovered in 1932 by the American Harold Urey, is called "heavy hydrogen" or "deuterium" (see A.14). Deuterium occurs in small concentrations in ordinary water, from which it can be separated only with great effort.
The Paris group conjectured that since a deuterium atom already has an extra neutron compared with a hydrogen atom, it is probably less inclined to capture any further neutrons. If so, then neutrons can more safely ricochet off deuterium atoms, losing momentum without being absorbed to the same degree. To test this hypothesis, one might replace all the hydrogen atoms in the water molecules with deuterium atoms instead. In other words, one might try "heavy water" (water made from heavy hydrogen: see A.15) as a moderator.
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C.8. Heavy Water from Norway
Early in 1940 the Paris group asked the French Minister of Armaments to obtain as much heavy water as possible. But heavy water is hard to come by. It must be manufactured using a tedious process requiring a lot of energy. The only place it was then being made was at a large hydroelectric station in Norway, using off-peak power. [41]
Already the Germans had offered to purchase the entire stock of Norwegian heavy water -- a frightening indication that they too might be pursuing research on an atomic bomb. Once the Norwegians learned of the possible military significance of heavy water, they entrusted it all to a French Secret Service man who smuggled it into France via England just before Germany invaded Norway in April 1940. A bomb-proof shelter was built near Paris to house the precious liquid.
There were only 200 kilos -- not enough for a chain reaction, but enough to determine if a chain reaction was feasible. Experiments were begun, then abruptly interrupted when the Germans launched a lightning attack through Belgium into France one month later. The entire inventory of heavy water was taken to England. Two and a half years later it would be moved to a secret laboratory in Montréal.
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C.9. Britain and the Bomb
In April, 1940, the British had set up a top-secret Committee of Experts to investigate the feasibility of an atomic bomb. Unlike the French, they were concentrating on fast (unmoderated) neutrons.
Originally the British thought that a chain reaction with fast neutrons was impossible because of the poisoning effect of U-238. They also figured that a chain reaction using slow neutrons would be too slow-starting to allow for a truly devastating explosion. Besides, the moderator would make such a "bomb" incredibly bulky. Throughout 1939, therefore, the British did not believe an atomic bomb was feasible.
PP Then in March 1940, Otto Frisch and Fritz Peierls -- two refugee German scientists living in England -- hit upon a startling new idea. On three typewritten pages, they outlined how an atomic bomb could be built and detonated using only a few kilograms of uranium-235, the lighter isotope of uranium, fissioned wholly by fast neutrons.
Frisch and Peierls observed that if U-235 were completely separated from U-238, there would be no need to slow the neutrons down. No moderator would be required. A chain reaction could be achieved with fast neutrons. An atomic bomb could then be made from a quantity of pure U-235 about the size of a grapefruit.
In the same paper they warned that no man-made structure could withstand the force of the resulting explosion; that no defence would be possible; that the intensely radioactive residues (fission products) would remain deadly to all living things for many years after the explosion; and that the Nazis were probably developing such a bomb.
British scientists were asked to verify that the Frisch/Peierls concept would work. In the summer of 1940 several research teams swung into action at four British universities. Amid all this excitement, the heavy water team, newly arrived from France, was cordially invited to continue its slow neutron research at Cambridge; but the project was given a low priority since it was not expected to produce a bomb.
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C.10. Uranium Enrichment
The most formidable task facing the would-be bomb-makers was that of "enriching" uranium -- that is, increasing the concentration of uranium-235 by separating out the unwanted uranium-238. Since there are no chemical differences between the two, the separation would have to be based on the slight mass difference between them.
Using uranium metal supplied by McGill University, British chemists produced a highly corrosive compound called "uranium hexafluoride" or "hex", which turns into a gas when heated. The British showed that as this gas diffuses through a very fine membrane, the lighter atoms pass through somewhat more easily than the heavier ones. The result is a very slight increase in the concentration of U-235. By repeating the procedure tens of thousands of times, any desired degree of enrichment can be obtained. This repetitive, energy-intensive process is known as "gaseous diffusion".
In principle, gaseous diffusion seems relatively simple; in practice it is extremely difficult. As Frisch commented, "It was like getting a doctor who had, after great labour, made a minute quantity of a new drug, and saying to him: 'Now we want enough to pave the streets.'" To complicate matters, any direct contact with the hexafluoride gas would quickly destroy metals, most lubricants, and rubber. [42]
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C.11. Nuclear Boilers
In August, 1940, a British delegation was sent to North America to explore the possibility of locating British military nuclear facilities across the Atlantic ocean, out of reach of the German Luftwaffe. Among others, the delegation visited Enrico Fermi, then at Columbia University. He was pursuing slow neutron research rather similar to that of the Paris group, but using very pure graphite as a moderator. They told him of the Frisch/Peierls concept for an atomic bomb.
Like most scientists in the U.S. at that time, Fermi was skeptical of the prospects for an atomic bomb. He was more interested in the possibility that a nuclear-powered boiler might eventually be used as an industrial energy source. In such a device, the energy from a carefully controlled chain reaction would be used to boil large quantities of water. The resulting steam could be used for a variety of purposes, including the generation of electricity.
Fermi told the delegation about some preliminary U.S. research on uranium enrichment -- not for bombs, but for boilers. In a boiler, a chain reaction using slow neutrons would be easier to achieve if the uranium fuel were slightly enriched. For example, if the concentration of uranium-235 were only slightly increased from 0.7 percent to 3 or 4 percent, then ordinary water could be used as a moderator. [43] This degree of enrichment would require much less effort than the 90 to 99 percent level of enrichment needed by the bomb designers.
In Ottawa, the British delegation met the Canadian George Laurence, a one-time student of Rutherford's. They learned that Laurence had secretly and single-handedly built his own slow neutron experiment in a room on Sussex Drive, using graphite as a moderator. Laurence, who was then on the staff of the National Research Council, had in fact anticipated Fermi's work at Columbia by several months.
Although Laurence never achieved a self-sustaining chain reaction (his graphite had too many impurities absorbing too many neutrons), his work placed Canada in the front ranks of experimentation on nuclear fission. This led to a regular exchange of secret information between British, American, and Canadian nuclear scientists.
The British felt that the slow neutron studies at Columbia, Ottawa, and Cambridge were irrelevant to the war effort. But nuclear boilers might have post-war industrial uses and Britain shouldn't be left out. Back home again, the British arranged a small donation in support of the Canadian fission experiments. They also urged the Americans to start producing heavy water, as the Cambridge research team had already shown that it was superior to graphite as a moderator.
Eventually, the Americans would build four heavy water plants. The first was in Trail, British Columbia. [44]
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C.12. The Plutonium Connection
Near the end of 1940, two British researchers named Feather and Bretscher stumbled on an unexpected link between nuclear boilers and atomic bombs. When a uranium-238 atom captures a neutron, it changes into uranium-239, which then transmutes by beta emission into neptunium-239, and thence, by another beta emission, into plutonium-239 (see A.23). On theoretical grounds, the two scientists predicted that plutonium-239, like uranium-235, must be fissile: that is, capable of sustaining a chain reaction with fast or slow neutrons. If so, then plutonium-239 can be used as a nuclear explosive.
The two men predicted that significant amounts of plutonium-239 would be produced inside any nuclear reactor fuelled with natural (or slighly enriched) uranium, as many stray neutrons are bound to be captured by uranium-238 atoms in the fuel. Since plutonium is chemically different from uranium and the fission products, it can be separated from the spent nuclear fuel by purely chemical means, without any need for the laborious process of isotope separation.
At the time, little importance was attached to these predictions. They could not be verified. No self-sustaining chain reaction had yet been achieved, so no inventory of plutonium was available for testing. But early in 1941, tiny traces of plutonium were meticulously prepared in California using the world's first cyclotron. In March, the American Glen Seaborg demonstrated that plutonium-239 fissions even more readily than uranium-235 does -- and with fast neutrons as well as slow ones. Thus plutonium-239 can, in principle, be used either as fuel for a nuclear boiler or as an explosive in an atomic bomb.
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C.13. The Atomic Bomb Project
In October, 1941, the Americans received a copy of the top-secret British report fully confirming the feasibility of the Frisch/Peierls scheme for making an atomic bomb. [45] The report urged that such bombs be produced as a matter or the highest priority. Churchill gave the project his stamp of approval. "Although personally I am quite content with the existing explosives," he wrote to his military advisers, "I feel we must not stand in the way of improvement."
Almost overnight the Americans changed their minds about the feasibility of an atomic bomb. They offered to work closely with the British, but Britain hesitated because of America's refusal to enter the war. They hesitated too long. At the end of November, just a week before Japan's bombing of Pearl Harbour, President Roosevelt committed the U.S. to an all-out unilateral effort to build atomic bombs.
The Americans decided to pursue every available technological option, since it was unknown which would work best. Uranium enrichment would be attempted, using not only the gaseous diffusion method pioneered by the British, but also centrifugal and cyclotronic methods. [46] In addition, uranium fuelled reactors (or boilers) would be built to produce plutonium for possible use as a nuclear explosive.
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C.14. From Radium to Uranium
Whichever option proved best, a lot of uranium would be needed. Since radium is a decay product of uranium, any ore rich in one is also rich in the other. But the Czechoslovakian radium deposits were in German hands. There was a rich Belgian-owned radium mine in the Congo, but Belgium was Nazi-occupied. A private Canadian company, Eldorado Mining and Refining, had the world's next richest radium mine at Great Bear Lake in the Northwest Territories (NWT).
Gilbert Labine founded Eldorado as an Ontario gold mining company in 1926 in partnership with his brother. He discovered radium during a prospecting trip to the NWT in May, 1930. At that time radium was selling at $70,000 per gram (down from $100,000 a few years earlier). In 1931, work on the mine began. The following year Marcel Pochon, a radium chemist from France, was brought in to design and build a chemical refinery at Port Hope, Ontario, using a process developed by the Canadian Department of Mines.
For the rest of the decade, ore from the North was shipped to Port Hope, where radium was extracted and sold on the world market. But by 1938, radium prices had dropped to $7,000 per gram and Eldorado was in trouble. Then came the war, closing off markets. In 1940, with large unsold inventories at Port Hope, Great Bear Lake, and Waterways, Alberta, Eldorado decided to shut down its radium mine.
Thousands of tons of residues, still containing uranium, were heaped up by the Port Hope docks, stored in farm silos, and dumped in the harbour. There was no real market for uranium. German potters used to use it to make a reddish glaze, but during the war there were few clients: local art schools (for color), Columbia University (for Fermi's work), and the National Research Council (for Laurence's work).
The atomic bomb project changed all that. In May 1941, the U.S. placed an order for eight tons of uranium. Later that year, the British ordered two tons. Early in 1942, after the bomb project had been officially (but secretly) launched, the U.S. ordered sixty tons of refined uranium oxide. That was enough to justify re-opening the mine and re-starting the refinery. Port Hope had the only plant in North America equipped to refine uranium.
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C.15. Ottawa's Involvement in Uranium
Since fuel, aviation and manpower were controlled during wartime, Gilbert Labine went to ask C. D. Howe, Canadian Minister of Munitions and Supply, for permission to re-open the Eldorado mine. When Howe learned that the Americans were developing some new kind of explosive, he gave Labine (whom he knew and trusted) the go-ahead. In secrecy, as required by both governments, Labine gathered the men and materials to produce uranium for the first atomic bombs.
That was in March, 1942. Then on June 15, Prime Minister Mackenzie King received a delegation from Britain. The British asked King to consider establishing international control over Eldorado's operations because of the strategic importance of uranium. They told him that uranium could be used to produce a bomb so powerful that any country possessing it would win the war. King was impressed. He entrusted the matter to C. D. Howe and C. J. (Jack) Mackenzie, President of the National Research Council (N.R.C.).
Mackenzie was familiar with Laurence's fission research, so he was able to corroborate the British claims and support their request. Howe agreed. He offered to enlist the help of Gilbert Labine to buy shares in Eldorado on behalf of the government -- quietly, to avoid publicity. Once Ottawa had a controlling interest, ownership would be split three ways, with Canada a junior partner to the U.S. and U.K.
Eldorado became a federal crown corporation in September 1943, and was eventually renamed Eldorado Nuclear Limited. The concept of international ownership was never implemented, however.
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C.16. The Montreal Laboratory
By 1942, the Americans were placing an increasing emphasis on plutonium as a nuclear explosive. Fermi was in Chicago building a large graphite-moderated uranium "pile" to produce plutonium. [47[ Seaborg and others from California gravitated to Chicago to study the complex chemical properties of this new element.
Slow neutron research, which Britain had put on a back burner, had acquired great military significance. It suddenly seemed urgent to move the heavy water team to Chicago, where all the experts on plutonium were. The Americans had now become very security-conscious, however. They were determined to keep the Germans and Soviets completely in the dark, so they were unwilling to confide atomic secrets to any non-British foreigners. (Only one of the six senior men in the heavy water group was British). They also feared that the U.K. might profit commercially after the war using U.S. technology.
On August 17, Malcolm Macdonald, the British High Commissioner in Ottawa, paid a visit to Jack Mackenzie. He proposed moving the heavy water group from Cambridge to Canada, creating a European-Canadian nuclear research team in the process. Mackenzie jumped at the chance. He saw it as a golden opportunity for Canadians to get in on the ground floor of a new technology, working closely with some of the finest minds in the world on the very cutting edge of science.
The British would pay the salaries of those they were sending over; the Canadians would pay for everything else. In scientific matters, van Halban would run the show. Administratively, Mackenzie would be in charge, acting for the N.R.C. Policy would be set by C.D. Howe and the British High Commissioner. It was decided to house the heavy water team in Montreal, where secrecy, lab space and living quarters were all easier to secure. A portion of the medical wing of the newly built Université de Montréal, on the slopes of Mount Royal, was secretly leased and refurbished to suit the purpose.
At first, U.S. authorities welcomed this joint venture. They requested the Montreal team to stay in close contact with the Chicago team. The Canadians, who had just learned about the heavy water plant in Trail B.C., managed to persuade the U.S. to earmark the first year's output (six tons of heavy water) for Montreal, as a necessary supplement to the Norwegian inventory which was soon to be shipped from Britain.
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C.17. Strained Relations
The Cambridge team began arriving in late November and early December, before the Montreal labs were ready. On December 2, Fermi's team achieved the world's first self-sustaining nuclear reaction in a squash court under the west stands of the University of Chicago football stadium. This news made the Montreal team eager to get to work to demonstrate the superiority of heavy water over graphite. But in January, a letter arrived from Washington stating the U.S. had decided to embark on its own intensive effort to produce plutonium using heavy water. The work was being contracted to the DuPont Company. Thus no heavy water would be given to the Montreal team unless they agreed to work under the direction of DuPont. Moreover, there would be no further sharing of sensitive technical information.
This blunt message reflected the fact that the U.S. Army had taken over the atomic bomb project in the person of General Leslie Groves. In conclusion, the letter asked for the cooperation of the Montreal group "in what is, after all, a joint aim -- namely the production of a weapon to be used against our common enemy in the shortest possible time under conditions of maximum security."
Seeing no alternative, the Canadians recommended compliance; but the British flatly refused. As a result of this deadlock, the Montreal heavy water project was set back by more than a year.
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C.18. Early Research at Montreal
The Montreal team carried out preliminary work early in 1943. Comparative neutron measurements were made using heavy water from Cambridge and graphite from Ontario, but there wasn't enough of either to obtain a chain reaction. Designs were drawn up for a nuclear boiler using a heavy water moderator, but they couldn't be tested. In February, the French chemist Goldschmidt brought back a sample of fissioned uranium from Chicago. From this sample, the Montreal team managed to separate three micrograms of plutonium. They began experimenting with various chemical methods for extracting plutonium.
In May, the British were exasperated to learn that Canada's entire uranium mining and refining capacity had been tied up by the U.S. Twelve hundred tons of high grade uranium ore from the Belgian Congo, found in a warehouse on Staten Island in the fall of 1942, had been sent to Port Hope for refining. In addition, the U.S. had placed orders for 800 tons of Canadian uranium -- about three years output. Apparently the Montreal team was to be deprived not only of heavy water from B.C., but also of uranium from Port Hope and the NWT.
Mackenzie tried to mollify the British by telling them how much the Americans were actually doing. Three new cities were being built to house the atomic bomb project. At Oak Ridge, Tennessee, enormous uranium enrichment facilities were under construction. At Hanford, Washington, a series of graphite moderated boilers were being built. At Los Alamos, New Mexico, bomb design and testing was taking place. Additional work was being done in Chicago and in California.
But by the summer of 1943, work in Montreal had almost halted. Morale was low. The Canadians were tempted to cancel the project.
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C.19. The Quebec Agreement
When General Groves visited Ottawa in July 1943 to discuss uranium supplies, Mackenzie told him the Montreal project deserved support because the heavy water route was the best way to get plutonium for bombs. Groves concurred. Meanwhile, in London, based on frank exchanges between American and British officials, Churchill drew up a set of proposals for nuclear cooperation to be handed to Roosevelt. Roosevelt and Churchill signed the Quebec Agreement on August 19 at Quebec City. Using guarded language, they committed the U.S. and U.K. to share resources "to bring the project to fruition at the earliest moment". They promised that "we will never use this agency against each other", that "we will not use it against third parties except by mutual consent" and that "we will not either of us communicate any information ... to third parties except by mutual consent."
Although Canada was not a signatory to the Quebec Agreement, Howe was given a seat on the six-person Combined Policy Committee set up to oversee its implementation. The C.P.C. first met at the Pentagon on September 10. By December, it had decided that British scientists could fully participate in the gaseous diffusion enrichment work at Oak Ridge, the cyclotron enrichment efforts in California, and weapons design work at Los Alamos. Nothing was said about the Montreal Lab.
The C.P.C. next drew up a Declaration creating a Combined Development Trust to control and allocate the world supplies of radioactive ores. Not being a party to the Quebec Agreement, Canada did not sign the Declaration either -- which nevertheless stipulated that one of the six trustees would be a Canadian. Still nothing about Montreal.
Unknown to the Canadians, the plant at Trail, B.C., had already started producing heavy water in June 1943. Montreal had not received an ounce of it. The Americans were secretly withholding several tons for their own heavy water reactor being built at Argonne, near Chicago. Behind the scenes, the Chicago group opposed giving the heavy water project to the Montreal group. They wanted it for themselves.
Realizing that the Montreal project would be more palatable to the Americans if it were run by the British rather than by van Halban, the Canadians arranged for Sir John Cockcroft (who had once been Rutherford's research assistant) to take over. They also introduced to the C.P.C. James Chadwick, the esteemed British scientist who had discovered the neutron. Chadwick worked hard to ensure an endorsement of the project from the C.P.C. by answering every objection, refuting every argument, and overcoming every obstacle.
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C.20. Canada's First Nuclear Reactor
Finally, on April 13, 1943, in Washington D.C., the Committee decided that a large-scale pilot plant for plutonium production would be built in Canada using heavy water as a moderator. Materials would be provided by the U.S., but no information would be imparted about the chemistry or biomedical hazards of fission products or plutonium. The Montreal team would have to discover all such information for itself using a few irradiated uranium rods donated from U.S. reactors.
The first necessity was to find a firm to design and build the pilot plant in Canada. The natural choice was Defence Industries Limited (DIL), a crown company involved in munitions manufacture, whose key staff was drawn from CIL. On May 18 the DIL Directors were briefed on the nature of a nuclear chain reaction and the basic requirements for a heavy water moderated reactor.
They were told the reactor could not explode like an atomic bomb because it would use only slow neutrons. It was nevertheless clear that an uncontrolled chain reaction would result in a violent release of energy, possibly scattering fission products over a wide area. On several occasions before the first atomic bomb was tested, it had been suggested that great damage might be done to an enemy by dropping fission products from the air, thereby contaminating food and water supplies and making strategic areas uninhabitable. Any similar contamination occurring by accident, it was pointed out, would have a deplorable effect on the future public relations of CIL.
Mackenzie retorted that CIL would surely not escape blame if DIL refused the contract and some less capable firm did the job. That could be seen as evasion of reponsibility. On May 26, DIL accepted the contract, and the search for a site began. For reasons both of safety and security, it would have to be isolated.
The shores of Georgian Bay, Lake Superior and the La Tuque region of Quebec were considered. Finally, in mid-July, a secluded spot on the Ottawa River was selected. It was two hundred miles northwest of the capital, near the small village of Chalk River. A townsite for employees was chosen at Indian Point a few miles away; the community is now known as "Deep River". Deep River was situated far enough upwind and upriver of the Chalk River research reactors to avoid radioactive fallout.
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C.21. The Chalk River Nuclear Complex
Meanwhile, the Montreal group had determined the basic features of the Canadian pilot plant, to be called NRX for "National Research Xperiment". The fuel would consist of 175 rods of uranium metal, each with a thin coating of aluminum. The coating, or "cladding", would protect the metallic fuel against chemical reactions and prevent the escape of fission products. The fuel rods would be suspended in an aluminum tank full of heavy water, surrounded by graphite to "reflect" escaping neutrons back into the tank. Since NRX would generate more than ten million watts of heat, the fuel would have to be cooled to prevent melting. Reliable shutdown systems would be required to halt the chain reaction abruptly if it began to get out of control. Biological shielding would be needed to protect workers from neutrons during operation. In addition, the gamma radiation from fission products would be so intense that massive shielding would be required to safeguard workers at all times, even when the reactor was shut down. These safety concerns greatly complicated the design of the plant. The NRX fuel rods were to be housed in double-walled tubes through which ordinary "light" water would be pumped at very high speed to cool the fuel. The tubes had to be thin, so as not to absorb too many neutrons, but strong enough to prevent a loss of coolant. The heavy water moderator would fill all the remaining space in the tank outside these vertical tubes. Between the tubes would be hundreds of adjustable "control rods" made of neutron-absorbing materials. When inserted vertically into the tank, these rods would soak up excess neutrons, slowing down or stopping the reaction.
Structural materials near the core of the reactor would inevitably absorb stray neutrons. Due to the resulting activation (see A.21), all of the internal structures, including the cooling tubes and control rods, would become intensely radioactive. Thus, maintenance could be performed only by remote control or after a lengthy shutdown. Given these complications, it was considered prudent to build a much smaller and simpler reactor -- using the same fuel, moderator and reflector as NRX, but not powerful enough to need cooling and not radioactive enough to prevent workers from approaching it. It was to be called ZEEP for "Zero Energy Experimental Pile". [48] In late July, Cockcroft sent for Lew Kowarski to take charge of the ZEEP project. A member of the original Paris group, Kowarski brought with him some other members of the Cambridge heavy water team who had chosen to stay in England because of personal difficulties with van Halban. The Canadian government had offered to pay the whole shot, but the final cost estimate was something of a shock. It included NRX, ZEEP, two chemical extraction plants, a huge water purification plant and a maze of labs and offices. It also included an entire planned community at Indian Point (the village of Deep River) complete with hospital, school, shopping centre, recreational hall and administration building. Ottawa gave its approval for the Chalk River complex on August 19, 1944, five days before the liberation of Paris.
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C.22. Advanced Reactor Concepts
There were now about 100 scientists in the Montreal group -- over forty Canadians, an equal number of British, and twelve others -- including five French citizens. During the fall of 1944 and the spring of 1945, while detailed design work for NRX and ZEEP was underway, these scientists also found time for other types of advanced research.
The Americans had suggested that in designing the NRX reactor, provision should be made for thorium rods to be inserted in the graphite reflector. By then it was known that thorium-232 changes into fissile uranium-233 by neutron capture, just as uranium-238 changes into fissile plutonium-239. A new man-made isotope of uranium, U-233 could be used as a nuclear explosive and so was worth investigating.
The British were beginning to plan for the post-war period. Having no heavy water in England, they elected graphite as their moderator of choice. A "graphite group" was formed at Montreal in December, 1944, and by the end of the war all the basic design work had been done for Britain's first experimental reactor at Harwell, called BEPO. The graphite for Britain's first few reactors came from Ontario.
Early in 1945 a "future systems group" was formed at Montreal to brainstorm on other possible reactor designs. Special materials able to resist corrosion, conduct heat and tolerate radiation were sought out. A variety of liquids and gases were investigated for possible use as coolants. In the end, this group anticipated every major conceptual development in reactor design for the next quarter century.
In particular, they perceived that economic deposits of uranium are likely to be rather scarce. Accordingly, if nuclear boilers were to last for more than a few decades as an energy source, they saw a need to "breed" a man-made substitute for uranium-235; either plutonium-239 bred from uranium-238, or uranium-233 bred from thorium-232.
They were thus led to conceive a futuristic type of nuclear reactor, fuelled by highly concentrated ("enriched") fissile material, fissioned by fast neutrons rather than slow ones, and surrounded by a blanket of uranium-238 or thorium-232. In principle, more fissile material can be bred in the blanket -- by neutron capture -- than is consumed in the fuel. This greatly extends the supply of nuclear fuel. Such advanced reactors are called "fast breeder reactors", and the stuff in the blanket is appropriately called "fertile material". Experimental breeders have since been built in the U.S., the U.S.S.R. and France.
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C.23. Reprocessing Spent Nuclear Fuel
In July, 1944, the U.S. delivered to Canada a few irradiated rods of uranium (containing plutonium), and of thorium (containing U-233). The Montreal chemists knew little about the U.S. method for seperating plutonium except that it was based on precipitation. The Americans would first dissolve the spent fuel in nitric acid, then chemically convert the dissolved plutonium into a solid which would slowly settle out, leaving fission products and uranium in solution.
Precipitation has one big disadvantage: it can only be done in batches. It is a stop-and-start operation. The Montreal team sought a process that would run continuously, producing a steady stream of plutonium. Over two hundred solvents were studied to find one that would strip plutonium away from the other radioactive materials, creating two liquid fractions which, like oil and water, do not mix. The plutonium-bearing fraction could then be separated mechanically and continuously, and from it the plutonium itself could be extracted at will. Similar concepts applied to the separation of U-233.
The British and French both gained a distinct post-war advantage over the U.S. in reprocessing technology -- the technique of recovering fissile material from spent nuclear fuel -- as a result of their Montreal experiences. That advantage persists to the present day.
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C.24. Biomedical Effects of Fission Products
Because of intense penetrating radiation from fission products, the irradiated fuel rods could not be handled directly. All work upon them had to be done behind a shield of thick concrete. When irradiated fuel rods are dissolved in acid, a fraction of the fission products are inevitably released into the air. Spills of radioactive liquids can also occur, as well as the spread of radioactive dust. Equipment and clothing can become contaminated. The Montreal Health Section developed instruments called "dosimeters" to record individual levels of gamma and beta exposure. They also began studying the biological effects of atomic radiation -- not only from external exposure, but also from ingested or inhaled radionuclides.
The task was a formidable one, involving as it did over a hundred new radionuclides not existing in nature, each having its own unique physical and chemical properties. For example, strontium-90 is chemically related to calcium -- like radium -- and is therefore a bone-seeker. However, it is a beta emitter whereas radium is an alpha emitter, and very little was then known about the biomedical hazards of beta emitters. Cesium-137, another beta emitter, is chemically similar to potassium, and is therefore stored in the muscle and the meat. Iodine-131, like non-radioactive iodine, concentrates in the thyroid gland, whereas the radioactive "noble gases" [49] such as krypton-85 and xenon-133 are not stored in bodily organs, yet they do dissolve in bodily fluids. There was much to learn and many other wartime priorities were clamouring for attention.
The Montreal team could not match the resources that the Americans brought to bear on this problem. In the U.S., hundreds of animal studies were under way to determine the biological characteristics of these new hazardous materials. In the vast majority of cases, it was found that the main dose from ingested fission products was delivered to the bone. Other organs at risk included the liver, kidney, spleen, lung, and gonads, depending on the particular radionuclide. One of the major U.S. findings was that, for "whole body" radiation, beta rays and gamma rays are about equally effective, per unit dose, in causing bodily harm.
- The phrase "critical configuration" would be more accurate than "critical mass", since the ability of a given mass of uranium to sustain a chain reaction depends on the presence or absence of other materials (such as a moderator) as well as the precise geometry of the fuel/moderator ensemble .
- During the war, this Norwegian heavy water plant was bombed several times by allied planes and sabotaged by commandos to prevent the Nazis from acquiring their own supply of heavy water.
- Considerable effort was made to find another, less troublesome uranium compound which is also a gas at reasonably low temperatures, but to no avail.
- All American "light water reactors" are in fact moderated by ordinary water, with the uranium fuel enriched to about 3 percent uranium-235. Those neutrons that are absorbed by water molecules are compensated for by the increased rate of fission caused by the higher concentration of uranium-235.
- They bought and coverted the old Zinc Electrolytic plant for this purpose.
- It was called the M.A.U.D. Report, and was written by the M.A.U.D. Committee. The name was inspired by a cryptic telegram from atomic scientist Neils Bohr in Nazi-occupied Norway referring to "Maud Ray", which British intelligence took to be a code name for "Radium". Not until 1943 did they learn that Maud Ray was the name of an English governess who had once minded Bohr's children.
- Centrifuges are devices for separating out the heavier constitutents in a mixture using the "artifically enhanced gravity" created by spinning the mixture at high velocity. Cyclotrons, by contrast, use electromagnetic fields to bend the path of accelerated ions into a circular shape. As the radius of the circular path depends on the mass of the ion travelling along it, physical separation can be achieved. Heavier ions bend more slowly, so -- making a 180 degree turn -- they end up further away from the original path than the lighter ions do. By July 1941, in California, the "father of the cyclotron" Ernest Lawrence had separated microgram quantities of plutonium-239 from neutron-irradiated uranium samples in this way.
- The Chicago nuclear pile was a large geometrically-formed pile of blocks alternating between graphite and uranium, with room for long-handled "control rods" made of neutron poisons needed to regulate the chain reaction.
- Nuclear reactors were called "piles" for many years, because the first reactor was in fact a pile of graphite and uranium blocks. See note 47.
- Noble gases do not undergo chemical reactions and are therefore considered chemically and biologically inert.
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