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p. 192. A new technology is developedlocked

  • Maxwell Irvine

Abstract

‘A new technology is developed’ explains the process of nuclear fission and charts the development of nuclear weapons during the Second World War. In August 1939 a letter to President Roosevelt drafted by Leo Szilard and signed by Albert Einstein stressed the danger that Germany might develop a weapon of enormous destructive capacity. By September 1941, the USA had been drawn into the war, and in 1942 the Allies' nuclear research was coordinated under the directorship of the American physicist Robert Oppenheimer. The resulting weapons destroyed the Japanese city of Hiroshima and Nagasaki and effectively brought the Second World War to an end.

The first technology to be developed from the discoveries of nuclear physics was driven by the exigencies of the Second World War. For most non-scientists, including politicians, the laboratory experiments of Fermi, Curie and Joliot, and Chadwick appeared remote from the pressing issues of the approaching war. Amongst the scientific community, however, there was a growing speculation about the possibility of the explosive release of nuclear energy. Fermi's Columbia experiments had confirmed the concept of a chain reaction. Fears grew that Germany might seek to develop an atomic bomb.

Albert Einstein was one of very many European physicists who had fled the Continent ahead of the growing threat of National Socialism and the persecution of the Jews. Until this time, frontiers of scientific development had been concentrated in Europe and the Jewish community had played a leading role in cultural and scientific endeavours, especially in Germany. Of all the scientists in the world, Einstein had by far the highest popular profile. The European scientists brought with them to the USA the latest ideas about the new science of nuclear physics.

As concerns grew that Germany might develop a nuclear weapons programme, it was agreed that Einstein, although not directly involved in the evolving nuclear science, was the person most likely p. 20to be listened to by the politicians. In August 1939, a letter to President Franklin D. Roosevelt was drafted by Leo Szilard and signed by Einstein. The letter stressed the dangers that Germany might be able to develop a weapon of enormous destructive capacity. However, the letter was not delivered until October. In September, Hitler invaded Poland and the Second World War had begun. Despite the fact that the USA was not immediately involved in the war, the US government created the Uranium Committee which awarded a research contract with funding of $6,000. However, such was the fear of foreign scientists doing secret research that the money was not released to the Fermi–Szilard collaboration until Einstein was persuaded to send a second letter to the president in the spring of 1940.

With the new funding, Fermi was able to build the first atomic pile to reach criticality. The primitive reactor was built on the squash courts of the University of Chicago and went critical in December 1942. The Chicago Pile 1 was an essential development in the understanding of the fission process and the technical difficulties that would have to be overcome if nuclear technology was to deliver its potential.

The concept behind the atomic pile (the term ‘nuclear reactor’ was not coined until 1952) was simple: if neutron bombardment of uranium could induce fission with the release of a number of neutrons, these secondary neutrons could be used to induce further fissioning and a chain reaction could be established (Figure 11).

The problems were that the total available supply of uranium to the USA in the 1940s was limited and even this was of doubtful purity. This was a problem because the neutrons had to hit another uranium nucleus before they escaped from the reactor core. Thus a critical mass of uranium was required. Second, the most efficient fission process was to strike uranium-235 with slow-moving neutrons with a kinetic energy of a few keV (see Figure 12). However, the secondary neutrons were emitted with much higher p. 21

11. The neutron-induced fission chain reaction. A neutron strikes a uranium nucleus causing fission into two fragments with the release of several neutrons that can induce further fission

energies of a few MeV, and these were ineffective at causing fission. A material that moderates the velocity of the neutrons had to be found.

The induced fission yields are greater for uranium-235 at all neutron energies but at low energies the difference becomes dramatic, with the yield from uranium-238 becoming vanishingly small while it increases sharply for uranium-235 for neutron energies in the keV region.

When two particles collide, they transfer energy and momentum between themselves. In the case of a billiard balls, the cue ball strikes a stationary target ball which takes energy and momentum from it and slows it down. If the target is much lighter than the projectile, the projectile sweeps it aside with little loss of energy and momentum. If the target is much heavier than the projectile, the projectile simply bounces off the target with little loss of energy. The maximum transfer of energy occurs when the target and the projectile have the same mass.

In trying to slow down the neutrons, we need to pass them through a moderator containing scattering centres of a similar p. 22

12. The probability of fission induced by neutron bombardment of uranium isotopes as a function of neutron energy

mass. The obvious candidate is hydrogen, in which the single proton of the nucleus is the particle closest in mass to the neutron. At first glance, it would appear that water, with its low cost and high hydrogen content, would be the ideal moderator. There is a problem, however. Slow neutrons can combine with protons to form an isotope of hydrogen, deuterium. This removes neutrons from the chain reaction. To overcome this, the uranium fuel has to be enriched by increasing the proportion of uranium-235; this is expensive and technically difficult. An alternative is to use heavy water, that is, water in which the hydrogen is replaced by deuterium. It is not quite as effective as a moderator but it does not absorb neutrons. Heavy water is more expensive and its production more technically demanding than natural water. Finally, graphite (carbon) has a mass of 12 and hence is less efficient requiring a larger reactor core, but it is inexpensive and easily available.

Another problem was that naturally occurring uranium is 99.3% uranium-238 and only 0.7% is in the fissile form uranium-235; p. 23being chemically identical, no chemical process could separate them.

The first of the problems solved by the Fermi Pile was that of slowing down the secondary emitted neutrons from their initial MeV energies to keV energies and thus increasing the fissionability. At the same time, the Pile was made large enough that there was a high probability that the neutrons would collide with a fissile nucleus before escaping from the Pile. Finally, a system was introduced to control the rate of nuclear reactions.

The initial sources of neutrons and the fuel for the Pile were pellets of natural uranium; these were separated by blocks of graphite which slowed down the neutrons as they passed through them. The assembly was roughly spherical and supported by a timber frame. The rate of the nuclear reaction was controlled by cadmium-coated rods (cadmium is a potent neutron absorber). Inserting the rods absorbed the neutrons and slowed down the fission rate. Removing the rods increased the number of neutrons until a self-sustaining nuclear chain reaction rate was achieved.

In Figure 13, a neutron strikes a uranium-235 nucleus and induces fission and the release of three neutrons. One of the neutrons escapes the core. Another is absorbed by a uranium-238 collision. The third collides with a uranium-235 nucleus, inducing fission and the release of two neutrons, both of which induce further fissioning, after being slowed by the graphite, in uranium-235 nuclei and the release of further neutrons. FFs are fission fragments.

Initially, there was little interest in the scientist's concerns in the USA. However, in the UK, two expatriot physicists, Otto Frisch and Rudolf Peierls, carried out a feasibility study of the possibility of fast fission of uranium-235 in 1940. This included an estimate of the amount of uranium required to create a critical mass and hence to make a bomb. The fast neutrons that were emitted during the fission process were most likely to strike the more abundant p. 24

13. A schematic of a nuclear chain reaction initiated by a slow neutron collision with an isotope of uranium-235

uranium-238 isotopes. This did not produce fission; however, it did result in the transmutation of uranium-238 into the first man-made element, plutonium-239, through the reaction

n + uranium-238 -〉 uranium-239 -β‐〉 plutonium-239.

Plutonium-239 has a half-life of 80 million years and does not exist in nature but lives more than long enough for any practical utilization. Plutonium had recently been identified by a team led by Glenn Seaborg at the University of California. By July 1941, the UK programme had shown that plutonium-239 was a much more potent fissile material than uranium-235.

p. 25Winston Churchill was so impressed by the Frisch–Peierls report that in September 1941 he authorized a programme to develop an atomic bomb. By September 1941, the USA had been drawn into the war, and in 1942 the Allies’ nuclear research was coordinated with the title of the ‘Manhattan Project’, under the directorship of the American physicist Robert Oppenheimer. For security reasons, all work was now transferred to the USA. Also, it was quickly evident that under the impoverishment of war the UK could not match the resources available in the USA.

The work was divided over several sites in North America; Oak Ridge, Tennessee, was chosen as the facility to develop techniques for uranium enrichment (increasing the relative abundance of uranium-235), the Hanford site in Washington State became the centre for plutonium production, a number of Canadian sites produced heavy water, and the weapon design construction and testing were located at Los Alamos, New Mexico.

Since no chemical process could distinguish between uranium-235 and uranium-238, it required a physical process that depended on the mass difference, only 1.3%, to enrich uranium. At Oak Ridge, a giant gaseous diffusion facility was developed. Gaseous uranium hexafluoride was forced through a semi permeable membrane. The lighter isotopes passed through faster and at each pass through the membrane the uranium hexafluoride became more and more enriched. The technology is very energy consuming and Oak Ridge was chosen because the Tennessee Valley Authority had been established by President Roosevelt as a make-work programme during the Great Depression and this had created a gigantic hydroelectricity capacity. At its peak, Oak Ridge consumed more electricity than New York and Washington DC combined. Almost one-third of all enriched uranium is still produced by this now obsolete technology. The bulk of enriched uranium today is produced in high-speed centrifuges which require much less energy. The centrifuge facilities are composed of a large number of rotating cylinders. The gaseous uranium p. 26hexafluoride in the cylinders is subject to large centrifugal forces that throw the heavier isotopes to the outside leaving enriched uranium to be collected at the centres.

At Oak Ridge, Fermi was able to build a much larger version of his graphite pile and demonstrated that it was capable of producing plutonium. Following this demonstration, the plutonium production programme was transferred to Hanford. At Hanford, a giant version of the Fermi Pile was constructed specifically to produce plutonium. The reactor was fed enriched uranium from Oak Ridge and during its operation the fast neutrons transmuted some of the uranium-238 into plutonium-239. When the partially spent nuclear fuel was removed from the pile, the plutonium could be chemically separated from the uranium. Throughout the war years, the Manhattan Project followed the parallel approaches to producing weapon-grade material, uranium enriched to 〉98% uranium-235 and pure plutonium.

While the graphite-moderated Fermi Pile had demonstrated the possibility of a sustained nuclear chain reaction, graphite (carbon) was not the most efficient neutron moderator. Water, with its high hydrogen content, was a better moderator but this had the disadvantage that the hydrogen could capture the neutrons to form the heavy isotope deuterium. This had the effect of reducing the neutron flux necessary to maintain the chain reaction. There were two solutions to this problem. The loss of neutron flux could be compensated for by further enrichment of the uranium, or the water could be replaced with heavy water, i.e. water in which the normal hydrogen is replaced by deuterium. Given the limited capacity to enrich the uranium at the time, interest shifted to the use of heavy water. It was suspected that this was the approach being taken in Germany, and indeed this was the case. A contributory factor to the German failure to produce a nuclear weapon was their reliance on the extremely scarce resource of heavy water aggravated by the combined action of the UK air force and Norwegian resistance which destroyed a major heavy water p. 27production plant in occupied Norway. The French had removed the store of heavy water before Norway was occupied, and when the Germans sought to take the remaining heavy water to Germany after the destruction of the plant, the Norwegian resistance sank the ship carrying it. With hindsight, there was never any real chance that Germany could have produced an atomic bomb before the end of the war. Its technical options were limited, and it never had the resources – financial, manpower, material, and industrial capacity – to devote to a nuclear weapons programme on the scale of the Manhattan Project.

Natural hydrogen contains deuterium at the level of 1 part in 3,200. The difference in mass between the two isotopes means that at a given temperature molecules containing the isotopes are travelling at different velocities. In addition, chemical reactions involving molecules containing the different isotopes proceed at different rates. Canada took responsibility for producing heavy water as its contribution to the Manhattan Project and established a plant at Trail, British Columbia, in 1943, yielding six tonnes of 99% pure heavy water per year.

The Canadians used a two-step process: first, hydrogen sulphide gas and demineralized and deaerated water were circulated between a high-temperature (1300C) tower and a cold-water (300C) tower. The gas and the water exchange hydrogen isotopes at a differential rate and a cascade process yields water with 15–20% heavy water content. The slightly enriched water is then fed into a cold distillation vessel consisting of a chamber in which the pressure is reduced to the vapour pressure of the water. This encourages evaporation, with the lighter molecules being given off preferentially. Repetition of the process can result in 99% pure heavy water.

In order to sustain a nuclear chain reaction, it is essential to have a critical mass of fissile material. This mass depends upon the fissile fuel being used and the topology of the structure containing p. 28it. For pure uranium-235, the critical mass of a sphere is 52kg with a radius 17cm. For pure plutonium, the corresponding figures are 10kg and 9.9cm. The chain reaction is maintained by the neutrons and many of these leave the surface without contributing to the reaction chain. Surrounding the fissile material with a blanket of neutron reflecting material, such as beryllium metal, will keep the neutrons in play and reduce the critical mass. Partially enriched uranium will have an increased critical mass and natural uranium (0.7% uranium-235) will not go critical at any mass without a moderator to increase the number of slow neutrons which are the dominant fission triggers. The critical mass can also be decreased by compressing the fissile material. An unmoderated supercritical mass of fissile material will build its spontaneous chain reaction into an explosion. In building the early atomic piles with relatively low enrichments of uranium and a relatively poor moderator (graphite), the structures at Hanford were enormous. This is not a great problem for a static ground level structure, but clearly it would be impossible to build a bomb of this size, nor, as was later to be the case, build a nuclear marine engine to power submarines and ice-breakers.

The Manhattan Project reached its final goal by developing two techniques for overcoming the size problem. First, using uranium from Oak Ridge enriched to greater than 99% uranium-235, a supercritical mass was created from two subcritical masses. One piece was shaped into a cylinder forming a ‘target’. The other was shaped into a hollow cylinder ‘bullet’ into which the target would fit neatly to produce the supercritical mass. The bullet and the target were kept apart until a trigger of conventional explosive fired the bullet at the target and resulted in an atomic (sic nuclear) bomb. This weapon known as ‘Little Boy’ (Figure 14) destroyed the Japanese city of Hiroshima. The second system was used with nearly pure plutonium-239 from the Hanford facility. In this case, a sphere of plutonium encased a neutron source. The whole was surrounded by a blanket of conventional explosive which, when ignited, compressed the plutonium core with a spherical p. 29

14. A schematic of the operation of the Little Boy bomb

shockwave (Figure 15). Fat Boy as this devise was codenamed destroyed the city of Nagasaki and effectively brought the Second World War to an end.

With the war at an end, thoughts turned to the peaceful development of nuclear power. Unfortunately, the Cold War saw a continuation of the nuclear weapons race, with the USA and the USSR as the principal protagonists.

The end of the Second World War saw a schism develop between the Manhattan Project scientists. Many had joined the war effort specifically out of fear of Hitler's regime. With victory in Europe achieved, they were only too happy to return to their laboratories across the USA and Canada or to return home to the UK and Continental Europe. Many took up the cause of nuclear disarmament. Some, particularly those from Eastern Europe, saw in Stalin's USSR a threat just as great as that posed by Hitler. Prominent amongst these scientists was Edward Teller, who strongly advocated pressing on to develop a superbomb based not on the uranium/plutonium fission chain reactions but on the fusion of isotopes of hydrogen to form helium-4.

The principle of the hydrogen bomb was similar in some ways to the Nagasaki bomb. The plutonium core was replaced by a mixture p. 30

15. A schematic of the operation of the Fat Boy bomb. A conventional high explosive generates a spherical shockwave that compresses a core of pure plutonium-239 around a central neutron source

of the hydrogen isotopes deuterium and tritium seeded with lithium. To trigger the fusion chain reaction, the core had to be heated to temperatures and pressures greater than those at the centre of the Sun. Conventional explosives could not achieve this, but a plutonium bomb could.

Without the technical advances made during the Manhattan Project, in four short years, it is unlikely that a civil nuclear plant would have been constructed in the 20th century. Nor would the prospects of a fusion reactor, now predicted for the middle of the 21st century, appear likely before the 22nd.