“THE ITALIAN NAVIGATOR HAS LANDED IN THE NEW WORLD …”
On 2 December 1942, in Chicago, Fermi built the first nuclear battery (CP-I). With this device, for the first time in history, it was possible to produce a controlled nuclear-fission chain reaction, using natural uranium as fuel and pure graphite as a moderator to slow down the neutrons. This is a decisive step for the exploitation of nuclear energy.
On 2 December 1942, Enrico Fermi, at the decommissioned Stagg Field sports facility of the University of Chicago, managed to produce a self-sustained and controlled nuclear-fission chain reaction, using natural uranium as fuel and pure graphite as a moderator to slow down the neutrons.
This is a decisive step for the exploitation of nuclear energy.
Forty-nine people were present that day. A Chianti fiasco, signed by those present, was opened to celebrate the event.
The phenomenon of nuclear fission was discovered with radio-chemical methods in December 1938 by Otto Hahn and Fritz Strassmann in Berlin, while Enrico Fermi was traveling with his family to the United States after receiving the 1938 Nobel Prize for Physics in Stockholm. By virtue of this phenomenon, nuclei of uranium, subjected to the bombardment of slow neutrons, can undergo a split into two lighter fragments. Otto Hahn received the 1944 Nobel Prize in Chemistry (awarded in 1945) for this discovery.
Subsequent research, carried out in the major laboratories all over the world, which also see Fermi involved in America, help to clarify the modalities of the phenomenon. In fission, a large amount of energy is released, which can be estimated at about 200 MeV per nucleus, to be compared with a few eVs per atom during chemical reactions. The energies involved in nuclear fission are therefore hundreds of millions of times larger than chemical ones.
The isotope of uranium that most easily undergoes fission by slow neutrons is 235 (U-235, 235 U, with mass number A = 235), present only in the percentage of 0.7% in natural uranium. Some fast neutrons are also released in the course of fission.
Research on the possibility of practical exploitation of the enormous energy produced by nuclear fission began in 1939.
Nuclear fission plays a marginal role among spontaneous natural phenomena. Any neutrons present in cosmic rays can certainly induce fission phenomena, for example, on the nuclei 235 of natural uranium. However, the additional neutrons produced are rapidly absorbed by the surrounding nuclei without having the possibility of producing appreciably new fissions, also because the neutrons are so fast.
It is therefore a question of building suitable devices, called “reactors”, in which the additional neutrons produced can be suitably slowed down to produce additional fissions in a self-sustained exponential chain reaction, which must then be suitably controlled. For this, large quantities of natural uranium need to be available, and also a so-called “moderating” substance that has the ability to slow down neutrons without absorbing too many.
The problem of the self-sustained and controlled nuclear-fission chain reaction is a formidable problem, from a scientific, industrial, and technological point of view. The neutrons produced must be slowed down, so as to be able to induce other fissions, without being lost in excessive quantities. The loss of neutrons occurs by absorption without fission on the uranium nuclei, or on the nuclei of the moderator, or by escape to the outside of the reactor. All these phenomena must be kept under control.
It is interesting to note, however, that in the distant past the fraction of U-235 then present was much higher than it is today, due to the difference in the average life of the two main isotopes, U-238 and U-235, which amounts to about 4.5 billion of years for the former and about 700 million years for the latter. This made it possible to trigger a spontaneous chain reaction, moderated by natural water.
In particular, there is evidence that a natural fission reactor operated spontaneously about two billion years ago in the Oklo uranium field in Gabon, West Africa. The discovery was made by the French Commissariat for Atomic Energy (CEA) in 1972.
The evidence for the existence of a spontaneous chain reaction in the Oklo field is based on two facts. First of all, there is an impoverishment of the ratio between the two isotopes of uranium, U-235 and U-238, compared to the current values in terrestrial samples. Furthermore there is a peculiar distribution of the isotopes of some elements, called “rare earths”, strong neutron absorbers. These rare earths, which constitute a particular group of metallic elements, heavier than Iron, with similar chemical properties, would have been produced and “burned” by the large flux of neutrons generated in the natural chain reaction.
It is estimated that the energy produced in the Oklo natural null reactor was about 100 billion kWh, equivalent to the global production of about 3,000 years of all industrial reactors currently operating around the world.
The moderating substance, properly mixed with uranium, must slow down the fast neutrons produced, without absorbing too many. It must therefore contain sufficiently light nuclei, so that the neutrons can lose energy in the course of elastic collisions.
Among the possible moderators, heavy water and carbon in the form of graphite acquire practical importance.
Of course, the most effective moderator for the slowing down would be hydrogen, which however absorbs too many incident neutrons to form deuterium. Heavy hydrogen, deuterium, whose nucleus is made up of a bound state of a neutron and a proton, has instead a small “cross section”, i.e., a small probability of neutron absorption. Heavy hydrogen is produced in the form of heavy water D2O. There will therefore also be oxygen nuclei, which however absorb few neutrons.
What are the prospects for these two main neutron moderators?
Heavy D2O water is the most effective moderator. Deuterium (with mass number A = 2) is light, so it slows down the neutrons well and absorbs them little. Oxygen does not cause much trouble because it also absorbs neutrons very little.
There are some notable drawbacks though.
Heavy water (D2O) exists in nature in water in very small percentages
Natural H2O has only about one part deuterium in 7,000. Complex and expensive separation processes are required to attain an appreciable percentage of heavy water. The most effective method is the electrolytic one. During the electrolysis of water, light hydrogen develops to a higher percentage, so the water that remains in the electrolytic cell is progressively enriched with heavy water.
The first industrial plant for the commercial production of heavy water was that of the Norsk Hydro company in Norway built in 1934, with a production capacity of about 12 tons per year. In his experiments in Rome on the behavior of slow neutrons in 1934–1938, Fermi also obtained his supply from this company.
During WWII, Germany occupied Norway, taking over the plant and reinforcing it. In addition to sabotage operations by the Norwegian resistance, the Allies also subjected it to intense bombing to prevent the heavy water from being used in the German nuclear project. The Germans even tried in 1943 to use Italian electrolytic plants in South Tyrol for the production of heavy water.
The German nuclear project, of which Werner Heisenberg was the leader from a scientific point of view, turned decisively towards heavy water, without success.
Fermi’s choice, on the other hand, was graphite.
A carbon core is a less efficient moderator than one of deuterium since carbon is heavier (it has mass number A = 12). Furthermore, the neutron-absorption cross section is higher. It should also be noted that industrial graphite, the most common crystalline structure containing carbon, has impurities that produce considerable absorptions for neutrons.
The graphite problem is therefore a typical industrial problem. It is necessary to ensure production of graphite with a very high degree of purity, a task to which American industry was perfectly capable in the 1940s.
Fermi is strongly oriented towards graphite.
Heisenberg, on the other hand, rejected graphite, certainly influenced by the difficulty of obtaining graphite at the necessary degree of purity by German industry, much tested by intense bombing, and oriented to the most immediate priorities of the war effort. It is also not clear whether Heisenberg knew the correct neutron-absorption cross section of the carbon core. Walther Bothe’s important experiments on absorption, carried out in Germany in 1941, were only published in 1944, when the problem had already lost its relevance. The values published in 1944 still seem to overestimate the absorption on carbon, compared to the real one.
The arrangement of the uranium is also of great importance.
Fermi’s choice is to use powdered uranium oxide, obtained at a high degree of purity, arranged in cubes immersed in graphite blocks. This arrangement, separating between the uranium and the moderator, is necessary. In fact, the fast neutrons produced in fission immediately leave the uranium and are slowed down in the moderator. Consequently, when they then re-enter the uranium block, they are sufficiently slowed down so as to produce new fissions and are almost not absorbed by the uranium in processes without fission.
Instead, in a homogeneous uranium–moderator arrangement, the neutrons produced, still fast, would be excessively absorbed by the uranium in processes without fission.
The calculation of the optimal size of the cubes and their distance is a theoretical problem of great importance, and it was brilliantly addressed by Fermi, thanks to his experience on the diffusion, absorption, and slowing of neutrons, gained in the experiments carried out in Rome in 1934–38..
Instead, Heisenberg’s choice was to use metallic uranium in plates, immersed in heavy water. This choice ensures great rigidity in the structure. However, metallic uranium requires very difficult industrial production, requiring very high-temperature furnaces. Furthermore, the plate arrangement, while making the theoretical calculations simpler, is nevertheless less efficient than the cubed one. Only in the final experiment, carried out in the spring of 1945, did Heisenberg decide to use the cubes arrangement. But the cubes must be cut from the available slabs, and these are not of optimal size. Furthermore, the amount of uranium and heavy water is not sufficient. The device multiplies the neutrons produced, but does not achieve the self-sustaining reaction.
Given the times, characterized by a bitter worldwide conflict, the first applications are of a military nature.
In fact, the American nuclear program had started after a letter from Albert Einstein to Franklin D. Roosevelt of 2 August 1939, where the president was informed of the concrete possibility of building very high-power explosives following the developments in nuclear research, and the consequent danger that Germany could be the first to achieve this goal.
The nuclear reactor is above all a very powerful source of neutrons. So it was then immediately used to produce plutonium, a recently discovered and highly fissile transuranic element. Bars of natural uranium, consisting mostly of U-238, are inserted into a functioning reactor. Uranium 238, irradiated by neutrons and activated, undergoes two beta decays in the chain, perfectly analogous to those hypothesized by Fermi in 1934 for the production of Experiment (Z = 94), transforming into plutonium. The real esperio is the reality plutonium.
The Fermi atomic pile was reproduced in various specimens of large-scale power reactors at the Hanford site, in Washington State. The Hanford site, active until 1989, spanned nearly 600 m2, with tens of thousands of people permanently employed.
The plutonium produced in the reactors is then extracted from the rods by chemical methods, still kept secret in their essential details, in large industrial plants.
Plutonium, which has an average life of about 40 years for alpha decay (an average life long enough for applications, even if very short on a geological scale, so much so that plutonium does not exist in nature), is a highly fissile element in which, once a critical mass is reached, an explosive fission chain reaction develops.
The first experimental bomb in history (the Trinity test) exploited these properties of plutonium. It was detonated on Monday July 16, 1945 in the Alamogordo Experimental Center, a few hundred km from the headquarters of the Manhattan Project, located in Los Alamos, New Mexico. The estimated power of the bomb is 20,000 equivalent tons of TNT (20 kilotons), equivalent to the load of about 2,000 B-29 bombers (the Flying Superfortresses of WWII). Fermi himself provided a detailed report on the explosion, which he had witnessed from a distance of a few km.
In parallel with the production of plutonium, massive programs for the production of
enriched uranium were also pursued.
The arrangement chosen by Fermi is of maximum efficiency. Uranium oxide cubes and graphite bricks can be progressively arranged in a very compact and stable structure, namely, the pile, until the critical dimensions are reached and the divergent chain reaction is triggered, in which the number of processes of fission gradually increases. The reaction is controlled by inserting cadmium cylinders, which is a highly absorbent substance for slow neutrons.
On 2 December 1942, at 15.25 pm, the triggering of the divergent reaction was observed, completely under control. The experiment was carried out by Fermi at the University of Chicago. Arthur Compton communicated the success of the venture through a phone call to James Conant, President of Harvard University, and who will later be a key figure in the Manhattan Project. No secret communication code had been agreed, so it was necessary to improvise. Compton told Conant: “The Italian navigator has landed in the New World.” “What were the natives like?” Conant asked. “Very friendly” was the reply.
This result was of the utmost importance, both scientific and practical. The reasons for success consisted of an effective confluence of various factors. First, Fermi’s full understanding of the physical phenomena involved. And then the possibility of using the extraordinary potential of American industry to produce the necessary amount of uranium oxide and graphite of great purity.
The Fermi CP-1 pile (Chicago Pile number 1) was just a prototype. Immediately a series of large reactors were then built for application purposes.
There is a famous graph of the intensity of neutrons that shows the beginning of the divergent exponential reaction. This graphic, reproduced here, has remained secret for a long time, due to the sensitive information contained therein.
Of course, for safety reasons, it was forbidden to take photographs of the CP-1 stack. However, numerous drawings were made, some of which are reproduced here.
After the successful explosion of the Alamogordo experimental bomb (Trinity test, 16 July 1945), the American political and administrative authorities made the critical decision to use the nuclear weapon against Japan. The Nagasaki bomb on 9 August was made of plutonium (Fat Man, 25 kilotons). A few days earlier, on August 6, the city of Hiroshima had been destroyed with a bomb (Little Boy, 20 kilotons) based on enriched uranium (isotope 235), for which no experimentation was necessary. The use of the two nuclear bombs on Japan put an end to WWII.
Obviously, in the following years, the main industrially advanced nations began the production of plutonium and equipped themselves with increasingly powerful nuclear weapons. The balance of terror maintained a state of Cold War, but of effective peace in the many following decades.
The first Soviet reactor, graphite like the American one, reached criticality on 25 December 1946, four years after the Fermi reactor. The plutonium produced was used in their first nuclear test, code named First Lightning Joe-1 (22 kilotons), carried out on 29 August 1949, at the Semipalatinsk experimental site in Kazakhstan.
The UK detonated the first atomic bomb (Hurricane, 25 kilotons) on 3 October1952, at the Monte Bello islands along the northwest coast of Australia. The plutonium came partly from the Harwell and Risley plants in the UK and partly from Canada.
The first French reactor (ZOE), using uranium and heavy water, was activated on 15 December 1948, through the decisive action of Frédéric Joliot. After a series of complex events, during which, among other things, Joliot was removed from his management responsibilities in the nuclear sector, France carried out its first nuclear test (Gerboise Bleue, with plutonium) on 13 February 1960 in Algeria, in the Sahara desert. The power was greater than 60 kilotons, very high for an experimental prototype.
It is not surprising to note that Enrico Fermi also received a Medal of Military Valor, with a resolution of the US Congress in January 1946.
The nuclear reactor is also a powerful neutron generator that can be used for research on the properties of neutrons in their interaction with nuclei.
Fermi was also involved in these research studies. In particular, these explored the reflection of neutrons on mirrors, the transmission of slow neutrons through microcrystalline materials, the phenomena of interference for slow neutrons, and the interaction between neutrons and electrons.
It is interesting to note that some of these experiments were closely linked with similar research done in Rome in the period 1934–1938. Only now is it possible to use the reactors as neutron sources, enormously more powerful than the available radon-beryllium ones, and which Fermi had used to win the Nobel Prize.
The nuclear reactor is also a powerful source of neutrinos coming from the beta decay of fission products. In modern terms, they are electronic antineutrinos.
The reactor was then used for the first neutrino detection. In 1956, Clyde L. Cowan and Frederick Reines were able to detect reactor antineutrinos. Cowan died in 1974. Reines received the 1995 Nobel Prize in Physics for his work on the neutrino.
The detection method is based on the reaction
The intense flow of antineutrinos from the beta decay of the reactor fission fragments impacted on a detector containing many target protons in a liquid-hydrogen scintillator.
A positron and a neutron are produced. These products are detected in the form of a pair of delayed signals. The first signal is given by the slowing down and consequent annihilation of the positron on an electron in a pair of photons. The second pulse, delayed compared to the first, is produced by the slowing down of the neutron in the hydrogen with consequent absorption in the cadmium dissolved in the scintillator, with the emission of a gamma ray.
It is interesting to remember that the first proposal for the detection of antineutrinos intended to use a nuclear explosion as a source, with consequent important problems of isolation of the detector apparatus, both against the radiation produced and against the mechanical shock wave. Later, however, Cowan and Reines were convinced to use a reactor as a source of neutrinos, less efficient, but safer.
The nuclear reactor, due to the high energy released in fission, is also a potential powerful source of heat. After the end of the war, there was also a shift towards civil applications.
In a lucid report dated 27 May 1946, entitled “The Future of Atomic Energy”, presented to the US Atomic Energy Commission, Enrico Fermi addressed the challenges of using atomic energy for peaceful purposes. These ranged from the production of electricity, using heat exchangers to operate turbines, to the production of artificial radioactive substances to be used for therapeutic medical purposes, similar to the products of natural radio, or as tracers to obtain information on chemical and metabolic reactions.
The first nuclear power plant for the production of electricity was that in Obnisk in the USSR, which, suitably upgraded, remained active for almost 48 years, from 1954 to 2002. In 1956, it was followed by the Calder Hall power plant, near Sellafield in the UK. On the other hand, the first American nuclear power plant for the production of electricity on an industrial scale was in Shippingport, whose reactor entered criticality on 2 December 1957.
Similarly, nuclear-powered engines can be built for the propulsion of submarines and ships.
The Nautilus, the first nuclear-powered submarine, was launched on 21 January 1954 in Groton, Connecticut, in the US. Among its feats was reaching the North Pole, under the ice of the pack ice, which took place on 3 August 1958.
In 1960, the Soviet Navy also boasted nuclear-powered submarines, armed with lethal ballistic missiles. However, in 1957, the Soviet Union built the Lenin, the first nuclear-powered icebreaker, but also the first nuclear-powered ship intended only for civilian use.
Nuclear-powered merchant ships have been impractical due to their very high costs.