Nuclear Fission energy for war and peace
In the fission process, the fragments and neutrons move away at high speed carrying with them large amounts of kinetic energy. The neutrons released during the fission process are called fast neutrons because of their high speed. Neutrons and fission fragments fly apart instantaneously in a fission process. No delayed liberation of neutrons was ever observed. Gamma rays (photons) equivalent to 8 MeV of energy are released within a microsecond of fission. As mentioned earlier, the two fragments are beta emitters. Recall that beta decays are accompanied by antineutrino emissions, and the two types of particles carry away approximately equal amounts of energy. Beta decays often leave the nuclei at excited states, and gamma emission follows. Estimated average values of various energies are given in a table here.
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The Cyclotron and Fission Research
The machine built by Cockroft and Walton accelerated protons, which smashed 7Li nuclei. Any machine that speeds up the velocity of particles are called particle accelerators. Particles from accelerators induced many nuclear reactions, and the value of accelerators in the study of nuclear reactions was soon realized.
Various types of particle accelerators have been built, using electric potentials or electromagnetic forces. Linear particle accelerators made particles moving faster along a straight line; whereas cyclotrons accelerated them as they travel along circular path. The cyclotrons built by Ernest O. Lawrence in Berkeley, California belong to the latter type, and they have given useful results.
A cyclotron has two hollow D-shaped (Dee) sections assembled together with a small space in between. A magnetic field deflects the particles into spiral motion. By applying alternated voltages between the Dees, the cyclotron accelerates charged particles to desirable energies. By changing the strength of the magnetic field, particles of various energies are made available. The first such cyclotron has a diameter of only 13 cm, and it accelerated protons to a maximum energy of 13 keV. Cyclotrons built later with larger diameter accelerated particles to energies between 10 and 100 MeV.
Accelerated particles are used to induce nuclear reactions as discussed in the last Chapter. Reactions between accelerated charged particles from cyclotrons and light nuclides produced neutrons of variable energy. The following are some of the reactions:
Reactions between tritium (3T or t) and deuterium (2D or d) are particularly important, because these are fusion reactions. Furthermore, they release neutrons of various energies.
Cyclotron induced nuclear reactions provide neutrons of controlled energy for the study of fission. For example, some reactions of protons with medium-weight nuclides are listed below together with their threshold energy and neutron energy range.
* The threshold energy is the minimum energy of proton required for the reaction.
Energy from the neutron source 27Al (, 1n) 30P mentioned earlier can not be varied. Neutron sources from the cyclotron have an advantage over neutron sources induced by natural radiation, because neutron energy can be varied. This enables the study of energy dependence of neutron induced fission reactions. The variation of cross sections for neutron-induced fission as a function of neutron energy is a vital piece of information for nuclear reactor design. The study showed that fast neutrons (energy ranges from 10 MeV to 10 KeV) are not effective to induce fission, but slow neutrons (0.03 to 0.001 eV) are very effective. Slow neutrons are also called thermal neutrons, because their energy corresponds to room temperatures.
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The Synthesis of Plutonium
The intention to synthesize transuranium elements by neutron bombardment of uranium split 235U nuclei.
Experiments to produce elements 93 and 94 by the (n, ) reaction are sound, but so much fission products were produced that they impaired the detection of transuranium elements. The cyclotron, however, provided high intensity neutrons of definite energy, and it gave a chance for success.
The cyclotron provided neutrons for E. M. McMillan (1907-) and P. H. Abelson (1913-), to bombard uranium. In the summer of 1940, they confirmed one product as element 93, and named it neptunium, Np, after the planet Neptune. They inferred that Np would decay by emitting a particle converting itself into element 94, named plutonium after the planet Pluto. These reactions are summarized bellow:
238U + n 239U +
239U (half life 23.5 m) 239Np +
239Np (half life 2.35 d) 239Pu +
or in short notations:
238U (n, ) 239U ( , ) 239Np ( , ) 239Pu
238U (n, 2) 239Pu
Actually, the neutrons with high kinetic energies are used to produce transuranium elements. The fission theory by Bohr and Wheeler suggested that 239Pu would undergo fission. Thus, the cyclotron in Berkeley was put to work to produce enough plutonium for experiments. By mid-1941, the fission characteristic of plutonium was well established.
Plutonium was first detected (1940) by Glenn T. Seaborg, Joseph W. Kennedy, and Arthur C. Wahl, from the reaction 238U (d, b) 240Np using deuterium from the 60-inch cyclotron at Berkeley, California. The most important isotope is 239Pu, because it has a long half-life (24,400 y) and it is a fission fuel. This isotope can easily be produced using a breeder reactor, which shall be described later in this Chapter.
Plutonium has a very high electrical resistivity and a density of 19.84 g cm-3. It is chemically reactive, dissolving in acids and forming various ions of characteristic colour in water: Pu3+, blue-lavender; Pu4+, yellow-brown; and PuO+2, pink. Many compounds of plutonium have been prepared, often starting from the dioxide, PuO2), the first compound of any synthetic element to be separated in pure form and in weighable amounts (1942). Isotope 244Pu gives a melting point of 912 K, boiling point 3,508 K. The concept of critical mass will be introduced later, and you may be interested in knowing that the critical mass of 239Pu is only 300 grams.
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Uniting Political and Nuclear Powers
Neutron induced fission reactions release energy and neutrons. The amount of energy in fission reactions is alarmingly large and the liberation of neutrons in the fission process gives the possibility of an explosive chain reaction, which releases a tremendous amount of energy. At that time, the scientists already foresaw the danger of nuclear power, especially if the technology falls in the wrong hand.
Since securing the presidency of Germany in 1933, Hitler became a dictator, and many top scientists in Austria, Hungary, Italy and Germany felt uncomfortable. Scientists with ethnic backgrounds other than German felt threatened. Many European scientists had escaped the Hitler regime and come to the United States. At the time fission was discovered, Hitler invaded Poland, Hungary, Slovak and other European countries. Many scientists were concerned that Hitler would make use of fission to build bombs. Such a move is a threat to the entire world.
Among the concerned people were three Hungarian refugee scientists Leo Szilard, Eugene Wigner, and Edward Teller, who thought the time has come to unite political force with nuclear power. They thought Hitler had the potential and possibility of developing atomic bombs. This matter should be brought to the immediate attention of the President of the United States, Roosevelt. To achieve this, they needed someone with a reputation. They convinced Albert Einstein that such an action was a necessity. Szilar, Wigner and Teller composed a letter for Einstein, and Einstein signed the letter* as the sender. They took the matter so serious that they convinced the economist Alexander Sachs to personally deliver the letter to the White House. (Use Einstein and letter and Roosevelt as keys to search the Internet will get many sites containing this letter)
Refugee scientists from Hungary, Germany and Italy (L. Fermi, 1955) have worked under a totalitarian political system, in which totalitarian leaders controlled everything including universities and researchers. The governments knew whatever went on in university laboratories. They were fearful of Germany being the first to develop the atomic bomb. Thus, they took the initiative to bring the issue to the president of the United States. Most American scientists at that time were usually unfamiliar with this type of political control, and they felt less threatened.
The likelihood that Germany might develop an atomic bomb caused President Roosevelt to act and he decided immediately to create an Advisory Committee on Uranium that would give financial assistance to universities engaged in uranium research. The sum of $300,000 (remember these are 1940’s dollars) was immediately allocated to Columbia, Princeton, MIT, Chicago, California, Virginia etc. Research on uranium and fission is complicated, and information on many aspects of uranium and of fission is required. Each group worked on one or more aspects of fission, and nuclear research was intensely carried out by many young and old scientists.
The Chicago group worked on uranium, and the California group worked on plutonium. Soon they realized that enriched 235U or pure 239Pu would be required for the construction of an atomic bomb. For building an atomic bomb, production of enough fissionable material is the most important task. However, other information such as identification of fission products, accurate cross section for neutrons as functions of energies, moderating neutron motion, and percentages of neutrons that induce fission are all required.
Fermi's group irradiated uranium samples with neutrons. They surrounded the samples with different materials at various times and found samples surrounded by water, wood, and paraffin more radioactive.
After Fermi's group has learned of nuclear fission, they attributed the fission radioactivity increase to the moderation (slowing down) of neutrons by hydrogen and light elements in water, wood, and paraffin. They thought that neutrons are slowed quickly by collision with protons, because the two particles have comparable mass. Neutrons can transfer almost all their kinetic energy to proton in a collision.
Materials used to slow down fast neutrons are called moderators. On the average, 20 collisions with protons are sufficient for neutrons to reach an equilibrium state that further collisions will no longer change their average kinetic energy. The average kinetic energy of neutrons depends on the temperature of the medium, in which they are in thermal equilibrium with. These neutrons are called thermal neutrons. In the following paragraphs, we further describe how energy of the molecules depends on the temperature.
Molecules in a medium are constantly in motion: vibration, rotation, and translation. The average kinetic energy of molecules is directly proportional to the temperature in K. At room temperature (293 K) the average kinetic energy of all molecules is 0.025 eV. Of course, some molecules have higher and some have lower kinetic energies than 0.025 eV. In fact, the kinetic energies of the molecules have a Maxwellian distribution. This skewed distribution is depicted here, and it is different from the normal or bell shaped distribution. As the temperature changes, the skewed distribution shifts slightly to give a higher average kinetic energy.
The neutrons collide with molecules and atoms in the medium constantly, and their energies have the same distribution as those of the molecules.
Neutrons are often classified as fast, thermal, and cold neutrons according to their kinetic energies. Fast neutrons have a kinetic energy exceeding some threshold, typically 0.1 or MeV. Neutrons just released from the fission reactions are fast neutrons. After some collisions with atoms in the medium, they become thermal neutrons and their typical average kinetic energy is 0.025 eV. The average kinetic energy of cold neutrons is less than 0.01 eV. Slightly different boundaries of division may be given in other literature due to differences in view points or definition of room temperatures, but these are typical values. Cold neutrons are either from super cold hydrogen moderated experimental reactors or selected by diffraction from crystals. There are some special applications for this type of neutrons.
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Thermal Neutron Cross Sections
Cross section () is a measure of the probability of a given reaction, as we have discussed elsewhere. Cross sections are further classified according to types and reactions. Since thermal neutrons are readily available thermal neutron cross sections (c), are important nuclear data. They are usually given for each nuclide to indicate its probability of thermal neutron capture. For possible fission material, the thermal neutron cross section for fission (f) is also given.
Thermal neutrons are much better than fast neutrons at inducing nuclear fission. Thus, thermal neutron cross sections for all nuclides have been studied, because many materials are required for fission device (bombs and reactors) constructions.
Around 1940, the Uranium Research Program measured thermal neutron cross sections for various reactions of almost all nuclides. In the following list, thermal neutron (capture) cross sections () and thermal neutron fission cross sections (f) are given for some key nuclides. Half-lives (t1/2) of the radioactive nuclides are also given, because they are important properties of the nuclides regarding fission device.
1H 2H 12C 14N 16O 113Cd 233U 235U 238U
c /b 0.33 0.00052 0.0034 1.82 0.0002 19,820 46 98 2.7
f /b 530 580 2.7×10-6
The extremely large thermal neutron cross section of 113Cd makes cadmium a good neutron absorber or eliminator. The element cadmium contains many isotopes. The abundance (in %) and thermal neutron cross sections (b) are listed below:
106Cd 108Cd 110Cd 111Cd 112Cd 113Cd 114Cd
c / b 1 1 0.1 24 2.2 19,820 0.3
Abundance /% 1.25 0.89 12.45 12.80 24.13 12.22 28.37
The thermal neutron cross section of fission of 235U is 160,000 times larger that that of 238U. Fission of 238U is negligible. This difference made it necessary to enrich 235U for nuclear energy and atomic bomb material. Research in the 1940s revealed another important fissionable isotope of plutonium 239Pu. Even though other isotopes of plutonium had higher cross sections than 239Pu, their half-lives are very short. The half lives and thermal fission cross sections of plutonium isotopes are given below for your reference:
236Pu 237Pu 238Pu 239Pu 240Pu 241Pu 242Pu
f 150 2100 17 742 0.08 1010 0.2
t1/2 2.9 y 45 d 88 y 24131 y 6570 y 14 y 3.8×105y
Other factors in nuclear energy considerations were methods and costs of production. All these factors led to the conclusion that only the production of 235U and 239Pu are feasible and practical. Production of 233U was not worth considering.
Fission products are nuclides produced in fission reactions. As suggested earlier, rubidium and cesium as two of the possible fission products. Finding out fission products is certainly a strategic project the fission research.
After capturing neutrons, the compound nuclei 236U are at excited states with excess energy. The 236U nuclei undergo fission; or emission. The half-life for fission is much shorter (10 14 s) than those of and emissions (half-life for decay is 2.3107 y). Fission is the preferred process.
Since many nuclides are produced in the fission process, the study of fission products requires the separation, identification, and quantitative determination of various elements and isotopes. Since heavy nuclides contain more percentages of neutrons than light nuclides do, fission products from the fission of heavy nuclides are too rich in neutrons. Thus, fission products emit particles until they are stable. This aspect has been illustrated when we estimated the energy of fission reactions.
Since the nuclei usually split into two pieces of different masses, the mass numbers of fission products range between 40 and 170. In terms of elements, they range from potassium, to tungsten, nearly all the elements in the 4th, 5th, and 6th periods, including the lanthanides*. They include alkali metals (K, Rb, Cs), alkaline earth metals (Ca, Sr, Ba), all the transition metals from scandium to tungsten, metalloids (Ge, Sb, Te, Se, etc) halogens (Br, I) and inert gases (Kr and Xe). Thus, separation of fission products into various elements is a complicated operation.
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60 90 110 140 170
Atomic bombs and nuclear reactors are two types of fission application. Fission-product data and their behavior are of fundamental importance, because they have a great impact on the environment and society. Fission products are left following bomb explosions and reactor accidents. For example, some typical long-lived fission products such as 90Sr and 129I are used for monitoring nuclear explosions and accidents. These data are also essential for reprocessing used nuclear fuels and nuclear waste management.
Management of used or irradiated fuels also depends on radioactivity of fission products. Most fission nuclides have very short half lives. After a decade, few nuclides remain radioactive. A very low yield nuclide 85K has a half life of 10.7 years, and two other nuclides, 90Sr and 137Cs have half lives of 29 and 30 years respectively. There are no fission nuclides whose half-life lies between 30 and 105 years. Fission products with half lives greater than 100 years with yields greater than 10–4 are 126Sn (1105 y), 126Tc (2.1105 y), 91Tc (1.9106 y), 135Cs (3106 y), 107Pd (6.5106 y), and 129Tc (1.6107 y).
Fission products affect the operation of reactors in many ways, one of which is the absorption of neutrons by fission products. The high-yield fission product 135Xe has a c of 2,640,000 b, and a half-life of 9.2 hours. The presence of this product lowers the level of fission, and this effect is often referred to as xenon poisoning. The chain reaction of the atomic pile in Hanford suddenly stopped in July, 1944. John Wheeler, the poisoning expert, was consulted. After checking the control parameters of the reactor before the interruption, he concluded that it was the xenon. A few hours later, the reactor resumed function, and this is consistent with the half-life of 135Xe.
The First Fission Nuclear Reactor
Research on uranium has been divided into several tasks. With strong financial support from President Roosevelt, some facts are well known to the inner circle of researchers involved with the uranium project. Neutrons are released in nuclear fission of 235U. Thermal neutron cross sections for many elements have been measured.
Since neutrons are released, uranium undergoes a chain fission reaction, when the neutrons are moderated, and sufficient number of them will cause the next generations of reaction. Firmly believed in this, Fermi’s group assembled uranium into an atomic pile to test the feasibility of a sustained chain fission reaction. They used natural uranium with graphite as moderator, cadmium in the control rod, and boron in the neutron detector. These are the key requirements for nuclear reactors.
Because this was the first atomic pile, only the trial and error method was available to them. They experimented with various materials as they assembled the atomic pile. They used water as the moderator at the beginning, and it did not work. They thought water absorbed too many neutrons. They switched to graphite, still not working. They attributed the failure to the impurity in graphite, so they purified graphite, and made it into bricks. Due to high thermal neutron cross section for nitrogen (1.82 b), they put graphite and uranium into cans and removed the air from them. Step by step, they identified and solved many problems. They placed alternate layers of graphite bricks and pieces of natural uranium and constructed an atomic pile in a racquet court at Stagg Field at the University of Chicago.
Another major problem for the first nuclear reactor was the size of the atomic pile. Various calculations have given an estimate of the amount of required uranium, but experiments give the ultimate test. Fermi’s group built up the pile, and tested the operation as the size grew.
After years of effort, the atomic pile had a sustained chain reaction of a fission nuclear reactor on December 2, 1942 (Fermi, 1955). This was the beginning of the controlled fission reaction. Its success not only provide the pile as a tool for other research, the reactor became a research tool for future reactor design. Its operation provides data for the construction of larger and more sophisticated reactors. It was indeed a great event.
The way they built the first reactor was risky and dangerous in today’s standards. For example, control rods were manually handled. When the reactor was powered up for testing, the emergency measures were solutions of boron and cadmium compounds ready to be poured on to the pile by people standing on guard. On the other hand, every step was handled carefully, and the reactor operation did not have any major problems.
In 1946 the first controlled nuclear chain reaction in Russia was achieved at the Kurchatov Institute, four years following Fermi's in Chicago.
As of 17 August 1995 there were 425 nuclear power reactors in operation worldwide. At that time, the U.S.A. had 107 nuclear reactors in operation, generating the most nuclear power, more than twice that of France, the world second largest. According to the Uranium Institute information, (www.uilondon.org), Belgium, France, Lithuania, and Sweden, had more than 50% power supplied by nuclear reactors in 1996, whereas the U.S.A., Canada and Japan had 22, 16, and 33% respectively.
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