1. First Human Made Reactor and Birth of Nuclear Age

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Unformatted text preview: CHAPTER 1 FIRST HUMAN MADE REACTOR AND BIRTH OF NUCLEAR AGE © M. Ragheb 2/5/2010 “The energy produced by the atom is a very poor kind of thing. Anyone who expects a source of power from the transformation of these atoms is talking moonshine.” Lord Ernest Rutherford, 1933. “The Almighty certainly never intended that people should travel at such breakneck speed.” About trains travelling at 15 miles per hour. Martin van Buren, 1830. 1.1 INTRODUCTION The events that accompanied the birth of the nuclear age are described. The construction of the Chicago Pile Number 1 (CP-1) as the first human made nuclear reactor and the milestones in scientific progress that preceded and immediately followed it are considered. The success of the first man made self-sustained chain reaction was followed by the Manhattan Project, which culminated into the first use of nuclear weapons in warfare by the USA against Japan. We discuss the distinction between a nuclear reactor and a nuclear device using an exponential neutron population growth model. Humanity's hope lies in using its acquired knowledge in constructive endeavors, and refraining from its use for destructive actions. This process still continues in the nuclear age and affects every human in some special way. 1.2 THE CHICAGO PILE NUMBER ONE, CP-1 REACTOR The following code message was phoned from the Stagg Field at the University of Chicago to government officials in Washington DC: “You will be interested to know that the Italian navigator has just landed in the New World, and the natives are friendly.” The Italian navigator referred to was not Christopher Columbus (1451-1506), but Enrico Fermi (1901-1954). “The natives were friendly,” alluded to the success of a major experiment designated as the Chicago Pile Number 1: CP-1. A painting by Gary Sheahan that reconstructed the event showing Enrico Fermi's team is shown in Fig. 1. Fig. 1: Painting by Gary Sheahan reconstructing the Chicago Pile Number 1: CP1, first reactor showing Enrico Fermi's team. Enrico Fermi was an experimental and theoretical physicist, born in Rome, Italy in 1901. He taught theoretical physics at the University of Rome in Italy. He was the co-inventor with Leo Szilard of the nuclear pile. He had received a Nobel Prize in Physics for atomic research in 1938: “On the absorption and diffusion of slow neutrons.” He directed the building of the first reactor at the Metallurgical Laboratory at the University of Chicago. Fermi adhered to the Catholic faith and was married to a Jewish spouse. He escaped the then prevalent nationalistic Fascism and Nazism in Europe to teach at Columbia University in the USA, where he became involved in the field of neutron physics. Humans initiated a self-sustained nuclear chain reaction and controlled it for the first time on December 2, 1942. This occurred beneath the West Stands of Stagg Field, Chicago in the State of Illinois in the USA, at 3:25 in the afternoon. Fig. 2: Enrico Fermi (1901-1954). In the center of the 30 by 60 feet squash court, where it was constructed, the reactor consisted of a pile of graphite bricks and wooden timbers. It was square at the bottom and flattened as a sphere near the top. As an extra safety feature, it was shrouded on all but one side by a gray balloon cloth envelope, provided by the Goodyear rubber company, to contain any unexpected radioactivity release. Its sides were straight up to half its height, and the top was domed like a beehive. The squash court was situated under the ivy covered stands of the University of Chicago's Stagg Field, named after baseball's Grand Old Man: Amos Alonzo Stagg. This experiment was part of an effort that started a few months earlier aimed at releasing energy from the nuclear fission process. The project was given the code name: “Manhattan District Project,” in short: “The Manhattan Project.” The objective was to build a fission device. However, the scientists here were at the preliminary stage of investigating whether a self sustained release of fission energy could be achieved in the first place. Starting in April 1942, two test piles were built at the Stagg field in Chicago. By November 1942, Fermi and his coworkers had constructed a lattice of carbon blocks in the form of a graphite cube, containing lumps of uranium in the form of spheres. The uranium was the fuel for the reaction while carbon, in the form of ultra pure machined graphite, slowed down the neutrons originating from the fission process in the uranium fuel through collisions with the graphite nuclei from their average fission energy of about 2 Million electron Volts (MeV) down to the thermal equilibrium energy with the surrounding medium of 0.025 eV. This is a factor of: 2 x106 = 80 x106 0.025 or an 80 million times reduction in energy. It was then known that fission is likely to occur in the U235 isotope, not the more abundant U238 isotope of uranium. In addition, the probability of fission of the isotope U235, or its nuclear cross section, is enhanced if the neutrons are slowed down from their fast 2 MeV kinetic energy to the slow 0.025 eV thermal equilibrium energy. This is contradictory to classical physics where a large amount of kinetic energy would be more capable of splitting the uranium nucleus. However, at these energies, the neutrons behave more like waves than particles. Under this circumstance, their wave behavior is best described by Quantum Mechanics, rather than by Classical Mechanics. A simplified heuristic model envisions the uranium nucleus as a potential well into which a sluggish slow neutron can drop and be readily absorbed as a wave, distributing its energy among the nucleons in the nucleus and causing it to fission. Conversely, a fast neutron as a wave would readily pass over the well, jumping over it without falling in. Fig. 3: Photograph of the layering of the graphite blocks containing the lumped uranium spheres at the level of the tenth layer, 1942. Graphite block from CP-1, Photo courtesy: Paul Mikols. It was also known that to use natural uranium for a chain reaction necessitates the use of a moderator material, which does not appreciably absorb the neutrons. From that perspective, carbon as graphite, beryllium or heavy water (HDO or D2O) could be used, but not ordinary water (H2O). Heavy water occurs in ordinary water, at a ratio of one molecule of heavy water in 6,700 molecules of ordinary water. The deuterium to hydrogen ratio in nature is about: D/H = 150 parts per million (ppm). Another important consideration was also known: that mixing a moderator with the natural uranium fuel in a homogeneous manner in the form of a slurry or a salt solution would not allow the neutrons to slow down to the required thermal energy. A neutron originating from fission would get absorbed through resonance absorption in the U238 nuclei distributed evenly in the homogeneous mixture. Distributing separate lumps of the fuel into spheres embedded into the graphite blocks allows a neutron born through fission in the fuel to exit the fuel lump, find the nuclei of the moderator to collide with while slowing down in energy, and escaping to a great extent the process of being absorbed in the U238 nuclear cross section absorption resonances. Having escaped resonance capture in U238 and slowed down in the moderator, the now slow neutrons can encounter another lump of fuel and fission some of the U235 isotope nuclei in it. Blocks of ultra pure graphite numbering about 4,000 and weighing 6 pounds each were carefully manufactured. Uranium spheres the size of baseballs were positioned into 22,000 holes that were carefully drilled in the graphite blocks. Figure 3 shows the layering of the graphite blocks containing the lumped uranium spheres at the level of the tenth layer. Instrumentation for measuring the neutron flux such as Geiger counters were built and calibrated to measure the radioactivity arising from the fission process. Fig. 4: Diagram that was part of the patent 2,708,656 application in 1955 for the CP-1 reactor. Source: USA Patent Office. Control rods consisting of the strong neutron absorption material cadmium (Cd) were inserted through the pile to control, through neutron absorption, the fission process. For safety reasons, if the control rods were not able to shut down the chain reaction, buckets containing boric acid, with boron (B) as a strong neutron absorbing material, were ready to be poured on top of the pile should the control rods fail in shutting down the chain reaction. Layer upon layer of the lumped moderator and fuel configuration, up to 56 layers, were added with the control rods withdrawn with the addition of each new layer. On December 2, 1942 the pile was almost complete, with only the last layer being added. At 3:20 pm the nuclear age was born, when after 28 minutes the neutron flux was high enough to sustain a critical mass of natural uranium in a graphite moderator. The first man made reactor, CP-1 generated just 1/2 Watt of power from the fission chain reaction using 771,000 lbs of graphite as a moderator material. As fuel material, it used 80,590 lbs of uranium dioxide (UO2), and 12,400 lbs of uranium metal (U). It cost $1 million to construct as a flattened rotational ellipsoid 25 ft wide and 20 ft high. Patent number 2,708,656 was issued on May 18, 1955 to Enrico Fermi and Leo Szilard after World War II, for the CP-1 reactor design. Figure 4 shows a diagram that was part of the patent application. 1.3 THERMAL AND FAST NEUTRONS The neutrons born from the fission process have an average kinetic energy of 1.99 MeV, and are designated as “fast neutrons.” They collide with the nuclei of carbon in the form of the graphite moderator multiple times and slow down in energy until they reach thermal equilibrium with moderator medium and are then designated as “thermal neutrons.” The kinetic energy of thermal neutrons, or “kT neutrons,” is given by: Ek = kT where: (1) k is the Boltzmann constant = 1.38 x 10-16 [erg/K] T = 273 + oC, is the absolute temperature on the kelvin scale. According to the 13th General Conference on Weights and Measures (CGPM) in 1967, on the kelvin (K) temperature scale, temperatures are called “kelvins” without capitalization and the symbol K is not preceded by the degree symbol in contrast to the degree Celsius (oC) and the degree Fahrenheit (oF) scales. This can be expressed as: E k kT = = 1 2 mv , 2 from which their speed v can be estimated from: v= 2E k = m 2kT m (2) EXAMPLE We use Eqn. 1 to calculate the energy in eV of a thermal neutron in equilibrium with the moderating medium at a room temperature of 20 degrees Celsius as: Ek = kT erg eV 1 (273 + 20) K −12 K 1.61x10 erg = 0.025114 eV 0.025 eV = 1.38 x10−16 EXAMPLE We can then use Eqn. 2 to calculate the speed of the thermal neutrons as: v= = 2E k mn Joule erg x107 eV Joule 1.675x10-24 gm 2x 0.025eVx1.6x10-19 = 2.185 x105 2, 200 cm sec m sec Thermal neutrons are thus also referred to as “2,200 m/sec neutrons.” Fast neutrons are close to having relativistic speeds and it may be argued that they warrant a relativistic treatment. From the Special Theory of Relativity, for a relativistic particle, the Total Energy is expressed as: Total Energy = Kinetic Energy + Rest Mass Energy (3) Expressed in terms of the mass of the particle m and the square of the speed of light, this equation can be written as; mn c 2 Ek + mn 0c 2 = (4) From this equation the kinetic energy of a relativistic particle is: = mn c 2 − mn 0c 2 Ek = ( mn − mn 0 )c 2 (5) = ∆mc 2 The relativistic mass of the particle depends on the ratio of its velocity v to the speed of light c as β= v c (6) It can be written as: mn = mn 0 v 1− c 2 (7) Substituting for the relativistic mass Eqn. 7 into the expression for the kinetic energy Eqn. 5, we get: mn 0 Ek = − mn 0 c 2 2 1− v c (8) 1 = − 1 mn 0c 2 2 1− v c The relativistic particle’s speed can be expressed from Eqn. 8 as a fraction of the speed of light c as: 1+ 1 Ek = 2 2 mn 0c v 1− c 2 1 v 1− = 2 c Ek 1 + m c2 n0 1 2 1 1 − c = v 2 1 + Ek 2 mn 0c (9) EXAMPLE The speed of a fission neutron born at an energy of 2 MeV can be calculated from Eqn. 9 as: 1 2 1 1 − 3x1010 cm v = 2 sec erg 2 x106 eVx1.6 x10−12 eV 1 + 1.675 x10−24 gm(3x1010 cm ) 2 sec 1 2 1 1 − 3x1010 cm = 2 2 x1.6 x10−2 sec 1 + 1.675 x 3x 3 1 1 2 10 cm = 1 − 3x10 sec 1.00424994541 1 cm = [ 0.00423195981034] 2 3x1010 sec cm = 0.06505 x 3x1010 sec cm = 1.95 x109 sec m = 1.95 x107 sec This amounts to 6.5 percent of the speed of light. Should the treatment has used Eqn. 2, we would have obtained the similar result: v= = 2E k mn Joule erg x107 eV Joule , 1.675x10-24 gm 2x 2x106 eVx1.6x10-19 cm sec m = 1.95 x107 sec = 1.95 x109 1.4 ETYMOLOGY OF THE WORD “SCRAM” The etymology of the word Scram, meaning the sudden fast shutdown of a nuclear reactor, is that it is an acronym reportedly coined by Enrico Fermi, when he placed one of his colleagues, Norman Hillberry, next to a rope used to raise and lower the control rods into the CP1, equipped with an ax. Hillberry's duty, if called upon, was to chop the rope with a single swing, immediately dropping the control rods, absorbing the neutrons, hence stopping the fission chain reaction. The story goes that Hillberry's title in the project was: “Safety Control Rod Ax Man,” hence the acronym Scram. There are not any ax men in the control rooms of modern nuclear power plants. There exist though plenty of red colored Scram switches sometimes labeled: “RX Trip,” RX standing for “Reactor Scram.” A 45 degree clockwise yank sends the control rods into the core and the reactor shuts down within seconds. 1.5 SCIENTIFIC MILESTONES The successful experiment at the CP-1 reactor was preceded by a rapid succession of events in the study of radioactivity and nuclear processes. Antoine Henry Becquerel discovered radioactivity in France around 1896. Pierre and Marie Curie followed by discovering the element radium, and later polonium, and the chain decays of uranium in 1898. Their work was helped by other discoveries such as the discovery of the electron through experiments conducted with cathode ray tubes by J. J. Thomson in 1897. The energy to mass equivalence equation was introduced by Albert Einstein in 1905. Ernest Rutherford conducted experiments where he bombarded a thin gold foil with alpha particles, and inferred the existence of the nucleus at the center of the atom's structure in 1912. This was followed by the discovery of the neutron by James Chadwick in 1932. The discovered neutrons were used to induce artificial radioactivity by Irène and Fréderic Joliot-Curie in the 1930s in France. Enrico Fermi in Italy had conducted experiments producing new artificial isotopes using neutrons. Otto Hahn and Fritz Strassmann in Germany bombarded uranium nuclei with neutrons, and found traces of new nuclei in the middle of the periodic table: this was the discovery of the fission process. They discovered that when a nucleus of uranium was bombarded by neutrons, the uranium nucleus splits or, in the parlance of biology, fissions or splits like biological cells would do. It was later noticed that additional neutrons were emitted in the fission process. These neutrons become available for inducing further fission reactions with other uranium nuclei. This fact implied the possibility of a nuclear chain reaction, similar to the fusion nuclear reactions occurring in the sun and the stars. This chain reaction could be made self sustaining, or critical, if a sufficient quantity of uranium could be brought together under the proper conditions that reduce absorptions within the volume and leakage from the surface of the material. If the system were supercritical, the nuclear energy release would increase exponentially until the material is no longer in a supercritical configuration. This situation can be the basis of a nuclear weapon device. On the other hand, if the release is released in the critical controlled state, this energy can be harnessed for great benefit in electrical power production, fresh water desalination from the salty oceans, ship and rocket propulsion, isotopes production for nuclear medicine, harbor and canal excavation, defense against comet and asteroid astral assailants, and other uses requiring energy densities far exceeding the known sources of chemical energy. Lise Meitner, who earlier worked with Otto Hahn and corresponded with him while in England, reported Otto Hahn’s findings and explained its implications to Niels Bohr in Norway, who fully understood its implications and in turn promptly communicated it to scientists in England and the USA. Fig. 5: Leo Szilard with Albert Einstein reenacting their writing of a letter to President Franklin Roosevelt in 1939. Leo Szilard convinced Albert Einstein, to jointly write a letter to USA President Franklin Roosevelt in 1939, urging him to initiate work on the possibility of building an atomic weapon. The Roosevelt Committee for the feasibility of an atomic weapon was established in 1941. In England, a similar committee was established: the Military Applications of Uranium Disintegration (MAUD) in 1941. The Army's Manhattan Engineer District Project, or Manhattan Project in short, was established in the USA under Brig. General Leslie Grove from the Corps of Engineers in 1940. As part of this project, work was initiated at the University of Chicago's Metallurgical Laboratory under Arthur Compton in 1942. The culmination of the effort was the first self sustained man made fission chain reaction designated as Chicago Pile Number 1: CP-1. 1.6 THE MANHATTAN PROJECT Upon success of the CP-1 experiment, a nuclear device development effort project was promptly started in July 1943 with 100,000 employees at three then secret sites at a cost of $2 billion. It was feared that Germany, under the leadership of its leading nuclear scientist Werner Heisenberg, where fission was first discovered under Otto Hahn and Fritz Strassmann, was also on track for building such a weapon. Heisenberg had suggested the construction of a nuclear reactor using heavy water (D2O) as a moderator, instead of the graphite used by Enrico Fermi. The Germans went on an unfruitful track and emphasized the use of thermal neutrons and were only able to build bulky subcritical assemblies which never achieved criticality. They also never reached the realization that fast neutrons can be used to construct a compact unmoderated fast neutron supercritical configuration and hence a weapon. Brigadier General Leslie Richard Groves, born in Albany, New York in 1896 from the USA Corps of Engineers (Fig. 6), who had earlier supervised the construction of the Pentagon building in Washington D. C., was chosen to direct the effort. He enlisted the help of scientists headed by Robert Oppenheimer (Fig. 7) who was a theoretical physicist, born in New York City in 1904. Oppenheimer, in turn, sought the help of an international team of scientists such as Enrico Fermi, Hans Bethe, Leo Szilard. Victor Weisskopf, Niels Bohr, George Kistiakowsky, and Edward Teller. Fig. 6: Brigadier-General Leslie Groves from the USA Corps of Engineers directed the Manhattan Project. Fig. 7: Robert Oppenheimer led the team of scientists at Los Alamos National Laboratory. Fig. 8: Cutout through the X-10 pile at Oak Ridge Tennessee, USA. Fig. 9: Face of the X-10 air-cooled graphite pile constructed at the Clinton Engineers Works at Oak Ridge, Tennessee, USA. A second pile designated as the Chicago Pile 2 or CP-2, using the uranium from the dismantled CP-1 pile, was constructed by March 1943 at Argonne National Laboratory near Chicago. This new pile was surrounded by a five-foot thick concrete shield. It was operated at a few kilowatts of power, compared with the ½ watt power of CP-1, without an internal cooling system, producing small samples of plutonium for basic nuclear physics data and chemical separation experiments. Another pilot experimental graphite moderated and air cooled pile at a design power level of 1,000 kilowatts or 1 Megawatt, designated as the X-10 pile was constructed at the Clinton Engineers Works near Oak Ridge, Tennessee in 1943. It was meant to provide enough plutonium for the chemical separation semi-works. It consisted of a large cube of graphite blocks surrounded by several feet of high density concrete as a biological shield against gamma ray radiation. The graphite blocks were pierced by hundreds of horizontal diamond shaped channels, in which rows of cylindrical uranium slugs were fed horizontally, forming long rods. The pile was air cooled with circulation through the channels on all sides of the slugs. After a period of irradiation, new slugs would be fed from one face of the reactor, pushing the irradiated slugs to fall through a chute off the other side into a bucket immersed in water. After a few weeks of storage under water to allow the short lived fission products radioisotopes to decay, the buckets were transferred through an underground canal to a chemical separation plant. A series of cells with thick concrete walls would contain chemical separation equipment operated remotely. Large underground tanks were used to store the radioactive waste. The facility was ingeniously located on a slope to make use of gravity for flow. Figures 8 and 9 show the X-10 air-cooled pile constructed at the Clinton Engineers Works. The graphite blocks composing the X-10 pile measured 24 feet on the side and weighed 1,500 tons. It contained 1,243 channels on eight inches centers. The uranium slugs canned in aluminum jackets were 1.1 inch in diameter and 4.1 inches long. Two boron steel rods, in the right side of the pile controlled the power level of the chain reaction through inserting them in and withdrawing them out. Four more rods on the left side of the pile were used to shut down the pile. A hydraulic system was designed with two suspended weighted pistons, which would fall and drive the rods into the pile within 5 seconds in the event of a power failure. A second line of defense consisted of four rods suspended above vertical holes in the pile and dropping in when the trip mechanism is energized. As a third level of safety, two hoppers were filled with small boron steel balls to be released into vertical columns in an emergency. Air circulation traversed the graphite, went under the pile through a filter system to a 200 feet stack besides the building. A fan house contained a small steam driven air circulator for emergency use, one 50,000 cubic foot per second fan, and another of 30,000 cubic foot per second capacity. Thermocouples to measure temperatures, a Pitot tube to measure air flow, and ionization chambers to measure the neutron flux levels were incorporated in the design feeding the information into an adjacent control room. The pile reached criticality with 30 tons of uranium, or half of its 60 tons capacity. With improved cooling capabilities and increased fuel loading to 36 tons, the power level reached 500 kilowatts. Five tons of uranium metal containing just 500 milligrams, or half a gram of plutonium, was discharged in November of 1943. The power level eventually reached 1,800 kilowatts, double its design level, but the amounts of plutonium produced were only sufficient for experimental purposes, and could not support a weapon's construction effort. The real weapons engineering was a massive industrial effort that was carried out at three geographically dispersed sites. Large industrial complexes were constructed and managed by different industrial corporations. The DuPont Corporation managed the chemical works on a cost-plus basis in return for a symbolic payment of just $1. 1.6 SITE W, THE HANFORD SITE, STATE OF WASHINGTON The first site was designated as site W where three large-scale reactors were built at the Hanford reservation in the state of Washington, to breed plutonium as a possible fissile material from natural uranium. The reactors' power increased from the 1/2 watt of CP-1 to the 250 Megawatts thermal (MWth) power level. These reactors were water cooled. Each pile needed a river pump house on the Columbia River, large storage and settling basins, huge motor driven pumps to deliver water to the faces of the reactors, and facilities for emergency cooling in case of power failure. The first pile area was designated as 100-B. The reactors were designated with letters as B, D, C, N and F. The tolerances in construction were exceptional. The allowable deviation in cross section measurements was 0.005 inch; in length, 0.006 inch; in the diameter of longitudinal holes for the process tubes, 0.003 inch. About 17 percent of the graphite used in the piles, was tested in a small 30 watts pile constructed at Hanford. The Hanford piles rose more than 120 feet above the desert floor. Adjacent to them were the water cooling treatment facilities, using the Columbia River for cooling the reactors. The irradiated uranium slugs were loaded into heavy shielded casks placed on special railroad cars operated by remote control, and moved about 5 miles away from the piles. The buckets were suspended in water inside low concrete structures isolated in the desert. When a significant amount of the short lived radioactivity has been allowed to decay, the buckets were moved to the chemical separation plant. Fig. 10: Views of the Hanford piles rising 120 feet above the desert floor with the water cooling treatment facilities, and the Columbia River. Fig. 11: Assembling the core of the C reactor at Hanford, state of Washington. Fig. 12: Face of the N Reactor, Hanford, Washington. Fig. 13: Spent fuel stored under water at the N reactor, Hanford, Washington. A bismuth-phosphate chemical separation process carried plutonium from the irradiated fuel in the separation building. The plutonium would be separated from the phosphate carrier and other gross impurities in a concentration building. The phosphate carrier was dissolved in hydrochloric acid and a rare earth fluoride was used to precipitate the plutonium. Disposal of radioactive gases and wastes from the process building were done in a ventilation building and a waste storage area. There were three of these plants designated as T, U and B. The separation buildings, called canyons, were 800 feet long, 65 feet wide and 80 high. They resembled a large aircraft carrier floating on a sagebrush sea in the desert. In each one, a row of forty concrete cells, fifteen feet square and twenty feet deep, ran the length of the building. Each cell was covered by concrete blocks six feet thick, and was separated from its neighbors by six feet of concrete. The 35 tons concrete lids had to be poured maintaining a 1/8 of an inch tolerance to provide adequate shielding. The chemical separation plants were so large that they were called canyons. Along one side of the cell row, and behind seven feet of concrete, were the operating galleries on three levels. The lowest level was for the electrical controls. The intermediate level was for piping and remote lubrication equipment. The upper level was for operating control boards. The area above the cells was enclosed by a single gallery sixty feet high and running the length of the building. Its five foot concrete walls and three foot roof slabs were designed to prevent the escape of radiation when the cell covers were removed. Even with all the covers in place, radiation levels in the gallery would be so high that unprotected personnel could not be present. Once operation started, this huge gallery, or canyon as it became to be called, would become a silent concrete no-man's land isolated from the outside world. Fig. 14: Purex process chemical separation canyons at the Hanford site. Fig. 15: Interior view of the H reprocessing canyon. Fig. 16: Canyon tank overhead view. Fig. 17: Reprocessing canyon F control room. Radiation needs remote control equipment. This mandated simplicity of design, mechanical perfection, maintenance free operation, and interchangeability of parts. Steam jets were used to transfer process materials from one tank to another, to avoid servicing pumps and valves. Centrifuges, being more reliable than filters, were developed for separating materials. Liquid level and density meters were used to follow the progress of the operation. Once the plant was operating, the only access to the cells would be by means of large bridge cranes, which traveled the length of the building. From the heavily shielded cab behind a concrete parapet above the gallery, operators used periscopes and video monitors to view the inside of the gallery. They could use a seventy-ton hook to lift off the cell covers and lighter equipment to work within the cell. With impact wrenches and special tools, they could remove connecting piping, lift the damaged pieces of equipment, and isolate it in a storage cell. They would then lower another new piece of equipment into the operating position and reconnect the process piping. All of this was done at sixty feet or more without direct vision, and with the requirement of extreme accuracy in the dimensions of cell components. Equipment and connections were standardized. Distinctive color codes were used on all units. All concrete faces were coated with paint that is easily washable, corrosion resistant and adequately adherent to the concrete. Because the chemicals were highly corrosive, a special grade of niobium (columbium) stainless steel was used. The final stage in plutonium recovery involved a peroxide method. This process was based on the fact that all nitrates, except those of uranium, thorium and plutonium, are soluble in hydrogen peroxide. The plutonium could be isolated, by separating it from the lanthanumfluoride carrier, converting it into a nitrate, and adding peroxide. The product would be pure plutonium nitrate, which was then sent to the Los Alamos laboratory in New Mexico for reduction to metal. The great value and high toxicity of the product required specialized laboratory techniques. 1.8 SITE X, OAK RIDGE, TENNESSEE Fig. 18: The Alpha I racetrack Calutron (left) and the Beta track (right) used in the Y12 electromagnetic separation process at Oak Ridge, Tennessee. The second site was designated as site X where scientists were isotopically separating the fissile U235 isotope from natural uranium using electromagnetic separation in 184 inch cyclotrons called Calutrons deriving their name from: “California cyclotrons.” Copper for manufacturing of the magnet coils was in short supply and was allocated to the war effort. An amount of 14,700 tons from the strategic metals silver stockpile at Fort Knox was used instead of copper, and was returned back at the end of the war. About 200 grams of the U235 isotope were produced for experimental investigations. The Electromagnetic separation process was a monumental white elephant designated as the Y-12 plant. It produced only a few grams of impure U235 and had to be closed after repetitively breaking down. Figure 18 shows a racetrack used in the electromagnetic process at Oak Ridge, Tennessee. The protruding ribs are the silver-wound magnet coils. The box-like cover on top contains a solid silver bus-bar. Two types of tracks were used: the Alpha I track for an initial enrichment, and the Beta rectilinear racetrack to reach higher enrichments and allowing a smaller scale of the equipment. Other enrichment processes such as thermal diffusion and centrifugation were tried at this time, without producing significant amounts of the U235 isotope. The main success in the production of U235 was carried out in the K-25 Gaseous Diffusion enrichment plant using 4,000 separation cells and uranium hexafluoride (UF6), consuming as much electricity as New York City. The electricity needs were provided by Coal and Hydroelectric plants operated by the Tennessee Valley Authority (TVA) government agency. Fig. 19: The K-25 gaseous diffusion plant, at Oak Ridge, Tennessee, was closed in 1987. Centrifugal pumps designs, with appropriate seals, operating in the highly corrosive environment of uranium hexafluoride had to be developed. Nickel plating and nickel steel alloys resisted corrosion by the gas, and were used. This was the basis of the development of stainless steel as a spinoff industrial product. A barrier material capable of maintaining a separation capability over a long period of time without being clogged had to be chosen. It had to be submicroscopic, but not susceptible to clogging. It had to be porous, but solid enough to be manufactured. Nickel barriers involving a complex ten steps manufacturing process were initially used. Later development of a nickel powder barrier, possibly sintered powder, replaced the initial metallic barrier. Figure 19 shows the K-25 gaseous diffusion plant. In the center lie different service buildings. Fig. 20: Uranium hexafluoride UF6 storage cylinders. Fig. 21: Depleted Uranium (DU) storage cylinders. About 50 kilograms of highly enriched uranium were produced at Oak Ridge over a year's time for the Little Boy device, which was dropped on Hiroshima in 1945. The East Tennessee Technology Park nowadays is a former K-25 uranium-enrichment site. Oak Ridge is currently the USA Department of Energy's largest science and energy laboratory. Between 1942 and 1945, as part of the Manhattan Project, it turned the rural countryside about 20 miles west of Knoxville, Tennessee into a secret city inhabited by 75,000 people. 1.9 SITE Y, LOS ALAMOS, NEW MEXICO The third site Y was situated at Los Alamos in New Mexico. Its objective was to create a critical mass using the fissile isotopes manufactured at the two other sites. There existed an uncertainty about which of the two fissile isotopes would be suitable for constructing a weapon. There was concern about the process of spontaneous fission generating a neutron source in the different plutonium isotopes, and whether a device could be built from it. Spontaneous fission would initiate the fission reaction before a supercritical mass is fully assembled and such a device would prematurely fizzle releasing a minor amount of energy. The assembly of such a device would have to proceed at a much faster speed than in a gun barrel, necessitating an inverse rocket process designated as implosion. The production of both the isotopes was thus pursued. The group working on one isotope did not know about the existence of the other group. Ultimately both approaches were successful, culminating in, not just one, but two fission device designs. The first design that was developed was a gun barrel type design using U235. The second design was an implosion type device using Pu239. Fig. 22: Inert gas plutonium glove box. Fig. 23: Finished plutonium puck or button (left) and enriched uranium ingot (right). The intended target for these weapons was unequivocally Germany, but the war with Germany had already ended with its surrender. The Manhattan project was initiated out of fear that Germany was developing a fission weapon. The Germans under Werner Heisenberg had a modest primarily theoretical effort directed at building a subcritical reactor using heavy water as a moderator material. The production of heavy water available to Germany and its separation from ordinary water required the use of electricity produced from hydroelectric power in Norway. Aerial bombardment and a commando raid sunk the shipments of D2O to Germany and disabled the heavy water plant twice. This concerted effort, whose purpose was not clear to Germany, but whose implications were fully understood by the Allies, brought a halt to the German effort. An investigative team, called the Alsos team, investigated the assumed German program at the end of the war in Europe in 1944. The team discovered that the much feared German weapon project just did not exist. The war with Japan was still ongoing, and the use of the developed weapon against Japan was promptly considered. Japan, realizing that it will eventually lose the war, fearing the entry into the war by Russia and its occupation of its northern islands, and preferring to reach an accommodation with the USA, now that the war in Europe had come to an end, was sending feelers and trying to negotiate an end to it. There was some opposition against the use of the atomic bomb, with suggestions of demonstrating it as a warning to Japan before its actual use. Questioning the need for using the atomic bomb in the war against Japan, and fully aware of its vast destructive potential, James Conant and Vannevar Bush, at the urging of scientists working on the Manhattan Project, addressed a letter on September 30, 1944, to Secretary of war Stimson. They urged a demonstration of the weapon’s capability before its use against Japan. Arthur Compton in May 1945, wrote a letter to superiors where he raised the “… question of mass slaughter for the first time in history.” Niels Bohr in July 1944 sent a warning to the USA president that: “… any advantage the atomic bomb might seem to possess would be outweighed by a perpetual menace to human security.” He had correctly predicted the nuclear arms race that would follow later. James Frank on June 12, 1945 led seven atomic scientists from the University of Chicago in presenting the Chicago Petition or the Frank Report. Farrington Daniels, director of the Metallurgical Laboratory at the University of Chicago, on July 12, 1945, polled 150 scientists working on the weapon project. A majority favored some form of demonstration of the device. Sixty eight scientists at Oak Ridge recommended demonstration of the weapon. In the diary of then President Harry Truman, he made the case that atomic devices are purely military weapons: “We have discovered the most terrible bomb in the history of the world. It may be the fire destruction prophesied in the Euphrates Valley Era, after Noah and his fabulous Ark. This weapon is to be used against Japan. [We] will use it so that military objectives and soldiers and sailors are the target and not women and children. Even if the Japs are savages, ruthless, merciless and fanatic, we as the leader of the world for the common welfare cannot drop that terrible bomb on the old capital or the new. The target will be a purely military one.” Leo Szilard who ironically had earlier written a letter with Albert Einstein to President Franklin Roosevelt, urging the start of the bomb development project, found himself with 69 other scientists writing a petition to President Harry Truman asking him to first demonstrate the use of the atomic bomb to Japan before using it. Fig. 24: Declassified Leo Szilard petition to President Harry Truman with 69 other scientists on July 17, 1945. Source: USA National Archives. Fig. 25: Assumed schematic of the Trinity plutonium device designated as “The Gadget” surrounded by the chemical explosive lenses initiators. Source: Gamow, “The Curve of Binding Energy.” From May to July 1945, a committee designated as the “Interim Committee’ studied the implications of using the atomic bomb. It was composed of the following members: Henry Stimson, George Harrison, James Byrnes, Ralph Bard, William Clayton, Vannevar Bush, James Conant, Arthur Compton, Enrico Fermi, Ernest O. Lawrence, and J. Robert Oppenheimer. The Interim Committee, after deliberation, recommended the use of the bomb on Japan on a dual target without prior warning. Navy Under-Secretary Ralph Bard dissented and withdrew his agreement. Fig. 26: Mockups of the Trinity device showing the core, tamper and surrounding explosive lenses and their initiators. .. Fig. 27: Photographs of the Trinity device on top and at bottom of its test tower showing the initiators wiring. Fig. 28: Energy museum model of the Trinity device at Albuquerque, New Mexico and assembly of Fat Man device.. Fig. 29: A photograph taken of the fireball and the horizontal Mach stem generated by the Trinity test at 0.016 second into the explosion. Fig. 30: Mushroom cloud from the Trinity test on July 16, 1945 at 5:29:45 am. Because of uncertainties about the behavior of the implosion type device, it was tested for the first time at the Trinity Test Site on July 16, 1945 at 5:29:45 am. The test was conducted at Jornada del Muerto (In Spanish: Journey of Death), 100 miles south of Los Alamos in the Alamogordo desert of southern New Mexico. It was a plutonium implosion device placed at an optimal height on a 100 foot tall tower, to maximize its shock wave or blast effects through the reinforcing interference of an incident and the reflected shock waves into the ground as a horizontally propagating Mach stem blast wave. The plutonium fission core was surrounded by an array of explosive lenses, in turn enclosed in a metal casing. Figure 25 shows a schematic, Fig. 26 shows a mockup, and Fig. 27 shows a photograph of the Trinity plutonium device, designated as “The Gadget” surrounded by the chemical explosive lenses' initiators. 1.10 NUCLEAR ENERGY RELEASE The yield of the Trinity test was about 19 kilotons (kT) equivalent of the high explosive Tri-Nitro-Toluene (TNT). The site is now a historical site open to the public only once a year on the first Saturday of October each year. Figure 29 is a photograph taken of the fireball generated by the Trinity test, and its Mach stem at 0.016 second into the explosion. An average value of the energy release per fission event in a fissile isotope such as U235 is 200 million electron volts (MeV). The MeV energy unit is equivalent to 1.6 x 10-6 ergs or 1.6 x 10-13 joules. The proportion in which this energy is distributed in a fission reaction is shown in Table 1. Only a part of the energy is available in a nuclear explosion. This includes the kinetic energy in the fission products, most of the energy of the prompt gamma rays, which is converted into other forms of energy within the exploding weapon primarily ionization and x rays, and most of the neutron kinetic energy, but only a small fraction of the decay energy of the fission products. There is some contribution from the energy released in reactions in which neutrons are captured in the device's fragments. If we exclude the antineutrinos, whose 10 MeV are unavaible, and the delayed gamma rays (6 MeV) and the beta particles decay energy (7 MeV), and allow about 3 MeV for the neutron gamma reactions with the device fragments, about 200 10 – 6 -7 + 3= 180 MeV are immediately available per fission event. Table 1: Apportionment of Energy Release from fission. Distribution of Fission Energy Kinetic energy of fission fragments Prompt gamma rays energy Kinetic energy of fission neutrons Beta particles from fission products Delayed gamma rays from fission products Anti neutrinos from fission products Energy release per fission event Neutron- gamma reactions with device fragments Unavaible antineutrinos energy Unavailable fission products beta particles Unavailable fission products gamma rays Energy available in fission device Energy (MeV) 165 ± 5 7±1 5 ± 0.5 7±1 6±1 10 ___________ 200 ± 6 +3 -10 -7 -6 __________ 180 There are 6.02 x 1023 nuclei or Avogadro's number in a gram molecular weight or mole or 235 grams of U235, or 239 grams of Pu239. The explosive energy released by Tri-NitroToluene C7H5N3O6 or TNT ranges from 900 to 1,100 calories per gram. The energy release from a molecule of TNT yields: C7 H 5 N 3O6 → 6 CO + 5 3 H 2 + N 2 + C + 608,000 J 2 2 Some of the released carbon oxidizes into CO2 yielding 393,510 J per mole of C. The hydrogen oxidizes into water releasing an extra 241,826 J per mole of H2. The carbon monoxide oxidizes into CO2 yielding 282,980 J per mole of CO. If all the carbon burns, and the nitrogen does not, this adds up to: 5 3 608,800 + 6 x 282,980 + 241,826 + 393,510 = 2 2 608,800 + 1,697,880 + 604,565 + 590, 265 = 3,501,510 J The molecular weight of TNT is: 7 x12.011 + 5 x1.0079 + 3x14.0067 + 6 x15.994 = 84.077 + 5.0395 + 42.0201 + 95.964 = gm 227.1 mol The reported value of the heat of combustion for TNT is 3,305,000 J/mol, or: J 1 mol 9 gm 10 = mol 227.1 gm kT J 1 calorie 1.455 x1013 = kT 4.184 J calorie 3.48 x1012 kT 3,305,000 A complication arises in that to optimize the energy of a blast, ammonium nitrate NH4NO3 (80 gm/mol) is added as an oxidizer to TNT actually yielding an explosive mixture designated as “amatol.” An optimal mixture is 21.3 percent TNT and 78.7 percent ammonium nitrate by weight yielding the reaction; 2C7 H 5 N 3O6 + 21 NH 4 NO3 → 14CO2 + 47 H 2O + 24 N 2 + 9,088,600 J For practical dynamic considerations a mixture for the amatol is 20 percent ammonium nitrate and 80 percent TNT. The molecular weight of ammonia is: 2 x14.0067 + 4 x1.0079 + 3x15.994 = 28.013 + 4.0316 + 47.982 = gm 80.003 mol In the case of the amatol mixture, the energy yield becomes: J 1 mol 9 gm 10 = mol (2 x 227.1) + (21x80.003) gm kT 1 J mol 9 gm 9,088,600 10 = mol (454.2 + 1680.063) gm kT 1 J mol 9 gm 9,088,600 10 = mol 2,134.263 gm kT 1 calorie J 4.2584 x1012 = kT 4.184 J calorie 1.018 x1012 kT 9,088,600 The kiloton could be considered as the short kiloton corresponding to 2 x 106 pounds, the metric kiloton would be equal to 2.205 x 106 pounds, or the long kiloton equal to 2.24 x 106 pounds. To avoid ambiguity, the energy released from one thousand tons or 1 kiloton (kT) of TNT is defined to be 1012 calories. This is equivalent to 1 short kT of TNT if the energy release is 1,102 calories per gram, and to 1 long kT if the energy release is 984 calories per gram of TNT. This resulted in the published energy equivalences shown in Table 2. Table 2: Energy equivalence of 1 kt of TNT. Device Energy Release Energy Equivalents 12 1 kT of TNT = 10 calories 2.6 x 1025 MeV 4.184 x 1019 ergs 4.184 x 1012 joules 1.16 x 106 kW.hrs 3.97 x 109 BTUs fission of 56.8 grams of fissile nuclei fission of 2.00 ounces of fissile nuclei Fission of 1.45x1023 fissile nuclei The number of fissions that would release the equivalent of 1 kT of TNT can be estimated from: 1 kT TNT=1012 calories x 4.184 Joule 1 eV MeV 1 fission x x10-6 x -19 calorie 1.60207x10 Joule eV 180 MeV =1.45x1023 nuclei where we used the average extractable energy of 200 – 10 – 6 -7 + 3 = 180 MeV per fission event, excluding the antineutrinos and the delayed beta gamma rays whose energies cannot be extracted, but allowing for about 3 MeV for the (n, gamma) capture reactions with the device debris.. Using Avogadro’s law: N= g Av nuclei M (10) where:g is the mass in grams, M is the atomic mass in amus, nuclei A v = 0.6 x1024 , mole the mass of fissile material resulting from the fissioning of N fissile nuclei is: g= N .M gm Av (11) EXAMPLE The mass of U235 nuclei leading to the release of 1 kT of equivalent TNT excluding the antineutrinos and delayed gamma rays is: g= N .M gm Av 1.45 x1023 x 235 gm 0.6 x1024 = 56.8 gm ounce = 56.8 gm x 0.3527 gm = 2.00 ounces = 1.11 DISTINCTION BETWEEN A NUCLEAR REACTOR AND AN EXPLOSIVE DEVICE MULTIPLICATION FACTOR AND CRITICALITY We try to identify and clarify the difference between a nuclear reactor and an explosive device in terms of the energy release as a function of the number of neutron generations in the fission process. Let us define: ν : the average number of neutrons released per fission event, a : the average number of neutrons lost by absorption in the active material per generation, : the average number of leakage neutrons released from the active geometry per generation. We can define a multiplication factor k as: k=ν -a- (12) Three situations present themselves according to the value of the multiplication factor k: k=1 ⇒ a critical system, k>1 ⇒ a supercritical system, k<1 ⇒ a subcritical system. (13) EXPONENTIAL NEUTRON GENERATIONS GROWTH MODEL The number of fissionned nuclei in 1 kT of TNT equivalent to the fission of 56.8 gms of U235 from Table 2 and Eqn. 11 is: N= 56.8 0.6x1024 = 1.45 x1023 nuclei. . 235 If N is the number of neutrons present at any instant, from Eqn. 12, the number of neutrons in the next generation will be: Nk=N(ν -a-) The increase in the number of neutrons from one generation to the next becomes: dN=Nk-N =N(k-1) =N(ν -a- − 1) Let the generation time be denoted by τ. Thus we can write the rate of neutrons increase as: dN N ( k − 1) = dt τ (14) Separating the variables followed by limit integration, assuming that k and τ are constant, yields: N (t ) ∫ N0 ln dN (t ) ( k − 1) = dt N τ ∫ 0 t N (t ) ( k − 1) = t N0 τ N (t ) = N 0e + ( k −1) τ (15) t by taking the exponential of the natural logarithm function. This shows that the neutron generation undergoes an exponential growth as a function of time. We can express the number of neutrons as a function of the number of generations as: n= t (16) τ Substituting from Eqn. 16 into Eqn. 15, results in a change of variables from t to n as: N(n)=N 0e + ( k −1) n (17) EXAMPLE Adopting the numerical values: ν = 2.5, a = 0.5, = 0, k = 2.5-0.5-0.0 = 2, then: N(n)=N 0e + n (18) This assumes no leakage of the neutrons. The number of generations in which 0.1 kT of TNT equivalent of energy corresponds to the fissions of 0.1 x 1.45 x 1023 = 1.45 x 1022 nuclei, and the release of 100 kT of TNT equivalent corresponds to the fissions of 100 x 1.45 x 1023 = 1.45 x 1025 nuclei. If we further assume that the chain reaction is initiated by a single neutron, then N0 = 1, and thus: N = e+ n (19) Taking the natural logarithm of both sides we get: ln N ln e n n= n = = ln e We can thus construct Table 3 relating the total energy release to number of neutron generations. Table 3: Energy release as a function of the number of neutron generations Energy Release Number of fissions Number of generations kT TNT 0.1 1.0 10 100 N 1.45x1022 1.45x1023 1.45x1024 1.45x1025 n = ln N 51.03 53.33 55.63 57.93 Hence the release of 1 kT of TNT equivalent requires the confinement of the fission reaction for at least 53 neutron generations, thus in nuclear devices the reaction must be confined by use of a tamper. In a nuclear reactor no tamper is used, hence it is impossible to confine a supercritical system for more than a few neutrons generations, since the heat release would disassemble, through melting followed by vaporization, the configuration into a subcritical one shutting down the chain reaction. The following observations can be made: 1. There is a need for a very large number of 51 neutron generations to release a mere 0.1 kT of TNT equivalent. 2. Most of the energy release: 100 − 0.1 99.9 = = 0.999, 100 100 or 99.9 percent is released in the last 58 – 51 = 7 generations in a short period of approximately 7 x 10-8 sec = 0.07 μsec, or less than a tenth of a microsecond. 3. Nuclear explosives are “tamped” inertially with a high strength high-density “blanket” material such as Be, BeO, W, Pb, Ta, Au or U to increase the inertial confinement time and obtain fissions and energy release from the latest number of generations. 4. Nuclear reactors are not tamped. The heat release will thus melt then vaporize any supercritical configuration and disperse it. Dispersion restores subcriticality of the configuration with stoppage of the energy release. 5. Thermal reactors cannot explode like nuclear explosive devices, since they are constructed differently. 6. The worst that can happen in a fast reactor accident is compaction of the core to supercriticality with a substantial nuclear energy release, but not a complete explosion, since dispersal and subcriticality would occur quite rapidly. OTHER TECHNICAL DIFFERENCES To initiate a nuclear explosion in a nuclear device, the fissile materials such as U235 or Pu , or a combination of them in a dual core configuration, are brought together very rapidly and then kept together for a long enough time, such that a significant fraction of their nuclei undergo the fission process. The fissile components of the device are driven together with explosive force producing rapid increases in the reactivity of the system way beyond the neutrons prompt critical state. This necessitates the use of Pu239 and/or U235 with a high enrichment in the fissile isotopes because the other isotopes act as a sink for the fast neutrons needed for the process. Under these circumstances, the fissions are due to fast neutrons, the time 239 between the neutron generations is very short, no delayed neutrons exist, high enrichments are used, and massive amounts of energy are released before the material has time to blow itself apart, overcoming the inertial resistance to its expansion by the surrounding tamper, terminating the fission reaction. In a thermal reactor, on the other hand, the situation is totally different. The fuel is low enriched uranium with a large proportion of U238, which absorbs neutrons, and a small proportion of U235 and Pu239 at only 3-5 percent. The fuel is also surrounded by a moderator that slows down the energy of the neutrons produced. In a power surge, even if a prompt neutrons critical state is attained, the presence of the U238 damps down the increase in reactivity by absorbing the neutrons as they are slowed down through the nuclear negative feedback mechanism of Doppler resonance broadening. This inherent negative Doppler coefficient of reactivity is a safety characteristic of all commercial nuclear plants and research reactors and describes the fact that as the fuel gets hotter it gets less effective at using neutrons to induce fission and produce power causing the reactor’s reactivity to fall. The nuclear reactors’ Doppler coefficient is always negative, causing its reactivity to decrease as its temperature increases. In addition, the majority of fissions in nuclear reactors are caused by slow neutrons for which the time between successive fissions is longer than in the case with fast neutrons. The effective neutron lifetime and hence the reactor period is increased even more by the presence of the delayed neutrons released from the decay of the fission products. The end result is that the energy released by a power surge will disrupt the fuel and terminate the chain reaction long before the reactivity reaches the very high levels needed in a nuclear device. The energy released in a power surge, including fast reactors, would be far lower: by as much as 100 million times less by volume than in a nuclear device. 1.12 NUCLEAR DEVICES USE IN WAR The decision was reached to use the atomic bomb against Japan on two targets without prior demonstration or warning. The argument that won the debate, concerning prior demonstration, was that it would save soldiers lives on both sides, as well as civilians on the Japanese side, who would have otherwise to incur heavy losses as collateral damage in a conventional invasion of the Japanese islands. The large human losses on both the American and Japanese sides in the invasion of the Japanese island of Okinawa supported this fear. There were opposing rumblings and accusations to the effect that the decision was based on the political need to use the weapons to justify the effort and expenses of building them. What is remarkable is that the two approaches that were followed to construct the bomb, the uranium235 enrichment approach as well as the plutonium239 breeding approach, both succeeded. Figure 32 shows a replica of the gun barrel design using U235 as a fissile material, designated as Little Boy. Figure 34 shows the actual Fat Man Implosion device employing Pu239 as a fissile material. Gun barrel casing Tamper Chemical propellant Accelerated two hemispheres configuration Chemical propellant Tamper Gun barrel casing Single accelerated hemisphere configuration Tamper Chemical propellant Gun barrel casing Accelerated plug configuration Fig. 31: Gun barrel configurations could contain one to two critical masses of highly enriched U235. Fig. 32: A replica of the Little Boy large metallic gun barrel device dropped on Hiroshima. It was 304 cm long and 71 cm in diameter. It was exploded at a height of 570 meters at 08:15 hour on August 6, 1945 generating about 15 kT of yield. It had a low efficiency and is reported to have fissioned only 1.5 lbs of its uranium content. The overall device weighed 4-5 metric tonnes. It was made of 64 kgs of uranium with an enrichment of 80 percent or 51 kgs of U235. At 50 kgs as critical mass, this corresponds to 7,000 kgs equivalent of natural uranium. This corresponds to the critical mass of an uncompressed bare unreflected sphere of U235. Sophisticated methods of uranium storage and controlled compression have reduced the critical mass by a factor of 2-3. The hazard of uncontrolled explosions limits the amount of U235 content of a modern warhead with 5-10 times the yield of Little Boy. Casing Explosive lenses Tamper Solid core implosion Casing Explosive lenses Tamper Spherical shell implosion Casing Explosive lenses Tamper Porous core implosion Fig. 33: Implosion configurations using solid, spherical shell or porous cores. With compression and a tamper, modern weapons are reported to use 15-20 kgs of Weapons Grade Uranium (WGU) instead of the about 50 kgs bare critical mass of U235. Fig. 34: The actual Fat Man Implosion device dropped on Nagasaki, being loaded on a bomber for delivery to its target. It was exploded at 503 meters height at 10:58 hour on August 9, 1945. It was 325 cm long and 154 cm in diameter. It generated 22 kT of yield mostly from plutonium. It weighed 4-5 metric tonnes of which the nonfissioning assembly material constituted a thick blanket surrounding the fission core. Colonel Paul Tibbets lead the 509th Composite Bomb group and prepared at Wendover air base in Utah for the bomb dropping. The bombers had to train on bomb dropping procedures, plane maneuvers and tactics that would allow the bombers to mitigate the effects of the released gamma rays traveling at the speed of light on the bomber crew, followed by the neutrons, and escape the blast wave from the explosion. On August 1, 1945, B-29 bombers arrived on the coral island of Tinian in the Pacific Ocean, 1,500 miles south of Japan. The USS Indianapolis heavy cruiser had delivered the bomb components. The war was still continuing, the Indianapolis was later sunk by a Japanese submarine on its trip back from its secret mission. On August 6, 1945, the Enola Gay B-29 bomber, on exhibit at the Smithsonian Museum in Washington D.C., dropped a gun barrel bomb design using U235, designated as Little Boy, on Hiroshima, Japan at 8:16:02 am. It measured 28 inches in diameter and 129 inches in length. It weighed 9,000 pounds. From 80,000 to 140,000 persons perished and 100,000 more were injured. The radiation dose resulted primarily from fast neutrons. The yield of the weapon was 12.5 kT of TNT equivalent. Fig. 35: The mushroom cloud that rose from the Little Boy Hiroshima device's fireball on August 6, 1945, to the left, and the one over Nagasaki, August 9, 1945 to the right. Fig. 36: Survivor burn victim from the Hiroshima explosion. Burns are more prominent where the x rays were preferentially absorbed in the dark clothing areas. Fig. 37: The effects of the Hiroshima weapon 4,000 feet from “ground zero.” A fire station and its equipment are shown. Fig. 38: The effects of the Hiroshima weapon showing a concrete movie theatre remaining structure. Fig. 39: The B-29 bomber Bockscar dropped the Nagasaki plutonium Fat Man device. Fig. 40: Aftermath of the bombing and ensuing fire at Nagasaki. Fig. 41: Catholic cathedral explosion remnant at Nagasaki. Fig. 42: Nagasaki ground blast effects. On August 9, 1945, the Bockscar B-29 bomber dropped an implosion device using Pu239, designated as Fat Man on Nagasaki, Japan. The latter measured 60 inches in diameter, and was 128 inches long. It weighed about 10,000 pounds. About 74,000 persons perished and 75,000 were injured out of the 286,000 inhabitants of the city. A photograph of a survivor burn victim in Fig. 36 shows the effect of x-ray absorption at the dark parts of the clothing. The hilly nature of the landscape at Nagasaki, compared with the flat one at Hiroshima, resulted in a lower number of victims despite the larger released explosive yield. The radiation dose was primarily from x and gamma rays. The weapon's yield was 22 kT of TNT equivalent. Figure 35 shows the mushroom clouds that resulted from the Hiroshima and Nagasaki devices fireballs, with the buildings of the cities appearing at the bottom of the photograph. Figures 37, 38 show its effects at “ground zero” directly under the explosion, and Figs. 40-42 show the blast effects at the second bomb explosion at Nagasaki. Table 4 gives estimates of the casualties caused by the atomic blasts at Hiroshima and Nagasaki compared with those from the fire storm bombing of Tokyo with conventional and incendiary explosives on March 9, 1945, and with the average of 93 aerial attacks on other Japanese cities. The fire bombing evidently caused higher casualties. However, the higher mortality and casualty rates per square mile destroyed by the atomic devices is also clearly apparent. Table 4: Casualties comparison between conventional and nuclear explosives. Hiroshima Population per square mile Square miles destroyed Killed and missing Injured Nagasaki Tokyo 1,667 tons TNT and incendiary 35,000 4.7 70,000 70,000 65,000 1.8 36,000 40,000 130,000 15.8 83,000 102,000 Average of 93 attacks 1,129 tons TNT and incendiary 1.8 1,850 1,830 Mortality per square mile destroyed Casualities per square mile destroyed 15,000 20,000 5,200 1,000 30,000 42,000 11,800 2,000 As a comparison, the “Dresden Commission of Historians for the Ascertainment of the Number of Victims of the Air Raids on the City of Dresden on 13-14 February 1945” has provisionally estimated the likely death toll at around 18,000 and definitely no more than 25,000. Dresden was a German city of 3/4 of a million people, its population further swollen by hordes of anonymous refugees from the Eastern Front. Its historic heart was destroyed in one apocalyptic night by aircraft armed with more than 4,500 tons of high explosive and incendiary bombs. The devastated area amounted to around 13 square miles or34 square kilometers. 1.13 NAVAL TRAGEDY OF THE USS INDIANAPOLIS CRUISER The USS Portland class heavy cruiser Indianapolis (CA-35) had sailed 5,000 miles across the Pacific Ocean in just 10 days to deliver the atomic bomb components to the Tinian Island on July 27, 1945. It had been repaired in a dry dock after being attacked and suffering a near fatal hit on March 31, 1945 by a Japanese army Oscar airplane on a Kamikaze, Divine Wind, mission on its naval strafing mission, prior to the invasion of the Japanese island of Okinawa, 400 miles from the Japanese mainland. Being on a secret mission, the crew did not know the nature of its cargo. It sailed alone and was not escorted by ships equipped to detect and fight the Japanese submarines. At the time of her sinking the ship’s commanding officer Captain Charles McVay III was blamed for her loss and the loss in its crew. However, information was withheld from Captain McVay about submarine activity in the area on its way to the Philippines and especially about how the Navy policies at the time precluded the navy from even realizing that the ship was lost for days. On July 30, 1945, it was intercepted and hit by two torpedoes at 12:05 am, one blowing off most of her bow, where the officers slept. About 900 of the 1,196 sailors on board initially survived its sinking, which lasted 12 minutes. The rest were trapped inside the sinking hull. However, only 316 or 316 / 1196 = 0.264 or 26 percent of its crew would ultimately survive the horrors of hallucination from dehydration, heat and terrorizing attacks by dozens of sharks. An account of their tragic ordeal over 4 ½ days and 5 nights was described in the Hollywood movie: “Jaws.” The Indianapolis was sunk by the Japanese submarine I-58 under Lt. Cdr. Mochitsura Hashimoto, who was later promoted to Commander for the same attack, in the Philippine Sea. The survivors were not missed until August 2, 1945; three days later. It was planned that the Indianapolis would return to Leyte on July 31, 1945. A reconnaissance aircraft on a routine flight accidentally spotted the survivors in the water on August 2, 1945. Rescue operations did not complete until August 8, fully 9 days after the sinking. The ship’s commanding officer, Capt. McVay survived, only to commit suicide in 1968 being upset about receiving hate mail. Fig. 43: The Portland class heavy cruiser USS Indianapolis, or The Indy, off Mare Island shipyard on July 10, 1945, after its repair from a Kamikaze attack off the island of Okinawa. It was lost on July 30, 1945. Reports of the Indianapolis sinking were buried by the news of the Japanese surrender and the end of World War II. Captain Charles McVay III, the commander of the Indianapolis was court-martialed supposedly for not adopting a zigzag sailing course to evade submarine detection. His men thought he was made a scapegoat. In the year 2000, thirty two years after he committed suicide, an act of the USA Congress cleared his name. 1.14 DISCUSSION Japan was fearful of Russia’s entry into the war with its territorial claims in the northern Japanese islands, accepted the Potsdam agreement and the Japanese general Nazami signed the documents with American general Westmoreland and other Allies representatives on the Missouri battleship in Tokyo bay, thus ending the ordeal of World War II on August 15, 1945. The Missouri is kept as a floating museum, open to the public, at the Pearl Harbor Naval Base in Hawaii. Fig. 44: Monument at the ground zero of the Trinity site. The site is now a historical site open to the public only once a year on the first Saturday of the month of October. The nuclear age was born in the building of the first reactor, followed by the Manhattan Project, the first nuclear test at the Trinity site and the first use of nuclear devices against Japan ending the Second World War. The forces and knowledge unleashed in these events is sure to affect human life forever. The genie was out of the bottle. The Nuclear Age was born. EXERCISES 1. Calculate the speed in meters per second of thermal or kT neutrons with an energy of 0.025 eV. 2. If a single fission reaction produces about 200 MeV of energy, use Avogadro’s law to calculate the number of grams of U235, U233 and of Pu239 that would be fissioned to release 1 kT of TNT equivalent of energy. Assume that all the energy release is available, except for the energy carried away by the antineutrinos, and the delayed gamma rays and beta particles in the fission products, which is not fully recoverable. 3. The yield from the Hiroshima device was 12.5 kT of TNT equivalent, and the yield from the Nagasaki device was 22 kT of TNT. Assuming that one critical mass of lead reflected U235 Oralloy at about 30 kgs, and one critical mass of Pu239 at about 10 kgs were used to generate these yields, compare the energy release efficiencies of the two devices as the fraction or percentage of the fissile material converted into energy in the case of the gun barrel versus the implosion process. 4. Using the exponential growth model to calculate the number of generations needed for the release of 1 kT and 100 kT of TNT equivalent, in what neutron generations are the last 99 percent of the energy released? Use the following data: ν = 3.0, a = 0.4, = 0.1. 5. Compare the power in MeV/sec and in Watts of: a) The experimental CP-1 pile. b) The pilot X-10 pile. c) The Hanford plutonium production reactors. d). A modern electrical nuclear power plant. -6 -13 Note: The MeV/sec power unit is equivalent to 1.6 x 10 ergs/sec or 1.6 x 10 Joules/sec or Watts. 6. Calculate the speed in meters per second of neutrons possessing the following energies: a. Fast neutron from fission at 2 MeV, b. Intermediate energy neutron at 10 keV, c. Thermal energy neutrons at 0.025 eV. 7. Plot the neutron population N as a function of the generation number n using the exponential model for the following values of the neutron multiplication factor k: a) k=1.0, critical system, b) k=0.5, subcritical system, c) k=2.0, supercritical system. REFERENCES 1. S. Glasstone, Ed., “The Effects of Nuclear Weapons,” U. S. Atomic Energy Commission, 1950. 2. S. M. Ulam, “Adventures of a Mathematician,” Charles Scribner’s Sons, New York, 1976. 3. A. C. Brown, and B. MacDonald, “The Secret History of the Atomic Bomb,” Dell Publishing Co., New York, 1977. 4. R. G. Hewlett and O. E. Anderson, Jr., “The New World, A History of the United States Atomic Energy Commission,” Volume 1, 1939/1946, U. S. Atomic Energy Commission, 1972. 5. D. S. Saxon et al., “The Effects of Nuclear War,” Office of Technology Assessment, Congress of the United States, Washington DC, May 1979. 6. William E. Loewe, “New Radiation Dosimetry Estimates for Hiroshima and Nagasaki,” Energy and Technology Review, Lawrence Livermore National Laboratory, pp. 25-36, November 1984. ...
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This note was uploaded on 06/16/2010 for the course NPRE 402 taught by Professor Ragheb during the Spring '08 term at University of Illinois at Urbana–Champaign.

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