Uranian

“ When a neutron strikes the nucleus of uranium-235 (235U) or plutonium-239 (239Pu) , it may cause the nucleus to split into two roughly equal fragments, releasing neutrons and energy. This is the well-known process of neutron-induced fission, the method in which nuclear energy is produced in both reactors and bombs. [14] The most common uranium isotope, 238U, also breaks up by fission, but does so all by itself, without the need for any external neutrons. That process is spontaneous fission and it goes on at random, very much like radioactive decay. ”

“ Lead of radioactive origin, or isotopic lead the final product of the decay of uranium is found to vary slightly in its atomic weight according to the uranium mineral from which it is extracted.
To radioactivity is ascribed the well-known “pleochroic haloes”—tiny spots or borders of deeper colour surrounding microscopic inclusions—long ago observed in certain rock-forming minerals (cordierite, andalusite, mica, etc.) when micro-sections of rocks are examined in polarized light. ”

“ The lead-210 will then decay with its 22-year half-life.”
The radioactive series that starts with uranium is like a series of lakes connected by waterfalls. As long as uranium, the big one on top, has water in it, the others will be full and the falls will keep flowing. ”

“ But, surrounded by a shield through which no neutron could escape, any mass of material on earth would become an atomic pile!
A single molecule of uranium in any mass of rock will sooner or later disintegrate, giving off high-speed neutrons. Normally they travel indefinitely and are harmless. Some go up into the air and may ionize a single molecule. Some may find a fissionable atom and disrupt it. ”

“ We should not only be able to use this uranium and thorium; not only should we have a source of power so potent that a man might carry in his hand the energy to light a city for a year, fight a fleet of battleships, or drive one of our giant liners across the Atlantic; but we should also have a clue that would enable us at last to quicken the process of disintegration in all the other elements, where decay is still so slow as to escape our finest measurements. Every scrap of solid matter in the world would become an available reservoir of concentrated force. ”

“ The nuclear force is so short-range, it barely reaches from end to end of a large nucleus like that of uranium. Left to itself, it holds together most of the time, but with the added energy of an entering neutron, we might imagine shock waves going through it and turning the nucleus into something like a quivering drop of liquid. Sometimes the uranium nucleus recovers, keeps the neutron, and then goes on to beta-particle emission. And sometimes the nucleus stretches to the point where the nuclear force doesn’t quite hold it together. ”

“ By 1910 it was well established that the earth must be extremely ancient. Analyses of some minerals containing uranium showed them to be hundreds of millions of years old, even though the uranium came from rocks that were known to be relatively young among geologic STRATA. Measurements still were inaccurate, however, and only a few rare and unusually rich radioactive minerals contained enough of the products of RADIOACTIVE DECAY [2] to allow analysis of their age by the crude methods then available. ”

“ The world appeared to have far greater sources of energy than had been expected. The “fossil fuels”—coal, oil and natural gas—were being used at such a rate that many speculated that the gas and oil would be gone in decades and the coal in centuries. Was it possible that uranium might now serve as a new source that would last indefinitely?
It was rather disappointing that it was uranium-235 which underwent fission, because that isotope made up only 0.7% of the uranium that existed. ”

“ Occasionally the random quivering of a nucleus would bring about a shape that the nuclear interaction could not bring back to normal and the nucleus would then break apart. In a gram of ordinary uranium, there is a nucleus undergoing such “spontaneous fission” every 2 minutes on the average. Therefore, enough uranium need only be brought together to surpass critical size and it will explode within seconds, for the first nucleus that undergoes spontaneous fission will start the chain reaction. ”

“ If a chain reaction could be started in a sizable piece of uranium, unimaginable quantities of energy would be produced. Just 1 gram of uranium, undergoing complete fission, would deliver the energy derived from the total burning of 3 tons of coal and would deliver that energy in a tiny fraction of a second.
Szilard, who had come to the United States in 1937, clearly visualized the tremendous explosive force of something that would have to be called a “nuclear bomb”. ”

“ One striking fact about uranium fission was the large amount of energy it released. In general, when a very massive nucleus is converted to a less massive one, energy is released because of the change in the mass defect, as Aston had shown in the 1920s. When the uranium nucleus breaks down through the ordinary radioactive processes to become a less massive lead nucleus, energy is given off accordingly. When, however, it breaks in two to become the much less massive nuclei of barium and krypton (or others in that neighborhood) much more energy is given off. ”

“ Teena said.
“Well, the reactor is about four stories high,” Eddie went on. “They call it a uranium ‘pile.’ It’s made up of hundreds and hundreds of graphite bricks. That’s where they get the name ‘pile’—from brick pile. Anyway, scattered around in between the bricks are small bits of uranium. Uranium atoms are radioactive. That is, they keep splitting up and sending out rays.”
“Why do they do that?” Teena asked.
37
“It’s just the way nature made uranium, I guess,” Eddie said. “Most atoms stay in one piece, although they move around lickety-split all of the time. ”

“ In this respect, as in some others, the universe seems like a clock running down, with no mechanism for winding it up again. All the uranium in the world is breaking down, and we know of no source from which new uranium can come. Under these circumstances it seems strange that there should be any uranium. But if, like some insects, we lived only for a single spring day, we should think it strange that there should be any ice in the world, since we should find it always melting and never being formed. ”

“ I was too close to the atom-rocket. My conductor screen grazed the warhead itself!
Its energies surged through the unstable elements in the warhead; a warning bell sprang into clamorous life. The group around me froze in their tracks, mouths open, faces mirroring fright and disbelief—and the frightful power of the strained atoms within the warhead began to grind toward nuclear fission! ”

“ The one per cent of argon found in the earth’s atmosphere comes almost entirely from the second reaction. These radioactivities are also interesting because appreciable amounts of potassium40 are always present in human tissue.
All nuclei at the heavy end of the periodic system are radioactive alpha emitters. Uranium, for example, has no stable isotopes; they all undergo alpha decay. But there is another 46 mode of spontaneous decay of uranium, which is much less frequent than alpha decay but is of much greater practical importance. This is the fission process. ”

“ The rays that get out strike the uranium plate and cause the surface layer of molecules to disintegrate, their products being driven off by the atomic explosions with a velocity about equal to that of light, and it's the recoil that deflects and swings the plate. The amount of uranium decomposed in this experiment couldn't be detected by the most delicate balance—small mass, but enormous velocity. See?"
"Yes, I understand," answered Thornton. "It's the old, 'momentum equals mass times velocity,' business we had in mechanics." ”

“ One isotope of plutonium, plutonium-239, which resulted from the absorption of a neutron by a 238U nucleus and the emission of two beta particles, was discovered to be as fissionable as 235U and hence theoretically just as feasible for a bomb. Since plutonium is chemically different from uranium, it offered the tremendous advantage that it could readily be concentrated by conventional chemical techniques.
The way to manufacture usable amounts of plutonium, an element that had never before been detected on earth, is to expose uranium to a very intense neutron bombardment. ”

“ In uranium, we have the one obvious derivative uranium-X, and the question remains whether this one descent can be connected with any other individual or family. Uranium is long-lived, and emits only alpha-rays. Uranium-X decays to half value in 22 days, giving out beta- and gamma-rays. Since our evidence goes to show that radio-activity is generally accompanied by the production of new elements, it is natural to search for the substance of uranium-X in other forms, and perhaps under other names, rather than to surrender immediately our belief in the conservation of matter. ”

“ Now you have lost completely, Hohmann, for the radiation from that paper when bombarded with the plutonium radiation, produces a whole string of secondary radioelements in your offices, in your desks, in your bodies, and in your air. The ash from burning is still hot, Hohmann, and the trucks that will carry the deadly paper will be as deadly. Your very country will be subject to slow fission if you start another uranium pile for several hundred years. ”

“ There's a big cylinder of metallic uranium inside, and we play our disintegrating rays on the under surface of this cylinder from those oblique tubes below. When the rays hit the uranium in the cylinder, the atoms explode, and the decomposition products are shot off downward at almost the velocity of light. A back pressure is thus produced which lifts the Ring exactly like a rocket. ”

“ Detailed descriptions of how the metal uranium slowly and regularly breaks down in a descending series into the metal lead and the gas helium, will be found in the literature on radio-activity. For our purpose, it suffices to say that according to Barrell, an atom of uranium which breaks up will ultimately give rise as a stable product to eight atoms of helium and one atom of lead. ”

“ It is always well to make a comparative test, timing the rate of collapse, with a piece of ordinary stone.
Determinations of the ratio of the amount of uranium to the amounts of the various products of its decay (radium, helium, lead, etc.) present in various radioactive minerals give (knowing the rate of the decay) some idea of the period of time during which these products have been accumulating. In this way estimates have been made of the age in years of these minerals and even of the age of the earth ”

“ It is, but we’re talking about such small quantities that its difficult for scientists using the most sensitive equipment to detect it. The uranium in the rock decays to another radioactive element and that one decays to another, and another, and another, and so 12 forth, in a series of elements that results in lead, which is not radioactive. In this series are two radioactive elements, radium and a radioactive isotope of lead, that help us to date paintings. ”

“ Then 2 beta particles are given off so that the thorium-233 becomes first protactinium-233 and then uranium-233.
If a thorium shell surrounds a nuclear reactor, fissionable uranium-233 is formed within it and is easily separated from the thorium. In this way, thorium is also added to the list of earth’s potential nuclear fuels. [4]
If all the uranium and thorium in the earth’s crust (including the thin scattering of those elements through granite, for instance) were available for use, we might get up to 100 times as much energy from it as from all the coal and oil on the planet. ”

“ If all the energy delivered by a gram of uranium in the course of its radioactivity over many billions of years was totalled, it was enormously greater than the energy produced by the burning of a candle with a mass equal to that of uranium. ”

“ This was the only chance and here, between the hidden place where determined men had worked tirelessly and ingeniously to refine only a small capsule of pure Uranium in a year and the hidden place where other learned men waited to incorporate the product into an atomic bomb that could destroy the city of the invaders completely, lay the greatest danger of defeat. ”

“ We might think of a single uranium atom breaking down and shooting off an alpha particle. We might also think of a single carbon atom combining with 2 oxygen atoms to form carbon dioxide. The uranium atom would give off 2,000,000 times as much energy in breaking down, as the carbon atom would in combining.
The energy of radioactivity is millions of times as intense as the energy released by chemical reactions. ”

“ One only has to add a neutron to common U238 to get radioactive U239. In the course of time this decays into plutonium. This element can be used like U235: It produces fission, a great amount of energy and enough neutrons to keep the process going. We shall also learn to extract energy from other nuclear fuels. Thorium acts like uranium, while deuterium can give energy by building up bigger nuclei rather than breaking them into smaller pieces. Therefore the source of energy will be universally available and quite inexpensive. ”

“ Each new neutron could, if properly slowed down by a moderating material, cause another nucleus to split and release more energy and still more neutrons, and so on, as illustrated in Figure 5. (A moderator is necessary because fast, newly released neutrons are too readily absorbed by uranium-238 nuclei, which rarely split.) Apparently all that was needed to achieve this spectacular kind of a chain reaction was to assemble enough uranium in one place so that the released neutrons would have a good chance of finding another 235U nucleus before escaping from the pile. ”