Self-sustaining chain reaction

Am undergo fission with thermal neutrons of these isotopes and Pu are the most important as they are most readily obtainable. Other heavy nuclei require fast neutrons to induce fission such neutrons are much more difficult to control into a self-sustaining chain-reaction. 

A key parameter in determining the possibiUty of a self-sustained chain reaction is the value of k for an infinite medium, k. In the four-factor formula,... 

In early 1941, 0.5 )-lg of Pu was produced (eqs. 3 and 4) and subjected to neutron bombardment (9) demonstrating that plutonium undergoes thermal neutron-induced fission with a cross section greater than that of U. In 1942, a self-sustaining chain reaction was induced by fissioning 235u... 

FIGURE 17.24 A self-sustaining chain reaction, in which neutrons are the chain carriers, takes place when induced fission produces more than one neutron per fission event. These newly produced neutrons can stimulate fission in increasingly greater numbers of other nuclei. 

The three neutrons produced when uranium splits have the ability to split other U-235 nuclei and start a self-sustaining chain reaction. Whether a chain reaction takes place depends on the amount of fissionable material present. The more fissionable material that is present, the greater the probability that a neutron will interact with another U-235 nucleus. The reason for this involves the basic relationship between surface area and volume as mass increases. If a cube with a length of 1 unit is compared to a cube of 2 units, it is found that the surface area to volume ratio of the 1 unit cube is twice that of the 2 unit cube (Figure 17.6). This shows that volume increases at a greater rate than surface area as size increases. The probability that neutrons escape rather than react also depends on the surface area to volume ratio. The higher this ratio is the more likely neutrons escape. When a U-235 nucleus contained in a small mass of fissionable uranium is bombarded by a neutron, the... 

Subcritical Mass when the quantity of radioactive fuel is insufficient to produce a self-sustaining chain reaction Sublimation process where a substance passes directly from the solid to gaseous phase without going through the liquid phase... 

Supercritical Mass when the quantity of a radioactive fuel is sufficient to produce a self-sustaining chain reaction Supernova massive explosion of giant star at the end of its life... 

The second method makes use of the fact that when a subcritical quantity of an appropriate isotope, that is, 239Pu (or 235U), is strongly compressed, it can become critical or supercritical. The reason for this is that compressing the fissionable material, that is, increasing its density increases the rate of production of neutrons by fission relative to the rate of loss by escape. The surface area (or neutron escape area) is decreased, while the mass (upon which the rate of propagation of fission depends) remains constant. A self-sustaining chain reaction may then become possible with the same mass that was subcritical in the uncompressed state. 

The paper of 1939 [1 ], On the Chain Decay of the Main Uranium Isotope, studies the effects of elastic and non-elastic neutron moderation and concludes that chain fission reactions by fast neutrons in pure metallic natural uranium are impossible. The 1940 paper, On the Chain Decay of Uranium under the Influence of Slow Neutrons [2 ], is classic in the best sense of this word its value is difficult to overestimate. The theoretical study performed showed clearly that the effect of resonance absorption of neutrons by nuclei of 238U is a governing factor in the calculation of the coefficient of neutron breeding in an unbounded medium it was concluded that a self-sustained chain reaction in a homogeneous natural uranium-light water system is impossible. 

The sequence of reactions 2,3, and 4 has a chain branching factor of 2, but, the overall chain branching factor for 1, 2, 3, and 4 is 2(1- a) where a is the ratio of the rate constant for reaction 2 to the sum of the rate constants of reactions 1 and 2. Since reaction 1 is faster than 2, a is less than one. This means that for a self sustaining chain reaction to occur another reaction which provides additional chain branching is necessary. It is generally agreed that in the Thermal DeNOx process reaction 5 has that role... 

The fact that the Thermal DeNOx chemistry has a lower temperature limit when NH3 alone is used as the reducing agent is explained by the temperature dependence of reactions 1 and 2. As the temperature is decreased, the ratio of the rates of these reactions shifts in favor of reaction 1. More of the interactions of NH2 and NO lead to chain termination rather than chain branching and maintaining a self sustaining chain reaction becomes more difficult. 

The neutrons produced by this fission reaction can potentially collide with other U-235 nuclei. The likelihood of the extra neutrons striking other nuclei increases as the mass of the sample increases. At a characteristic mass, the neutrons are assured to collide with U-235 nuclei, and as a result, a chain reaction begins. In this chain reaction, the neutrons from one fission will strike other nuclei and cause additional fission reactions. The mass at which a self-sustaining chain reaction will occur is known as the critical mass. Fission reactions are responsible for the production of nuclear power and for the design of nuclear weapons. 

The very first nuclear reactor built, where the main objective was to perform condensed matter research, was the High Flux Beam Reactor (HFBR) at Brookhaven National Laboratory, Upton, NY. The first self-sustaining chain reaction at the HFBR took place on Halloween, 1965. For over 30 years, the HFBR was one of the premier beam reactors in the world, matched only by the ILL reactor in Grenoble, France. These reactor-based sources have been a continuous and reliable source of thermal neutrons for research in a wide range of different scientific fields from physics, chemistry, materials science, and biology to engineering and isotope emichment. The instrumentation that is in place at these sources has seen steady improvement from the days when Nobel laureates, Brockhouse and Shull, performed their pioneering work at these facilities. 

Representation of a fission process in which each event produces two neutrons, which can go on to split other nuclei, leading to a self-sustaining chain reaction. 

Critical mass the mass of fissionable material required to produce a self-sustaining chain reaction. (21.6)... 

Fermi s responsibility was to study the reaction that occurs when uranium atoms are bombarded by neutrons. His team did most of this research at the University of Chicago. On December 2, 1942, the team made an important breakthrough. They produced the first self-sustaining chain reaction in history. A self-sustaining chain reaction is one in which neutrons split uranium atoms apart. Large amounts of energy are produced in the reaction. Additional neutrons are also formed. These neutrons can be used to make the reaction repeat over and over again. The reaction eventually formed the basis of the first atomic bombs built three years later. 

Nuclear energy cannot be produced by a self-sustained chain reaction in thorium alone because natural thorium contains no Bssile isotopes. Hoice the thorium-uranium cycle must be started by using enriched uranium, by irradiation of thorium in a uranium- or plutonium-fueled reactor or by using a strong external neutron source, e.g. an accelerator driven spallation source. 

The fission of U-235 is used exclusively in nuclear power plants located in the United States. There are many different fission reactions of U-235, but all the fission reactions are self-sustaining chain reactions. Explain. Differentiate between the terms critical, subcritical, and supercritical. What is the critical mass How does a nuclear power plant produce electricity What are the purposes of the moderator and the control rods in a fission reactor What are some problems associated with nuclear reactors What are breeder reactors What are some problems associated with breeder reactors ... 

In many publications, any reactions of oil decomposition are named using the term cracking. For the purpose of this chapter, cracking of hydrocarbons will be defined as a self-sustaining chain reaction of hydrocarbon molecule disintegration initiated by thermal or other action. 

A periodic comprehensive and targeted check of the state of nuclear safety by a board of Russian Federation Gosatomnadzor [the Federal Inspectorate for Nuclear and Radiation Safety]. It should be noted that thanks to the well-honed system for nuclear-safety assurance, there has not been a single instance of a self-sustaining chain reaction at the radiochemical plant in its entire operating life (more than 34 years), and there have been no cases in which safe parameters were exceeded with respect to the mass or concentration of fissile material in equipment. 

Therefore, the problems which faced the would-be designers of chain reactors early in 1941 were (1) the choice of the proper moderator to uranium ratio, and (2) the size and shape of the uranium lumps which would most likely lead to a self-sustaining chain reaction, i.e., give the highest multiplication factor. In order to solve these problems, one had to understand the behavior of the fast, of the resonance, and of the thermal neutrons. We were concerned with the second problem which itself consisted of two parts. The first was the measurement of the characteristics of the resonance lines of isolated uranium atoms, the second, the composite effect of this absorption on the neutron spectrum and total resulting absorption. One can liken the first task to the measurement of atomic constants, such as molecular diameter, the second one, to the task of kinetic gas theory which obtains the viscosity and other properties of the gas from the properties of the molecules. The first task was largely accomplished by Anderson and was fully available to us when we did our work. Anderson s and Fermi s work on the absorption of uranium, and on neutron absorption in general, also acquainted us with a number of technics which will be mentioned in the third and fourth of the reports of this series. Finally, Fermi, Anderson, and Zinn carried out, in collaboration with us in Princeton, one measurement of the resonance absorption. This will be discussed in the third article of this series. 

In addition to the above-mentioned losses which are inherently a part of the nuclear chain reaction process impurities present in both the slowing material and the uranium add a very important neutron loss factor in the rhaiti. The effectiveness of various elements as neutron absorbers varies tremendously. Certain elements such as boron, cadmium samarium gadolinium and some others, if present even in a few parts per million could prevent a self-sustaining chain reaction from taking place. 

---