Neutrons release

The nuclear chain reaction can be modeled mathematically by considering the probable fates of a typical fast neutron released in the system. This neutron may make one or more coUisions, which result in scattering or absorption, either in fuel or nonfuel materials. If the neutron is absorbed in fuel and fission occurs, new neutrons are produced. A neutron may also escape from the core in free flight, a process called leakage. The state of the reactor can be defined by the multiplication factor, k, the net number of neutrons produced in one cycle. If k is exactly 1, the reactor is said to be critical if / < 1, it is subcritical if / > 1, it is supercritical. The neutron population and the reactor power depend on the difference between k and 1, ie, bk = k — K closely related quantity is the reactivity, p = bk jk. i the reactivity is negative, the number of neutrons declines with time if p = 0, the number remains constant if p is positive, there is a growth in population. 

The two nuclei on the right side are just two of the many possible products of the fission process. Since more than one neutron is released in each process, the fission reaction is a self-propagating, or chain reaction. Neutrons released by one fission event may induce other fissions. When fission reactions are run under controlled conditions in a nuclear reactor, the energy released by... 

The moderator component of a reactor slows neutrons without capturing them. Moderators are used because the neutrons released in fission have such high kinetic energies that they are difficult to capture. The critical mass of a nuclear fuel is much smaller for slow neutrons than for fast neutrons, so considerably less fuel is needed in a... 

The neutron bursts take place in helium burning shells surrounding the inert carbon-oxygen core. The neutrons released here are grafted onto iron and its kin. 

The neutrons released from the fission of first uranium atom can hit other uranium atoms and which again release neutrons, each of which can further hit are uranium atom. In this way, a chain reaction is set up resulting into the liberation of a tremendous amount of energy. 

Control Rods The energy can be controlled by controlling the fission which, in turn can be controlled by controlling the number of neutrons released during fission. To control the fission cadmium or boron rods are used which absorb the neutrons... 

The three neutrons released by fission of a 235U nucleus can induce three more fissions yielding nine neutrons, which can induce nine more fissions yielding... 

The historical context of uncertainty estimation in exposure assessment can be traced to the convergence of developments in multiple disciplines. For example, Stanislaw Ulam and John von Neumann are typically credited with creation of the Monte Carlo method for simulation of random events in 1946 (see Metropolis Ulam, 1949 Eckhardt, 1987). However, a paper by Lord Kelvin in 1901 appears to apply concepts similar to Monte Carlo to a discussion of the Boltzmann equation, and there are other precedents (Kelvin, 1901). The modem incarnation of Monte Carlo was first used for prediction of neutron release during nuclear fission and has since been applied in a wide variety of disciplines. 

The development of cyclotrons and nuclear reactors in the middle of the 20th century made possible the production of radioactive isotopes which are not naturally present in any significant quantity on Earth. Thus in a nuclear reactor some of the neutrons released by uranium fission may be absorbed by leading to the formation of Pu. 

By reaction (11.4) Pu is produced in all reactors operated with uranium. Special types of reactors are designed with the aim of producing larger amounts of Pu or respectively. The concept of these reactors is to use one of the neutrons released by fission to initiate another fission, and a second one to produce another fissile atom. The ratio of the number of fissile atoms produced to the number of atoms used up by fission is called the conversion factor c. If c> 1, the reactor is called a breeder reactor if c < 1, it is called a converter. 

Merging of primordial matter into elementary particles, such as protons and neutrons release of huge amounts of energy, beginning of rapid expansion... 

The control rods are, in a sense, the dial by which the rate of fission is maintained within the core. When the rods are inserted completely into the core, most neutrons released during fission are absorbed, and no chain reaction occurs. As the rods are slowly removed from the core, the rate at which fission occurs increases. At some point, the position of the control rods is such that the 1 1 ratio of produced to used up neutrons is achieved At that point, the chain reaction goes forward, releasing energy, but under precise control of human operators. 

Release of this energy in a large-scale way is a possibility because of the fact that in each fission process, which requires a neutron to produce it, two neutrons are released. Consider a very great mass of active material, so great that no neutrons are lost through the surface and assume the material so pure that no neutrons are lost in other ways than by fission. One neutron released in the mass would become 2 after the first fission, each of these would produce 2 after they each had produced fission so in the nth generation of neutrons there would be 2" neutrons available. 

The energy distribution of the neutrons released in the fission process is shown in Fig. 8.2. The mean energy is about 2 MeV but an appreciable fraction of the neutrons released have less than 1 MeV of energy and so are unable to produce fission in U. 

The chance of predetonation is dependent on the likelihood of a neutron appearing in the active mass while v is still small and on the likelihood that such a neutron will really set off a chain reaction. With just a single neutron released when v > c it is by no means certain that a chain reaction will start, since any particular neutron may escape from the active material without causing a chain reaction. 

A sample of fissionable material must have sufficient mass in order for a fission chain reaction to occur. If it does not, neutrons escape from the sample before they have the opportunity to strike other nuclei and continue the chain reaction— the chain reaction never begins. A sample that is not massive enough to sustain a chain reaction is said to have subcritical mass. A sample that is massive enough to sustain a chain reaction has critical mass. When a critical mass is present, the neutrons released in one fission cause other fissions to occur. If much more mass than the critical mass is present, the chain reaction rapidly escalates. This can lead to a violent nuclear explosion. A sample of fissionable material with a mass greater than the critical mass is said to have supercritical mass. Figure 25-18 shows the effect of mass on the initiation and progression of a fission reaction. 

Number of fission neutrons released per neutron absorbed. 

A fusion reactor s impact on the environment will be limited to the site it occupies, and the waste heat left over when the reactor heat is used to generate electricity. The only significant radioactive waste disposal problems occur when the reactor has worn out and must be dismantled. In the decommissioning of fusion reactors, the internal parts will be radioactive from years of exposure to the neutrons released by the fusion reaction. The total amount of waste remaining will depend critically on the materials used in the fabrication of the reactor. With selection of the proper elements the radioactive waste disposal problem will be in the range of 10,000 to 1,000,000 times smaller than that involved in the dismantling of a fission reactor and its waste. 

Some of the neutrons released in the controlled chain reaction strike the nuclei of non-fissionable U-238 atoms. In this case, the U-238 captures the neutron and becomes a heavier uranium isotope, U-239, which eventually decays to produce plutonium-239. 

Objective 40 many of the neutrons released in the fission reactions of uranium-235 that a chain reaction cannot be sustained. One part of the solution to this problem for nuclear power plants is to create a uranium mixture that is enriched in uranium-235 (to about 3%). A typical 1000-megawatt power plant will have from 90,000 to 100,000 kilograms of this enriched fuel packed in 100 to 200 zirconium rods about 4 meters long (Figure 18.6). 

When fresh fuels rods are introduced, the control rods are partially inserted to absorb some of the neutrons released. As the uranium-235 reacts and its percentage of the total mixture in the fuel rods decreases, the control rods are progressively withdrawn. In this way, a constant rate of fission can be maintained, even as the percentage of the fissionable uranium-235 diminishes (Figure 18.6). 

The production of energy by nuclear fission in a nuclear reactor must be a controlled process. Neutrons released from the fission of lose most of their kinetic energy by passage through a moderator (graphite or D2O). They then undergo one of two nuclear reactions. The first is capture by leading to further fission the second... 

Nuclear fusion became important on Farth with the development of hydrogen bombs. A core of uranium or plutonium is used to initiate a fission reaction that raises the core s temperature to approximately 10 K, sufficient to cause fusion reactions between deuterium and tritium. In fusion bombs, LiD is used as Li reacts with fission neutrons to form tritium that then undergoes fusion with deuterium. It is estimated that approximately half the energy of a 50 megaton thermonuclear weapon comes from fusion and the other half from fission. Fusion reactions in these weapons also produce secondary fission since the high energy neutrons released in the fusion reactions make them very efficient in causing the fission of... 

In order for a chain reaction to occur at least one of the neutrons released in fission must produce a new fission event. This condition is defined by the multiplication factor k ... 

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