Neutrons produced in fission

Of the fast neutrons produced in fission, some of them will be moderated to thermal energies and will induce other fission reactions while others will be lost. The ratio of the number of neutrons in the next generation to that in the previous generation is called the multiplication factor k. If the value of k is less than 1, then the reactor is subcritical and the fission process is not self-sustaining. If the value of k is greater than 1, then the number of fissions will accelerate with time and the reactor is supercritical. The goal of reactor operation is to maintain the system in a critical state with k exactly equal to 1. The extreme upper limit for the multiplication factor would correspond to the mean number of neutrons per fission ( 2.5 for 235U(n,f)) if each neutron produces a secondary fission. 

Some nuclear reactors are fueled with a mixture of fissile and fertile thorium. Figure 3.2 is a similar diagram showing the principal nuclear reactions that take place in such fuel. The effect of irradiation on U is the same as in Fig. 3.1. However, neutrons produced in fission of U are now absorbed in Th to produce short-lived Th, which decays to 27-day Pa. Most of this decays to fissile U, but in reactors with a thermal-neutron flux above 5 X 10 an appreciable fraction absorbs a neutron to make Pa, which then decays to U. In a... 

Are all neutrons produced in fission available to cause other fission reactions Explain. 

The fact that the fission process involves the emission of secondary neutrons leads immediately to the possibility of setting up a chain-reacting system. We start by considering the problem of designing a nuclear reactor in which the fuel is natural uranium. The criterion for a successful chain reaction is the following starting with a certain number of fission events taking place per unit time, it is necessary that the fraction of the secondary neutrons produced in fission which survive to cause further fissions should be sufficient to maintain the fission rate in the system at a constant level. 

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. 

Neutrons produced in a chain reaction are moving very fast, and most escape into the surroundings without colliding with another fissionable nucleus. However, if a large enough number of uranium nuclei are present in the sample, enough neutrons can be captured to sustain the chain reaction. In that case, there is a critical mass, a mass of fissionable material above which so few neutrons escape from the sample that the fission chain reaction is sustained. If a sample is supercritical,... 

The most efficient way to make elements 93 and 94 uses neutrons produced during fission in nuclear reactors... 

A chain reaction is a reaction that sustains itself once it has begun and may even expand. Normally, the limiting reactant is regenerated as a product to maintain the progress of the chain. Nuclear fission processes are considered chain reactions because the number of neutrons produced in the reaction equals or is greater than the number of neutrons absorbed by the fissioning nucleus. 

A hydrogen bomb produces a lot of fi ssion energy as well as fusion energy. Some of the fission is in the fission bomb trigger used to ignite the thermonuclear reaction and some is in fissionable material that surrounds the thermonuclear fuel. Neutrons produced in fusion cause more fission in this blanket. Fallout results mainly from the fission. 

Because of the large number of possible neutron-rich fragments produced in fission, the study of the y rays emitted by the fragments can lead to useful information about the nuclear structure of these exotic, short-lived nuclei far from stability. 

Thus, some of the neutrons emitted in fission of IT285 produce further fissions, and others form U239, which shortly decays to Pu239, which may be separated from the uranium sections of the reactor chemically and used in nuclear bombs. 

The problem is that the actual ratio of neutrons produced to neutrons used up in a fission reaction is closer to 2 1 or 3 1. That is, neutrons are produced so rapidly that the chain reaction goes very quickly and is soon out of control. By correctly positioning control rods in the reactor core, however, many of the excess neutrons produced by fission can be removed from the core and the reaction can be kept under control. 

Reactors with other functions are also in use. For example, a breeder reactor is one in which new reactor fuel is manufactured. By far the most common material in any kind of nuclear reactor is uranium-238. This isotope of uranium does not undergo fission and does not, therefore, make any direct contribution to the production of energy. But the vast numbers of neutrons produced in the reactor core do react with uranium-238 in a different way, producing plutonium-239 as a product. This pluto-nium-239 can then be removed from the reactor core and used as a fuel in other reactors. Reactors whose primary function it is to generate plutonium-239 are known as breeder reactors. 

The average number of neutrons produced per fission is denoted by v. At that time it was not known whether v has the same value for fission processes in different materials, induced by fast or slow neutrons or occurring spontaneously. 

In a nuclear reactor, represented in Figure 12, the fuel rods are surrounded by a moderator. The moderator is a substance that slows down neutrons. Control rods are used to adjust the rate of the chain reactions. These rods absorb some of the free neutrons produced by fission. Moving these rods into and out of the reactor can control the number of neutrons that are available to continue the chain reaction. Chain reactions that occur in reactors can be very dangerous if they are not controlled. An example of the danger that nuclear reactors can create is the accident that happened at the Chernobyl reactor in the Ukraine in 1986. This accident occurred when technicians briefly removed most of the reactor s control rods during a safety test. However, most nuclear reactors have mechanisms that can prevent most accidents. 

The fission within a nuclear reactor is started by a neutron-emitting source and is stopped by positioning the control rods to absorb virtually all of the neutrons produced in the reaction. The reactor core contains a reflector that acts to reflect neutrons back into the core where they will react with the fuel rods. A coolant, usually water, circulates through the reactor core to carry off the heat generated by the nuclear fission reaction. The hot coolant heats water that is used to power stream-driven turbines which produce electrical power. 

MeV = 10 eV = 8.07 x 10 cm = 9.65 xlO kJ mol ). Of the 2.5 neutrons produced per fission event, one is required to maintain the nuclear reaction, 0.5 neutrons are lost to absorption and one is available to leave the core and be used experimentally. Since occurs naturally at only 0.7% abundance, the use of enriched (>90% U) uranium is required. This has lead to concerns about nuclear weapons proliferation and there is a drive to use lower levels of enrichment in research reactors. 

In thermal reactors fueled with plutonium, the number of neutrons produced per neutron absorbed is less than 2.0 and breeding is impossible. For U, on the other hand, this number is substantially greater than 2.0, and breeding is practicable in a thermal reactor. In fast reactors, the number of neutrons produced per neutron absorbed is close to the total number of neutrons produced per fission, so that breeding is possible with both and plutonium. Breeding as here defined is not possible with U, because there is no naturally occurring isotope from which can be produced. 

Neutrons produced from fission are absorbed in to produce short-lived U, which decays successively to Np and fissile Pu. In most fuel-cycle analyses, it is permissible to assume that neutron absorption by U results in immediate formation of Pu. 

In thermal-neutron reactors has an important advantage over or Pu in that the number of neutrons produced per thermal neutron absorbed, tj, is higher for than for the other fissile nuclides. Table 6.1 compares the 2200 m/s cross sections and neutron yields in fission of these three nuclides. Thorium has not heretofore been extensively used in nuclear reactors because of the ready avaUabihty of the U in natural or slightly enriched uranium. As natural uranium becomes scarcer and the conservation of neutrons and fissile material becomes more important, it is anticipated that production of U from thorium will become of greater significance. 

Another radiation problem arises from fast neutrons produced in spontaneous fission of the even-mass plutonium isotopes. Half-lives and specific activities for spontaneous fission of the plutonium isotopes are listed in Table 8.17. 

Fission cross sections are denoted by For fissionable isotopes of thorium and elements of higher atomic number, the average number of neutrons produced per fission is listed in the same row as the fission cross section, in the same column as the mass, to conserve space in the table. The average number of prompt and delayed neutrons produced by fission with a thermal neutron is denoted by V. The average number of prompt neutrons produced by fission with a thermal neutron is denoted by Vp. The average number of neutrons emitted per spontaneous fission is denoted by t jp. ... 

To obtain absolute yields directly, it is necessary to determine, not only the absolute number of atoms of a nuclide produced in fission, but, also, the total number of fissions. The determination of the number of fissions in a sample of a fissile element irradiated with neutrons is a most complex problem and cannot be discussed in detail here. In general, the number of... 

Very small amounts of Np, as well as of Pu, have been discovered on earth the half-lives of Pu (in the 4/i -t- 3 series) is 2.411 x 10 y. Both isotopes are too short-lived to have survived the 4 eons since the solar system was formed. However, they are always found in minerals containing uranium and thorium and it is believed that the neutrons produced in these minerals through (o(,n) and (y,n) reactions with U and Th as well as by spontaneous fission of form the neptunium and plutonium through n-capture and 3-decay processes. The n-production rate in the uraniuniJ eisl pitchblende (containing —50% U) is about 50 n/kg s. The typical value for the " Pu/" U ratio in minerals is 3... 

Because the NIZ ratio for 92 (the fissioning nucleus) is 1.57, while the ratio necessary for stability is 1.2 - 1.4 in the elements produced in fission, fission fragments always have a too large NIZ ratio. This is partially compensated by the emission of several neutrons in the act of fission, prompt neutrons. However, the number of neutrons emitted is not sufficient to lower the NIZ ratios to stable values. To achieve further lowering, the fission fragments, after neutron emission, undergo a series of radioactive decay steps in which j8 -particles are emitted. Since the jS-decay occurs with no change in A, successive 8-decay... 

The first fusion reactors probably will use the DT-reaction, as the DD-reaction requires higher temperatures. Figure 17.5. The DT-reaction yields on the average 0.5 neutrons. Provided the blanket consists of only Li, this produces 0.5 new T-atoms (per consumed T-atom) if the blanket also contains Li, the yield of new T will be less. Therefore, the DT-fiision reactor must be fed continually with new tritium, produced in fission reactors. This demand will not be eliminated imtil the DD-ftision reactor comes into operation. 

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 ... 

The fission neutrons produced in nuclear reactors have a continuous kinetic-energy spectrum, mostly in the range of 1-10 MeV. Since (n, y) reactions are of more widespread analytical use, fission neutrons must be slowed to thermal energies by passing them through HjO, D2O, or graphite, which act as moderators. Depending on the type of nuclear reactor and the irradiation position in the reactor, the neutron spectrum may vary widely. Therefore, both (n, y) and threshold reactions can occur in samples placed in nuclear reactors. Threshold reactions may produce interferences, of which the experimenter should be aware. 

Not all the induced fission reactions involving are equally likely, and experiments have shown that, on average, there are 2.5 neutrons produced for every atom that is broken up. This means that more neutrons are generated in nuclear fission than are used up (Fig. 21.4). It is this fact that allows, in principle, the fission of to be self-sustaining, the fission occurring at a faster and faster rate as the neutrons produced induce fission in the remaining U-235 nuclei, causing a chain reaction. 

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