Reaction, absorption fission

Absorptive reactions are (n, a), (n, p), in,y), or (n, fission). In the case of an (n, y) reaction, the neutron may be detected through the interactions of the gamma emitted at the time of the capture, or it may be detected through the radiation emitted by the radioisotope produced after the neutron is captured. The radioisotope may emit /3 or or y or a combination of them. By counting the activity of the isotope, information is obtained about the neutron flux that produced it. This is called the activation method. If the reaction is fission, two fission fragments are emitted being heavy charged particles, these are detected easily. 

The type of collision is determined from the macroscopic cross sections of the material present at the collision site. If the reaction is a neutron absorption reaction, the particle history is terminated if the absorption reaction is fission, the location of the collision is stored as a potential starting point for the next generation of neutrons. 

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 analysis of steady-state and transient reactor behavior requires the calculation of reaction rates of neutrons with various materials. If the number density of neutrons at a point is n and their characteristic speed is v, a flux effective area of a nucleus as a cross section O, and a target atom number density N, a macroscopic cross section E = Na can be defined, and the reaction rate per unit volume is R = 0S. This relation may be appHed to the processes of neutron scattering, absorption, and fission in balance equations lea ding to predictions of or to the determination of flux distribution. The consumption of nuclear fuels is governed by time-dependent differential equations analogous to those of Bateman for radioactive decay chains. The rate of change in number of atoms N owing to absorption is as follows ... 

Radioactivity occurs naturally in earth minerals containing uranium and thorium. It also results from two principal processes arising from bombardment of atomic nuclei by particles such as neutrons, ie, activation and fission. Activation involves the absorption of a neutron by a stable nucleus to form an unstable nucleus. An example is the neutron reaction of a neutron and cobalt-59 to yield cobalt-60 [10198 0-0] Co, a 5.26-yr half-life gamma-ray emitter. Another is the absorption of a neutron by uranium-238 [24678-82-8] to produce plutonium-239 [15117 8-5], Pu, as occurs in the fuel of a nuclear... 

Jeveral aspects of the photolytic behavior of aqueous complex ions have been studied in this laboratory over the past few years. One continually interesting question has been the extent to which the photochemistry of a complex depends on the absorption band irradiated. In the case of Co(III) acidopentamines, such as Co(NH3)5Br+2, we found that irradiation of Ajg —> g) bands showing appreciable charge transfer led to redox and aquation reactions which were competitive. It was reasonable to suppose that the common precursor was the species formed by a prompt heterolytic bond fission (7). The ( Aig —> Tig) band was far less photoactive, and in model cases, irradiation led only to aquation. Each excited state or excited state manifold thus tended to show a distinct photochemistry, which meant that conversion from one excited state to another was not important. 

The first two equations represent the fact that the D-D reaction can follow either of two paths, producing tritium and one proton or hehum-3 and one neutron, with equal probability. The products of the first two reactions form the fuel for the third and fourth reactions and are burned with additional deuterium. The net reaction consists of the conversion of six deuterium nuclei lnlo two helium nuclei, two hydrogen nuclei, and two neutrons along with a net energy release of 43.1 MeV. The reaction products—helium, hydrogen, and neutrons—are harmless as contrasted with the myriad fission products obtained in a fission reactor. The neutrons produced may be absorbed in sodium to produce an additional 0.25 MeV per cycle. Therefore, the D-D reaction produces at least 7 MeV per deuterium atom (deuteron) and, with absorption in sodium, more than 10 MeV per fuel atom. 

Fission Reaction. In nuclear fission, the nucleus of a heavy atom is split into tv/o or more fragments. The reaction is initiated by the absorption of a neutron. A typical reaction is... 

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 constant C was initially calculated for the nuclide zqzPu based on the first major resonance at 259 kJ/mol (2.68 eV) (12). For some nuclides, values of C were assumed based on the peak absorption cross section in the major resonance. Others were assumed based on proportionality to the resonance integral (which can be measured empirically without knowing the detailed energy-dependent spectrum). Then, these assumed values for C and also < 2200 were adJusted by trial and error procedures to produce reasonable agreement with experimentally determined tranmutation reactions. Table I shows values presently in use for the parameters 02200 and for both capture and fission for the transuranic nuclides considered in this program. 

One potential problem facing the nuclear power industry is the supply of " fU. Some scientists have suggested that we have nearly depleted those uranium deposits rich enough in U to make production of fissionable fuel economically feasible. Because of this possibility, breeder reactors have been developed, in which fissionable fuel is actually produced while the reactor runs. In a breeder reactor the major component of natural uranium, nonfissionable -9zU, is changed to fissionable Pu. The reaction involves absorption of a neutron, followed by production of two particles ... 

Fast breeder reactors are not operated, as e.g. light-water reactors, with slow neutrons, but with unmoderated fast neutrons as they occur immediately upon nuclear fission. These fast neutrons are necessary to sustain the chain reaction. The neutron yield per fission is here larger, since more neutrons are left over for the breeding process, once the neutrons lost by absorption and leakage have been subtracted. They are absorbed by or which are... 

The operation of the first atomic bomb hinged on the fission of uranium in a chain reaction induced by absorption of neutrons. The two most abundant isotopes of uranium are and whose natural relative abundances are... 

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. 

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