Neutron-induced fission

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... 

Some heavy nuclei will fission spontaneously. Others can be induced to fission through interaction with a neutron. In both spontaneous nuclear fission and induced nuclear fission the pool of neutrons and protons is conseiwed. For example, the nucleus "" Cf (Californium) fissions spontaneously. The 98 protons and 154 neutrons in the nucleus of Cf are reconfigured into other nuclei. Usually a few neu-... 

The nucleus """U (uranium) does not fission spontaneously, but It can be induced to fission through interaction with a neutron. Pictorially, a typical neutron-induced fission of " U producing two nuclei and three neutrons is depicted in Figure 2. 

The pictorial depiction of the neutron-induced fission of " U can be summarized as an equation ... 

Any combination of reaction products consistent with these conseiwation principles is possible. For example, in the neutron-induced nuclear fission of it is possible to produce Xe, Sr, two neutrons, and 185 MeV of energy. The most likely reaction products are close in atomic number to xenon pCe) and strontium (Sr), but the possibilities number in the hundreds. 

Describing a neutron-induced nuclear fission reaction such as... 

Fission gives a range of product nuclides. Neutron-induced fission of U yields the distribution shown... 

Fission chambers use neutron-induced fission to detect neutrons. The chamber is usually similar in construction to that of an ionization chamber, except that the coating material is highly enriched U235. The neutrons interact with the U235, causing fission. One of the two fission fragments enters the chamber, while the other fission fragment embeds itself in the chamber wall. 

The r-process path is terminated by (neutron-induced or yd-delayed) fission near A max = 270, feeding matter back into the process at around Amax/2, followed by recycling as long as the neutron supply lasts, assuming sufficient seed nuclei to start the process off. The number of heavy nuclei is thus doubled at each cycle, which could take place in a period of a few seconds, yd-delayed fission also occurs after freeze-out, when the yd-decay leaves nuclei with A > 256 or so with an excessive positive charge (see Eq. 2.90). 

Nevertheless, the earth s crust contains technetium. Tc is formed by spontaneous fission of U as well as by slow neutron-induced fission of The first isolation of naturally occurring technetium was reported by Kenna and Kuroda , who isolated about 10" ng Tc from 5.3 kg of pitchblende. 

Among the long-lived isotopes of technetium, only Tc can be obtained in weigh-able amounts. It may be produced by either neutron irradiation of highly purified molybdenum or neutron-induced fission of uraniimi-235. The nuclides Tc and Tc are exclusively produced in traces by nuclear reations. Because of the high fission yield of more than 6%, appreciable quantities of technetimn-99 are isolated from uranium fission product mixtures. Nuclear reactors with a power of 100 MW produce about 2.5 g of Tc per day . 

Additional interactions of neutrons with nuclei include die release of charged particles by neutron-induced nuclear disintegration, Commonly known reactions are n-p. n — d. and n—ct. In these cases, the incident neutrons may contribute part of their kinetic energy to the target nucleus to effect the disintegration. Hence, more than mere neutron capture is involved, Then, there is usually a lower threshold for the neutron energy below which the reaction fails to occur, Another important reaction involving neutrons is fission, which may occur under different conditions for eidier slow or fast neutrons with appropriate fissionable material. 

Tritium is also produced in ternary fission and by neutron-induced reactions with 6Li and 10B. Tritium is a very low energy (3 emitter with a half-life of 12.33 y. The global inventory of naturally produced tritium is 9.6 x 1017 Bq. Tritium is readily incorporated in water and is removed from the atmosphere by rain or snow. Its residence time in the stratosphere is 2-3 y after reaching the troposphere it is removed in 1-2 months. The natural concentration of 3H in streams and freshwater is 10 pCi/L. 

Figure 11.8 Neutron total and subbarrier fission cross sections of 240Pu as a function of neutron energy between 0.5 and 3 keV. (From H. Weigmann, Neutron-Induced Fission Cross Sections in C. Wagemans, The Nuclear Fission Process. Copyright 1991 CRC Press. Reprinted by permission of CRC Press.)...

Figure 11.13 Smoothed fragment mass distributions for the thermal neutron-induced fission of 233U, 235U, and 239Pu. [From Seaborg and Loveland (1990).]...

Figure 11.18 Yields of products from the thermal neutron-induced fission of 235U. (From A. C. Wahl. Nuclear Charge Distribution in Fission, in New Directions in Physics, N. Metropolis, D. M. Kerr, and G. C. Rota, Eds. Copyright 1987 by Academic Press, Inc. Reprinted by permission of Elsevier.)...

The yield of each member of the isobaric series integrates, by virtue of the intervening (3 decay, the yields of its precursors. Such yields are referred to as cumulative yields. For example, the cumulative yield of the mass 140 chain in the thermal neutron-induced fission of 235U is 6.25%. 

Example Problem In the above example, what is the independent yield of 140Ba for the thermal neutron-induced fission of 235U and what is its cumulative yield ... 

Figure 11.19 Average total number of neutrons, vT, as a function of the mass number of the fissioning system for spontaneous and thermal neutron-induced fission. The values for thermal neutron-induced fission have been corrected to zero excitation energy (spontaneous fission) assuming dv jdt = 0.12 MeV-1.

The fact that neutrons can be absorbed by nuclei without overcoming a threshold (1 = 0 or s-wave reactions) makes neutrons extremely effective nuclear reactants. Neutron-induced reactions are the energy source for present-day commercial nuclear power (fission reactors) while charged-particle-induced reactions remain under study as power sources (fusion reactors). In this chapter we will consider the general features of nuclear fission reactors, following by the general features... 

Nuclear fission reactors ( nuclear power reactors ) are devices that use controlled neutron-induced fission to generate energy. While a complete description of the design of these devices is beyond the scope of this book, there are certain basic principles related to nuclear reactors that are worth studying and that can be described and understood with a moderate effort. 

Figure 14.1 Neutron-induced fission cross section for 235U and 238U as function of the neutron energy, En. (From D. T. Hughes and R. B. Schwartz, Neutron Cross Sections, 2nd ed., Brookhaven National Laboratory Report 325, 1958.)...

Let us review some aspects of fission discussed in Chapter 11. Consider the case of the thermal neutron-induced fission of 235U, that is,... 

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