Neutron reactions

In fission reactors the transmutation reactions of principal importance involving neutrons are capture and fission. All nuclides (except He) take part in the radiative capture reaction ( , y), an example of which is 

This reaction produces an isotope of the reacting nuclide with mass number increased by unity and one or more gamma rays, which carry off most of the energy of the reaction. Other capture reactions, possible for a few nuclides (mostly those of low mass number), result in emission of an alpha particle (n, a)  

The fission reaction is responsible for the sustained production of neutrons in a nuclear reactor and for most of the energy released. In this reaction, one neutron is absorbed by a heavy nuclide, which then splits into two nuclides each in the middle third of the periodic table, and several neutrons, which are available for initiating additional fissions. All elements beyond lead undergo fission with neutrons of sufficiently hi energy the only readily available long-lived nuclides that undergo fission with thermal neutrons are U, Pu, and Pu. 

The number of nuclei reacting in a specified way with neutrons in unit time is proportional to the number of nuclei present and to the concentration of neutrons. In the language of chemical kinetics, neutron reactions are first-order with respect to concentration of nuclei and neutrons, and it is because neutron reactions are simple first-order Irreversible processes that a very detailed quantitative treatment of the rate processes in a nuclear reactor can be given. 

The expression for the rate of reaction of neutrons with reacting nuclei N is 

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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... 

This NAA technique is based on the nuclear reactions 23Na(n,7)24Na and 41K(n/y)42K. Half-lives of the activated products are 15.0 hrs and 12.4 hrs, respectively. For Na analysis, the samples were irradiated in a specially designed thermal column to suppress the fast neutron reaction of 27Al(n,a)24Na which interferes with the reaction for Na. For K analysis, the proplnt samples were irradiated at a standard irradiation position of the reactor. For the Na irradiations, the neutron flux in the thermal column was in the order of 1010, whereas for the K assays it was approx 1012 neutrons/cm2-sec... 

Thru 1967, emphasis was given to the use of neutrons as the bombarding source of radiation. Almost all possible neutron reactions were considered including moderation of fast neutrons by hydrogen in the expl, thermal capture reactions, elastic and inelastic scattering of neutrons and neutron activation reactions. These neutron reactions are listed as follows ... 

Tritium. In natural hydrogen it occurs in amounts of 1 in 1017-1018. It is continuously formed in nuclear reactions induced by cosmic rays, and it is radioactive. It may be made, from lithium, in nuclear reactors by the thermal neutron reaction 6Li( ,a)3H. 

The data of Table I are derived from early time radiochemical data reported by Stevenson (5). The linearity of the radionuclide ratios was first pointed out in that report. The aerial filter samples were taken at successively later times, 1 and 2, below the reported cloud base, and 3, 4, and 5 in the cloud. The tabulated values of rA correspond to atom ratio of isotope A to an arbitrary refractory isotope normalized by dividing by the atom ratio in which the two species were formed. Refractory species include the isotopes of the rare earths Eu and Tb as well as 45Ca, 89Zr, Sc, and others produced by neutron reactions on stable isotopes. The tabulation has been limited to fission product species. However, the... 

Side Neutron Reactions in Water One of the interesting side reactions that occurs in water-moderated nuclear reactors is the (n,p) reaction on lfiO, which occurs with a cross section of 0.017 mbams. The 16N product rapidly decays back to lfiO with a half-life of 7.13 s thus, the net reaction can be called a catalysis of the neutron (3 decay. 

Neutron activation This is the term used for neutron reactions that result in the formation of radioactive nuclides. This type of reaction is used in solid-state use-once devices containing one or more materials that are activated by neutron radiation. The induced activity of each material can be measured and the neutron exposure can then be calculated from these activities. 

In 1972, Ziegler, et al. (p first reported the development of a near-surface technique complimentary to those mentioned above. The technique uses neutron reactions to measure absolute concentration versus depth profiles of a number of the light elements. Neutron... 

We are limited in this modeling process by the accuracy with which measurements can be made and by the accuracy of the fission yields and neutron reaction cross sections which are used to interpret the results. As an example consider the Nd- Nd fission product pair, which has been used as an indicator of thermal neutron fluence because the capture cross section for the former is large and for the latter is small. The thermal cross section for l53Nd has recently been listed as 325 ( 10) barns (20), and more recently as 266 barns (11). Using the 325-barn value we deduce an age of about 2 to 27T billion years from neodymium to uranium ratios in the Oklo reactors, while an age of about 1.8 billion years is obtained using the 266-barn figure. 

Table 2.6 Examples of neutron reaction cross sections...

The change in number of atoms of neutron-absorbing nuclide N with time due to neutron reactions alone, and in the absence of a source of this nuclide, is... 

Cm, Cm, etc., and by radionuclides formed by neutron reactions with fuel structural material, such as metal cladding. Methods and illustrative data that can be used in estimating the concentrations of such radionuclides and their contributions to decay heat are discussed in Chaps. 3 and 8. 

However, we may wish to calculate nuclide amounts in a chain wherein some members may be formed by neutron reactions with their individual precursors. We define here a linear chain as one in which each nuclide other than the first is formed directly only from a single precursor, illustrated as follows ... 

To calculate the growth and decay of these nuclides after reactor shutdown, the assumed equilibrium amounts at the time T of shutdown are calculated as above, using Eq. (2.114). These become the initial amounts for application of the batch decay, Eq. (2.18) for time t after shutdown. During shutdown the branching and convergence involving neutron reactions disappear, and we have only four simple linear chains to solve by applying Eq. (2.18). 

In addition to tritium produced by ternary fission, as shown in Table 8.1, tritium is also produced in reactors by neutron reactions with lithium, boron, and deuterium. Reactors can be designed to produce tritium by irradiating lithium targets with thermal neutrons, resulting in the (/I, a) reaction ... 

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