Neutron-induced reactions moderators

In a point neutron source, surrounded by a moderator, the thermal neutron flux (particles per unit area and unit time) as a function of the distance from the source is found initially to increase and then to decrease see Fig. 12.5. The distance from the source at which the flux is at maximum is given in Table 12.1. This is the optimum position for thermal neutron induced reactions. 

The is one of the radionuchdes produced by neutron-induced reactions in aU tjrpes of nuclear reactors. In a nuclear power facility the production of can occm in the fuel, the moderator, the coolant, and the core construction materials mainly by the reactions 0(n, a) C, and N(n, p) C. Part of the created in reactors is continuously released as air-borne effluents in various chemical forms (such as CO2, CO, and hydrocarbons) through the ventilation system of the power plant during normal reactor operation. Another part of the C produced is released into the atmosphere from fuel reprocessing plants. The enviromnental release of this reactor-derived C leads to an increase in atmospheric specific activity and hence, to an increased radiation dose... 

The fission process releases the large amounts of energy used in nuclear reactors and weapons and also several neutrons. The number of these neutrons depends on both the process and the fissioning atom, as indicated in the decay equations above. When the number of neutrons exceeds one per fission and the energy of the neutrons is suitably moderated to induce further fission, a chain reaction is induced that can be used for energy production. 

The first two-hour lecture session starts with a discussion of the fission process, toe production of neutrons during fission, the role of neutrons in inducing fission. Cross sections are introduced as measures of reaction probabilities, including capture-to-fission ratios. This leads into a discussion of fast fissioning systems, followed by neutron slowing down, moderator effects, and the criticality of solutions. Mixtures of finely divided fissile material, such as foil, wire, or powders, in moderating media are dtocussed as pseudo-solutions. 

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. 

In such systems (Fig. 7.5), spallation reactions induced by a high-intensity beam (10 to 250 mA) of GeV protons on a heavy target produce an intense neutron flux. These neutrons, after being more or less moderated, are used to drive a sub-critical blanket. The extra neutrons provided by the accelerator allow the maintenance of the chain reaction while burning the long-lived nuclear waste. The plant generates electricity, part of which is used to supply the accelerator. 

When used in place of hydrogen, deuterium or (sometimes designated as D) results in water approximately 10 percent denser than normal. Termed "heavy water," D O is harmless in small doses and can therefore be used safely as a tracer in the body, most commonly in measuring a subject s metabolic rate. Heavy water is also used as a neutron moderator, meaning it is able to slow neutrons by collisions without absorbing them.This process is crucial for the chain reaction in nuclear reactors, where fast neutrons are produced by the fission process, but slow or thermal neutrons are more likely to induce fission. 

There are two new ideas in the breeder program which are particularly worthy of attention. The first of these concerns the use of beryllium as moderator, as was first proposed, I believe, by Krasin et al. in Russia 11). It has been estimated that the (w,2n) reaction in Be may increase the total number of neutrons produced by fission by a factor as high as 1.12. A more realistic estimate is, perhaps, 1.07. The proposition has two drawbacks one economic and the other nuclear. The economic problem concerns the availability of beiyllium. This seems questionable. The nuclear problem centers about the facts that the (w,a) reaction, which leads to a neutron loss, can be induced by neutrons above 0.695-Mev energy but the (n,2w) reaction, which leads to the increase in the neutron number, only by neutrons with 1.85 Mev or more. However, the high-energy neutrons can induce also fast fi on in U and the question is, therefore, not really, Shall we make use of the n,2n) reaction in Be Instead, the proper question to be posed is, Does the (w,2w) reaction in Be or the fast fission reaction in U jdeld more neutrons ... 

Hydrogen occurs naturally in three isotopes. The most common ( H) accounting for more than 99.98% of hydrogen in water, consists of only a single proton in its nucleus. A second, stable isotope, deuterium (chemical symbol D or H), has an additional neutron. Deuterium oxide, D2O, is also known as heavy water because of its higher density. It is used in nuclear reactors as a neutron moderator. The third isotope, tritium, has 1 proton and 2 neutrons, and is radioactive, decaying with a half-life of 4500 days. T2O exists in nature only in minute quantities, being produced primarily via cosmic ray-induced nuclear reactions in the atmosphere. Water with one deuterium atom HDO occurs... 

An intimate mixture of an a emitter and Be will produce neutrons with a range of energies. When these are moderated i.e. slowed to thermal energy, they can induce an (n,y) reaction in some nuclei. This can be represented by ... 

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