Neutron absorbers induced reactions

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... 

The principles of balancing nuclear equations apply to all nuclear reactions. Nuclear fission occurs when a highly unstable isotope splits into smaller particles. Nuclear fission usually has to be induced in a particle accelerator. Here, an atom can absorb a stream of high-energy particles such as neutrons, Jn. This will cause the atom to split into smaller fragments. 

Instead of direct counting, neutron fluxes can also be determined by activation methods. Activation by (n, y) reactions (chapter 8) and subsequent measurement of the induced activity is a widely used technique. Au, In and Co are frequently applied as flux monitors. The presence of epithermal neutrons makes corrections necessary. Epithermal neutrons may be measured independently by wrapping the flux monitors in Cd or Gd which absorb the thermal neutrons. Fluxes of high-energy (fast) neu-... 

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. 

In order to estimate the induced activity, the neutron dose value and the effect of interfering reactions, the distributions of thermal, epithermal, and fast neutron flux densities should be determined in a bulk media or a phantom. The most commonly used dosimetry reactions are given in O Sect. 36.3 of the Appendix of this Volume. In the case of a phantom, the combination of the activation detectors and the physical integration methods is recommended for the determination of the volume averaged dose equivalent rates and through it the total dose absorbed by a given organ (Csikai et al. 1988). 

Fission-produced neutrons can be absorbed by any of the materials present in the reactor, with relative probabilities proportional to the neutron-absorption cross sections of the nuclei in the materials. Even for the fissile nuchdes, neutron-capture reactions compete favorably with neutron-induced-fission reactions. For example, low-energy neutron irradiation of a sample of results in the production of about half as often as it results in fission. Fuel containing the fissile actinides always contains some of the even-mass actinide nuclides as well, which are... 

The Th—cycle is of interest due to that the abundance of thorium in the Earth s cmst is between three to five times that of uranium (OECD/NEA and IAEA, 2014). In addition, there are large thorium deposits in some countries such as India, Brazil, Australia, and the USA (WNA, 2015a). The Pu cycle is the most effective with fast neutrons. For Pu, the number of emitted neutrons in a fission reaction per absorbed neutrons is greater when fission is induced by fast neutrons rather than thermal neutrons. The additionally emitted neutrons can be utilized for transforming more nuclides to Pu. Hence, the FBRs are based on Pu in which... 

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