3. The main types of interactions in the microphysics
Microphysics - a world on the level of elementary chastits.Processes involving different elementary particles vary greatly in their flow characteristic times and energies . According to modern ideas , occurs in nature are four types of interactions that can not be reduced to other more simple types of interactions : strong, electromagnetic , weak, and gravitational .These types of interactions are called fundamental.
Strong ( or nuclear ) interaction
- This is the most intense of all types of interactions. They creates an exceptionally strong bond between protons and neutrons in the nuclei of atoms. In the strong interaction may participate only heavy particles - hadrons ( mesons and baryons ) . Strong interaction is manifested at distances of less than 10-15 m and therefore it is called a short-range .
electromagnetic interaction
. In this kind of interaction can participate any electrically charged particles as well as photons - quanta of electromagnetic fields . Electromagnetic interaction is responsible , in particular, for the existence of atoms and molecules. It determines many properties of substances in solid, liquid and gaseous states . Coulomb repulsion of protons leads to instability of nuclei with large mass numbers . Electromagnetic interaction causes the processes of absorption and emission of photons by atoms and molecules of a substance , and many other physics processes of micro-and macrocosm.
weak interaction
- The slowest of all the interactions occurring in microcosm. It may participate in any of the elementary particles except photons. The weak interaction is responsible for the flow of processes involving neutrinos or antineutrinos , for example , β- decay of the neutron and the neutrinoless decay processes of particles with long lifetimes (τ ≥ 10-10 s).
gravitational interaction
common to all , without exception, the particles , but because of the smallness of the masses of elementary particles forces of gravitational interaction between them are negligible and in the processes of the microcosm of their role is inessential.
12. Collisions of neutrons in the reactor core
The reactor core has a volume ratio of water to fuel, which is less than 1, preferably, about 0.5 or less, and is remarkably smaller than the conventional reactor core of a light water reactor having a volume ratio of water to fuel, which is about 2.0 to 2.5. A ratio of coolant channel cross section to the fuel cross section of the reactor core is set preferably to about 0.5 or less. Accordingly, in the reactor core, a fissionable material such as plutonium or the like in the fuel is subjected to a fissile reaction by a neutron, and the heat and neutrons are generated.
A part of high energy neutrons (fast neutron) produced through the fissile reaction leaks outside the reactor core. However, most of high energy neutrons is moderated and scattered by the water as a coolant flowing between fuel rods, between these fuel rods and the channel box, and between the channel box and the control rod, and then, are again incident upon the fuel rod, thus contributing to the fissile reaction or the neutron absorption reaction.
In the case where the volume ratio of water to fuel is about 0.5, a moderation (slow-down) effect by water is small, and an average neutron energy is an energy for a water cooling reactor close to sodium fast breeder reactor. For this reason, the ratio of neutron capture reaction by fissionable material is small like the existing light water reactor, and the neutron per neutron absorption is much generated, for example, two or more. Thus, the neutron absorbed in a parent material (element) such as uranium 238 (U-238) or the like is much increased, and it is possible to set the breeding ratio to about 1, preferably, to a range from 1.0 to 1.1.
6. The fission cross section. The nuclear cross section of a nucleus is used to characterize the probability that a nuclear reaction will occur. The concept of a nuclear cross section can be quantified physically in terms of "characteristic area" where a larger area means a larger probability of interaction. The standard unit for measuring a nuclear cross section (denoted as σ) is the barn, which is equal to 10−28 m² or 10−24 cm². Cross sections can be measured for all possible interaction processes together, in which case they are called total cross sections, or for specific processes, distinguishing elastic scattering and inelastic scattering; of the latter, amongst neutron cross sections the absorption cross sections are of particular interest.
In nuclear physics it is conventional to consider the impinging particles as point particles having negligible diameter. Cross sections can be computed for any sort of process, such as capture scattering, production of neutrons, etc. In many cases, the number of particles emitted or scattered in nuclear processes is not measured directly; one merely measures the attenuation produced in a parallel beam of incident particles by the interposition of a known thickness of a particular material. The cross section obtained in this way is called the total cross section and is usually denoted by a σ or σT. The typical nuclear radius is of the order of 10−12 cm. We might therefore expect the cross sections for nuclear reactions to be of the order of πr ² or roughly 10−24 cm² and this unit is given its own name, the barn, and is the unit in which cross sections are usually expressed. Actually the observed cross sections vary enormously. Thus for slow neutrons absorbed by the reaction, the cross section is much higher than 1,000 barns in some cases (boron-10, cadmium-113, and xenon-135), while the cross sections for transmutations by gamma-ray absorption are in the region of 0.001 barn.
Nuclear cross sections are used in determining the nuclear reaction rate, and are governed by the reaction rate equation for a particular set of particles (usually viewed as a "beam and target" thought experiment where one particle or nucleus is the "target" [typically at rest] and the other is treated as a "beam" [projectile with a given energy]).
For neutron interactions incident upon a thin sheet of material (ideally made of a single type of isotope), the nuclear reaction rate equation is written as:
