The analysis of neutron cycle characteristics which are determined the value of reactor effective multiplication factor.
Students Name: Matviichuk Oleksii
Qualification Level: magister
Speciality: Nuclear Рower Еngineering
Institute: Institute of Power Engineering and Control Systems
Mode of Study: full
Academic Year: 2023-2024 н.р.
Language of Defence: ukrainian
Abstract: Matviychuk O.V., Lys S.S. (head). The analysis of the characteristics of the neutron cycle, which determine the value of the effective multiplication factor of the reactor. Master’s thesis. - Lviv Polytechnic National University, Lviv, 2023. The main component of the fuel - 235U - is split by neutrons of any energy - thermal, intermediate and fast, but most efficiently - by thermal neutrons, and (thanks to the slowing down process organized in the thermal reactor) more than 95% of fissions of 235U nuclei occur under the influence of thermal neutrons. 239Pu, contained in the fuel, is also effectively split by thermal neutrons and, to a lesser extent, by epithermal neutrons. And 238U splits under the action of only fast (and far from any fast) neutrons. Threshold nature of fission of 238U in relation to core conditions means that fast fission neutrons born in fissions of 235U and 239Pu nuclei under the action of thermal neutrons and having initial energies above the fission threshold of 238U, starting to slow down inside fuel cells, have the opportunity to collide with 238U nuclei and cause their division. Therefore, the total number of fissions of fuel nuclei under the action of neutrons of all energies increases due to fissions of 238U nuclei by fast above-threshold neutrons. And this means that the total number of fast fission neutrons obtained in the fission of fuel nuclei by thermal neutrons increases due to the neutrons obtained in the fission of 238U nuclei by fast above-threshold neutrons. Moreover, the total number of distribution neutrons also increases due to distribution neutrons, The object of research is the active zone of a power reactor. The subject of research is the characteristics of the neutron cycle of a power reactor. The purpose of the work is to analyze the characteristics of the neutron cycle, which determine the value of the effective multiplication factor of the power reactor. To achieve these goals, it is necessary to solve the following tasks: - conduct an analysis of the propagating properties of the reactor; - to determine the coefficient of use of thermal neutrons; – to analyze uranium-238 and the propagating properties of the reactor; - determine the multiplication factor for fast neutrons; - establish the probability of avoiding resonance capture. In this master’s qualification work, it is shown that uranium-235 and plutonium-239 exert their influence on the propagation properties of the active zone through the influence on the values of two coefficients that determine the value of the effective propagation coefficient (ke), the constant ? and the thermal neutron utilization coefficient ?. Analytical dependences were obtained for calculating the coefficient of use of thermal neutrons ? in a homogeneous breeding medium and in heterogeneous cells. The order of estimated calculation of ? in multi-zone cells of active zones of thermal power reactors using the two-zone homogenization method is substantiated. Qualitative dependences of the value ? in power reactors on its main determining factors - fuel enrichment, uranium-water (uranium-graphite) ratio and temperature - were established. It is shown that with an increase in the value of the initial enrichment of the fuel, the concentration of uranium-235 nuclei in it increases, therefore the share of thermal neutrons absorbed by them also increases, i.e., the value of the coefficient of use of thermal neutrons. An increase in the value of the uranium-water ratio (it does not matter if it is due to an increase in the volume of the fuel composition in the cell or a decrease in the volume of water) also leads to an increase in the value of the coefficient of use of thermal neutrons in the reactor. The presence of uranium-238 in the active zone of a power reactor affects the reactor’s multiplication factor with the help of two coefficients - the fast neutron multiplication factor ? and the probability of avoiding resonance capture ?. It has been proven that the multiplication factor for fast neutrons is the value of the relative increase in the number of fission neutrons obtained in the fission of fuel by thermal neutrons, counting the fission neutrons obtained in the fission of fuel by epithermal neutrons. Its value ranges from 1 to 1.192, and practically in power reactors it does not exceed 1.05?1.06. Resonance capture of neutrons in the fuel element is selective: strong resonances of 238U absorb slowing neutrons in the relatively thin peripheral layers of the fuel element, thus forming a strong internal block effect in the distribution of the density of the resonant neutron flux; weak resonances absorb neutrons to varying extents in the entire volume of fuel oil and do not produce a significant internal block effect. According to this, the value of the effective resonance integral is divided into two unequal components, one of which has a constant value and is a measure of unblocked resonance absorption in fuel cells, and the other, which corresponds to the blocked resonance capture of neutrons by strong resonances, is variable in magnitude and depends on temperature. A functional scheme of automation of a group of low-pressure heaters of a nuclear power plant with a VVER-1000 type reactor has been developed and presented. Key words: core, neutron cycle, reactor multiplication factor, resonant neutron capture, homogenization, fuel enrichment, power reactor.