Computational Modeling of the Neutronic Characteristics of the Core of a Water-Cooled Reactor with a Variable Neutron Spectrum

A calculated estimate of the neutronic characteristics of a light-water reactor core with a changing neutron spectrum is presented. The software tool DESNA-7 neutron physics module, designed for three-dimensional modeling of the core in a two-group approximation, is used. The calculation of the neutron cross sections is carried out in the software tool SAPHIR-95.1. Fuel assembly models with guide channels for the placement of absorbing elements or mobile displacers is developed, forming two cores with the same thermal power and different ways of regulating reactivity: boron and spectral (with partial boron). To compensate the excess reactivity partially, gadolinium fuel rods in the form of Gd2O3 with a concentration of 5 wt % are used. Uranium oxide with natural content of U235 is used as the material for the displacers. The two methods of displacer extraction are considered: linear displacement during the campaign and complete extraction of displacers (stationary from the beginning of the campaign) on day 326. They do not lead to a significant change in the critical concentration of boric acid. It is shown that the use of spectral regulation makes it possible to reduce the concentration of boric acid in the coolant by 35%. It is shown that the reactivity coefficients in terms of temperature of fuel and coolant and density of coolant increase in absolute value during the campaign, maintaining their sign. The value of the reactivity coefficient for the concentration of boric acid in the first reactor circuit, depending on the water–uranium ratio, decreases in absolute value. The maximum value of the axial power peaking factor in the core with spectral regulation is 1.6. The possibility of using absorbing rods placed in the fuel assembly guide channels as emergency protection devices is considered.