The influence of the heterogeneous effect in vaporization on the neutron-physical characteristics of BWR fuel assembly

Traditionally, when conducting neutron physics calculations in cases where the moderator is a steam + water system, the effect of vaporization, for example, in boiling reactors of the BWR type, is taken into account homogeneously by reducing the density of water in accordance with the proportion of steam. The paper attempts to investigate the effect of the heterogenic structure of water + vapor bubbles on the neutron-physical characteristics of the fuel assemblies of the BWR reactor. To create the model, data from the OECD/NEA Burnup Credit Criticality Benchmark Phase IIIB are used. The research is carried out using the SERPENT-2 software package which allows randomly scattering spheres of various radii filled with various materials in different areas of the reactor core. The calculations are performed using the JEFF-3.1.1 library. The dependence of the observed effect on the radii of steam bubbles and on the proportion of steam in the coolant is investigated. It is found that the differences for homogeneous and heterogeneous fuel assembly models can be up to 0.3% in the value of Kinf, which is significantly higher than the accuracy with which calculations are performed (~0.01%). It is shown that α decreases with increasing vapor bubble size when compared with the homogeneous model. Thus, a change in the neutron spectrum is justified, which affects the change in the multiplication coefficient. A calculating study of the void effect revealed that discrepancies reach 8% when considering models with different vapor contents and identical vapor bubble sizes. This value is important for the correct interpretation of the reactivity power effect in fuel optimization problems. In particular, it is relevant to justify the introduction of new fuels.

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