On Accounting for the Heterogeneous Effect in Vaporization on the Neutronic Characteristics of BWR Fuel Assemblies

Traditionally, in neutronic calculations, when the moderator consists of a steam–water system, the steam generation effect, such as in boiling water reactors (BWR), is accounted for homogeneously by reducing the water density in accordance with the steam fraction. This work attempts to investigate the influence of the heterogeneous structure of water and steam bubbles on the neutronic characteristics of the BWR fuel assemblies. For creation of the model, data from the OECD/NEA Burnup Credit Criticality Benchmark Phase IIIB were used. The studies were conducted using the SERPENT-2 software package, which allows random placement of spheres of various radii filled with different materials in different regions of the reactor core. Calculations were performed using the JEFF-3.1.1 nuclear data library. The dependence of the observed effect on the steam bubble radius and the steam fraction in the coolant was examined. It was found that differences between homogeneous and heterogeneous fuel assembly models can reach up to 0.3% in the value of Kinf, which is significantly larger than the accuracy of calculations (~0.01%). It was shown that α decreases with increasing steam bubble size when compared to the homogeneous model. Thus, a change in the neutron spectrum is substantiated, which affects the multiplication factor. In the computational study of the void effect, discrepancies of up to 8% were revealed when comparing models with different steam contents but identical steam bubble sizes. This value is important for the correct interpretation of the power reactivity effect in tasks of optimizing fuel utilization. In particular, it is relevant for justifying the implementation of new fuel types.