Аналіз критичності під час протікання важкої аварії у басейнах витримки відпрацьованого ядерного палива ВВЕР-1000

The article presents the results of the calculation analysis of the criticality of the reactor pool compartment for VVER-1000 reactors at different stages of a severe accident. The criticality analysis of fuel-containing systems formed during the development of a severe accident is considered on the example of several discrete configurations that can be described by certain models. In reality, the course of a severe accident will be a continuous process of transition from the initial stage of the accident to its steady state at the end. However, depending on the fuel characteristics and conditions of development of a severe accident, any considered configuration of fuel assemblies and reactor pool materials may become final. The article presents a criticality analysis of the reactor pool compartment of the VVER-1000 reactor unit at the severe accident stages following the deformation of fuel rods and fuel assemblies at the initial stage of the accident due to water boil-off and temperature increase. On the basis of the models developed, the multiplication properties of the reactor pool compartment in the conditions of degradation of hexagonal tubes were investigated. Supercriticality (i.e. multiplication factor was greater than 1) was achieved in all two considered configurations of the water-uranium mixture. That is, under such conditions, the required level of subcriticality can be provided by a boron solution in water. The considered homogeneous single-layer and two-layer model of corium spreading on the support plate is safe in terms of criticality. However, the support plate has holes for the passage of the coolant to the fuel assembly. In the case when the melt or fuel debris enters the space under the support plate through these holes, they can form a uranium-water mixture with the remaining water, which at certain fuel and water ratios can become supercritical. In this case, the multiplication properties of the reactor pool compartment will depend on the ratio of impurities of the reactor pool and fuel assembly structural elements in the material under the support plate and/or the concentration of boric acid in the water. After damage of the reactor pool compartment floor, the melting of concrete begins. The considered configurations and material composition of the mixture of corium, concrete and water allow us to make a conclusion about the safety of this phase of a severe accident in terms of criticality.