The State of the Scientific Problem Concerning the Study of the Main Characteristics of the Molten Corium-Concrete Interaction Products
Authors:   Institute for Safety Problems of Nuclear Power Plants, NAS of Ukraine, Sushchenko K. O. , Nosovskyi A. V. 
Published: December 2023

In severe beyond-design basis accidents with loss of coolant and destruction of the core, the fuel melt (corium) can escape from the vessel and move to the concrete base of the reactor shaft. An analysis of the calculations results and modeling experiments shows possible options for the formation of unorganized compositions with a critical mass of fuel in the molten corium-concrete interaction products (MCCI). In the presence of water as a mobile and uncontrolled component in such compositions, there is a possibility of a self-sustaining chain reaction (SCR). To ensure the subcriticality control of such compositions, computational and experimental studies of the SCR occurrence in the volume of MCCI cluster are carried out. The problem of eliminating the probability of SCR occurrence in the volume of MCCI cluster was solved by choosing sacrificial materials (SM) used in melt localization devices (MLD). Effective neutron multiplication factors (Kef) were obtained for the MCCI, depending on the mass of the SM and its type. On the basis of computational and experimental studies, it was shown that when corium interacts with concrete, an interlayer of destroyed concrete is formed at the boundary of which there is a ceramic crust, which allows water to be retained in the breeding system. Also, MCCI is a twolayer structure containing immiscible metal and oxide components. After the accident at the Chornobyl nuclear power plant, as information about fuel-containing materials (FCM) was accumulated, work was carried out on the nuclear safety (NS) estimation of the Shelter object. Calculations of neutron-physical parameters of homogenous FCM mixtures during water intake made it possible to estimate the range of fuel parameters at which the existence of a critical composition and the occurrence of SCR in the process of water intake are possible. With mixed fuel burnup 11,5÷12,5 MW · day/kgU the critical mass of the FCM breeding medium should be 15÷30 t with a mass fraction of uranium (U) 45÷60% and the moisture capacity is no less than 40% by volume. It is known that Kef for FCM clusters depends on the mass and mass content of uranium, elemental composition of the mixture, geometric dimensions, reflectors, temperature, concentration of water as a mobile component. All the above calculations were carried out only for FCM with a fuel mass content of 5÷15%. It is shown that all FCM clusters localized inside the Shelter object are nuclear safe, except for the cluster in the south-eastern part of the subreactor room 305/2. A nuclear dangerous cluster (NDC) was formed in the two-meter monolith of a reinforced concrete base plate (BP) penetration zone. NDC is a water-uranium system with low-enriched fuel. The estimation of the fuel mass in the NDC by thermometric measurements, as well as the determination (by the downholes of research boreholes) of its geometry allow us to state that there is the critical mass of fuel (18–20 t of U) located in a limited volume. The specificity of water-uranium breeding systems with low-enriched fuel is the existence of two criticality values. In June 1990, re-criticality (cold critical incident) occurred when water entered the cooling NDC. SCR self-extinguishing occurred due to overmoistening of the breeding medium. Before the installation of NSC “Arch” (November 2016), the NDC was in a waterlogged state (according to the second criticality value) due to the rainwater inflow in the NDC localization zone. Water was a stabilizing factor. After the NSC installation and the termination of water access to the NDC localization zone, a constant increase in neutron activity is observed, which can only be related to the processes of water loss through the porous breeding medium of the NDC. A decrease in the concentration of water in the breeding medium can lead to “recurrent” criticality with consequences that are difficult to predict. According to the results of the calculations, a range of the main parameters of the NDC model was obtained, which ensure the presence of two criticality values in the process of water inflow to the breeding medium and do not contradict the available experimental data. All calculations of the neutron-physical characteristics of the NDC were carried out on the basis of modeling of the version of critical assemblies for the uranium silicate version of the physical model. Its chemical composition was determined on the basis of mixing fuel tablets into the silicate matrix of the FCM cluster. Taking into account the impossibility of the NDC formation in this way, since the concentration of uranium reaches no more than 15% during admixture, an uranium-zirconium model of the NDC was proposed as a MCCI product. Taking into account the growth of neutron activity in the south-eastern part of room 305/2, and assuming the possibility of “recurrent criticality” in the NDC breeding medium, it is necessary to revise the calculations of the NDC neutron-physical characteristics, to form and investigate a new uranium-zirconium NDC physical model as a MCCI product.