METHODOLOGY FOR THE STUDY OF CORIUM AGING PROCESSES

To date, the corium research is one of the main issues in the framework of improving nuclear safety and is one of the tasks of conducting a successful procedure to eliminate the consequences of an accident with a core meltdown at the NPP. One of the important tasks for the procedure of eliminating the consequences of an accident at the NPP is to understand the physical state of the core melt of an emergency reactor (corium) in order to make decisions on its removal from the contents and further handling. The difficulty in assessing the structure and properties of the corium, which undergo the changes as a result of cooling with water and prolonged exposure in a melt content or trap (the process of corium “aging”), is in its high radioactivity. Corium includes elements of the core (uranium fuel, zirconium cladding), elements of metal structures, structural materials, concrete, etc. There are five known cases of non-design accidents with the formation of corium: at the reactor of the Three Mile Island-2 NPP (USA, 1979), at the Chernobyl NPP (Ukraine, 1986) and three cases of corium formation were observed during the accident at the Fukushima -1 NPP (Japan, 2011). All these incidents have shown the relevance of improving safety systems at nuclear installations, as well as the need to study the properties of corium in order to work with it. Due to the high radiation hazard, the studies of the corium properties are carried out on model samples. The model corium in the “Institute of Atomic Energy” Branch of RSE NNC RK can be obtained both under laboratory conditions and at the experimental benches of the Institute. The corium properties are different and depend on the initial composition of the charge (components modeling the core and structural materials) and the conditions for modeling an out-of-design accident (melting temperature, the principle of the melt cooling, the presence of residual power density, the method of retaining the melt, etc.). The paper presents the results of the analysis of modern achievements in the field of corium study, as well as methodological recommendations for the study of corium aging processes based on the experience of foreign specialists (Japan, Russian Federation) [1–4] and own long-term developments of the specialists of the branch “Institute of Atomic Energy” RSE NNC RK. The methodological recommendations given in this paper can be used directly to study the process of changing the corium properties under various conditions, as well as to predict the aging process for a given period of its exposure in the content of a reactor installation or a subreactor melt trap.