Scientists of the ISP NPP of the NAS of Ukraine and Chornobyl NPP specialists conducted joint scientific workshop on October 22, 2020, where issues on the modern state of the Shelter object fuel containing materials (FCM) were discussed. Experimental data on researches results received by the scientists during the last couple of years were represented.
Among the representatives of the SSE “Chornobyl NPP” were V. Seida, Senior Executive Vice-President; A. Bilyk, Senior Engineer of the Station; K. Shefer, Senior Executive Vice-President in charge of Licensing and Institutional Control; S. Kondratenko, Deputy Technical Director on Infrastructure; O. Novikov, Deputy Technical Director on Safety; L. Yakovenko, Shop Manager on Radiation Safety; and representatives of the Shop on the Shelter object operation and New Safe Confinement (NSC) – S. Klepikov, A. Siryi, S. Deriuga, S. Sverchkov.
S. Paskevych, Deputy Director on Research, V. Krasnov, Head of the Nuclear and Radiation Safety Division, S. Gabielkov, Head of the Department, R. Godun, acting Head of the Department, represented the ISP NPP.
Researches of the Shelter object FCM, conducted in the ISP NPP, have shown that there are processes in the FCM behavior, which in the future may influence on the existing state. It is connected with the change of the conditions of the environment after the NSC installation as well as processes going on with FCM. These processes are slow and not obvious on condition until now. However, for the choice of optimal strategy of the works on the Shelter object transformation into an ecologically safe system and development of specifications for the future FCM extraction technologies, it is necessary to take into account processes running in the FCM and that will lead to the change of their physical state.
Two reports were presented at the workshop:
The first one is “Experimental and calculated researches of the FCM state inside the Shelter object”, R. Godun. Up-to-date views on FCM accumulations formation, experimental and calculated properties of low-enriched multiplying systems, assessment of parameters that will have impact on the state of the Shelter object nuclear safety were represented in the report. Up-to-date data on the state of the FCM accumulations in the premise 305/2 were studied. According to the results of world analytical and experimental studies of products interaction between corium flux and concrete, possible structure of accumulations with enhanced concentration of fissible materials in premise 305\2 was presented.
The second one is “New data on phase composition of brown ceramics. Updated model of evolution of lava-like fuel-containing materials microstructure”. The main attention was paid to the issues of phase composition of crystalline phase of brown lava-like FCM. Silica glass-phase characteristics were received. Urania is derivative from “outgoing” materials of reactor before accident, cubic and tetragonal zirconium oxide as well as zirconium silicate were formed during the accident, and number of silicon oxides, silicates of uranium, zirconium, aluminum and calcium were formed in the crystallization process of glass matrix during 33 years after the accident. Uranium oxide in inclusions oxidized to UO2,34 and has cubic lattice which is metastable at this oxygen content. These inclusions exist due to their compression by glass matrix. Increase of their volume due to the oxidation of uranium oxide is the cause of formation of cracks in material. When brown ceramics fail, 20-25% of uranium oxide transfer from inclusions to U3O8 oxide with grains failure to micron and submicron level. Now the process of crystallization of glass matrix is at the stage of sustainable development.
S. Gabielkov, Scientific doctor, reported on updated model of evolution of microstructure of lava-like FCM developed in 2020 and which takes into account new data on phase composition of FCM. Forecast of the state and behavior of lava-like FCM is represented in the report. In the nearest future there won’t happen lava-like FCM destroy, long term – they will be destroyed totally. Terms of lava-like FCM destruction and particles sizes on which the glass phase will fail were evaluated. All inclusions of uranium oxides will get outside the lava-like FCM. Uranium oxides grains will break down to the sizes of a few microns and some of them, probably, to the submicron level. Up to 50 t of uranium oxides micron powders inevitably will participate in aerosols formation which will pose major hazard to the human. Some methodical and technological approaches to the creation of methods of lava-like FCM solid-phase conditioning were suggested.
According to the results of discussion among specialists and scientists it was held that such form of the workshops is effective and allow to work on common decisions between organization-scientific supervisor (ISP NPP) and SSE “Chornobyl NPP” concerning further researches and key decisions making on NSC-SO operation and the Shelter object transformation into an ecologically safe system.
It was suggested to conduct the second common scientific workshop in December 2020 and discuss results of the ISP NPP researches on behavior of radioactive dust in the NSC-SO complex and radioactive water that accumulated at the bottom marks of the Shelter object.