Modelling problems of emergency natural convection heat removal in the upper plenum of fast reactors using water

The problem of clearly defined purposes of modeling is posed. Conditions for an approximate modeling developed by the Japanese researchers are analyzed. Problems of modeling are considered in the events of different Euler criteria in liquid metal cooled reactor and in the water model; in deciding on structure of simulating heat exchanger etc. It is emphasized that there's no way to simulate the overall transient process from the moment of reactor shutdown up to an establishment of natural convection in the same model. Analysis of the results obtained in water experiments carried out in Japan and independently in the State Scientific Center of Russian Federation IPPE has shown, that violating the modeling rules is observed at small Peclet numbers in laminar flow because of strong dependence of hydraulic resistance from Reynolds number. It is proved that transient process may be simulated by the methods acceptable for the stable natural convection. Also it is shown, that modeling reactors with lead-bismuth alloy cooling (similar reactors are discussed at present as perspective ones) can be done easier, than in sodium cooled reactors. The lines of future researches are proposed as a contributory points for development of the natural convection modeling theory. (author)