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## DEVELOPMENT OF MATHEMATICAL MODEL AND COMPUTATIONAL ANALYSIS OF SEVERE BEYOND DESIGN ACCIDENTS AT FAST REACTORS

**Authors & Affiliations**

Kascheev M.V., Ashurko Y.M.

*A.I. Leypunsky Institute for Physics and Power, Obninsk, Russia*

**Abstract**

A rather complete mathematical model has been developed for the first time for numerical analysis of severe off-design accidents in sodium cooled fast reactors. Unlike the presently existing models, the developed model enables one to answer the question of the possibility of containment of molten fuel in the reactor vessel. The computational domain under consideration is multiply connected. The mathematical simulation of sub-domains as porous bodies is performed using the laws of conservation of mass, momentum, and energy, written in the form of equations of continuity, motion, and energy in two-dimensional cylindrical coordinates. The problem of formation of heat-generated layer on the bottom face shield has been solved. The zones of heat-generated layer were simulated. In particular, the melting of particles of steel, and then of those of fuel, was included by simulating the heat sinks in the heat-generated layer. A formula is derived for heat sinks in a zone with heat exchangers. There was obtained solution of the problem on the movement of variable mass vapor bubble in liquid. The result have been used to describe the heat sources originated due to vapor conden-sation above heat-generated layer. The developed numerical model is implemented in the form of BRUT computer code. Veri-fication of the code’s individual blocks has revealed an adequate agreement of the calculation results and available experimental data as well as analytical solution. Using BRUT code there was performed calculation analysis of fast reactor severe accident wherein complete melting of fuel subassemblies in the center of the reactor core and partial melting of peripheral subassemblies are considered. At subassemblies melting there are formed two heat-generated layers, located on the lower face breeding zone. Due to generation of heat in the layers there takes place melting of the lower face breeding zone at first, followed by slow melting of the collectors. The central heat-generated layer, after melt-through of the collectors and pressure-header upper plate, displaces at the lower plate of the pressure-header. At that, melt front comes to rest and heat-generated layer is cooling off. Thereby, in accordance with the calculation results on the considered above accident melted fuel is retained inside of the reactor vessel. As well, using code BRUT there have been performed a calculation of low-power fast reac-tor severe accident, wherein took place destruction of 18 fuel subassemblies in the core first and second rows. It was demonstrated that at initial stage only the bottom face shield undergoes meltdown process, which ceased in time. So, in the considered accident there is guaranteed retention of partially degraded core inside of reactor vessel.

**Keywords** fast reactor, mathematical model, vessel of reactor, heat-generated layer, molten fuel, heat sinks, severe accident, computational domain, vapor bubble, vapor condensation

**Article Text** (PDF, in Russian)

**UDC 621.039.51:621.039.586**

Problems of Atomic Science and Technology. Series: Nuclear and Reactor Constants", issue 1, 2014