SIMULATION OF THE FUEL ELEMENT CLADDING MELTING AND THE MELT TRANPORT IN A FUEL ASSEMBLY OF THE BN-TYPE REACTOR USING THE SOKRAT-BN CODE
I. S. Vozhakov, E. V. Usov, V. S. Zhdanov, M. E. Kuznetsova, A. E. Kiselev, R. V. Chalyi
VANT. Ser.: Mat. Mod. Fiz. Proc 2015. Вып.4. С. 15-21.
The paper subject is the development of the model of melting and transport of a fuel element cladding and its implementation in the SOKRAT-BM code used for the analysis and safety justification of NPPs with the BN-type reactors. The code allows numerically simulating the dynamics of fast neutron reactors with sodium coolants under abnormal operation conditions, in cases of the design basis and beyond the design basis accidents. The numerical simulation of the normal operation and emergency operation conditions is one of the major approaches to the safety justification of reactor operation. The specifics of fast neutron reactors is in high values of the specific energy release and low working pressures and, as a result, with the coolant boiling under the emergency conditions with flow loss the vapor flow rate may reach several hundred meters per second thereby significantly affecting the flow dynamics of the molten materials of fuel elements. The motion of the molten fuel elements and structural elements is followed by the reactivity changes in reactor and may cause the generation of both the lower and upper locks in some fuel assemblies and, hence, to the redistribution of the coolant flows in the first loop of the reactor core.
For the simulation of the molten cladding motion it is suggested to take into account the main specific features of the cladding flow process in fast reactor. In particular, it is assumed that the molten material moves along the given fuel element as a film of a varying thickness depending on the amount of melt and the molten material dynamics is governed by the force of gravity, friction between the melt and gas flow, and friction between the melt and solid wall. The specific feature of the model is in strict identification of the molten material boundaries and this allows circumventing numerical diffusion, i.e. the melt is not spread over the entire computational domain, but have clearly defined boundaries moving in accordance with the physics of these processes.
To demonstrate the described model applicability, results of simulations are presented both for the problems having analytical solutions and those modeling the real reactor plants. It is shown that the numerical simulation with the model of melting and transport of a fuel element cladding adequately describes the processes in a number of problems, such as the problem of a melting cylinder and the problem of a melt flowing under gravitational forces and friction between the molten material and coolant flow.Keywords: fast reactor, fuel element, cladding, melting, simulation, SOKRAT-BN.
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