METHODOLOGY OF CALCULATING THE AXIAL LEAKAGE FOR NUCLEAR REACTORS OF AXIAL SYMMETRY
24th Symposium of AER on VVER Reactor Physics and Reactor Safety (2014, Sochi, Russia)
Advances in spectral and core calculation methods
Abstract
METHODOLOGY OF CALCULATING THE AXIAL LEAKAGE FOR NUCLEAR REACTORS OF AXIAL SYMMETRY
Poveschenko T.S., Laletin N.I.
NRC “Kurchatov Institute”, Moscow, Russia
ABSTRACT
The problem of reactivity effect of coolant voiding is fundamental for theory and calculation analysis methods of nuclear reactors because it conditions reactor safety. Calculation analysis of axial leakage in nuclear reactor is very important part of this study. Especially it seems to be significant for VVER-type nuclear reactor because pitch size of the hexagonal lattice is small in comparison with neutron free path. So if we are going to model coolant voiding cell under accident conditions we have to do careful analysis of physical approximation of mathematical model. In this paper approach to solution of 3-D neutron transport equation is described. Direction collision probabilities are defined as a calculation tool to estimate axial diffusion coefficient Dz. Important aspects of boundary condition formulation on nuclear rector cell surface for the problem of axial leakage are investigated
Obtained results show that if influence of 2-D neutron flux spectrum into current along Z-axis is taken into account then axial leakage is changed in reflector area for VVER cell. Also the results show effect of increasing of axial diffusion coefficient when the closed lattice contains “void” gap. This effect vanishes if approximation of cylindrical cell is used and boundary conditions are formulated as isotropic reflection at the surface of equivalent cylindrical cell.