Authors & Affiliations
National Research Centre "Kurchatov Institute", Moscow, Russia
Axial distribution of neutron flux density in light water reactors is characterized by high gradients in the area of upper and lower reflectors. In intermediate neutron spectrum reactors the maximum value of thermal neutron flux in axial reflectors can exceed its maximum value in the core several times. High gradients of neutron flux density available in the area of axial reflectors may result in significant errors of 3D calculations of core power distribution in the vicinity of the reflectors. In order to avoid the direct calculation of neutron flux density in axial reflectors, it is suggested that the matrices of effective conditions should be used at the core and reflector boundaries similarly to the matrices of effective conditions specified at the radial boundaries of cells in the heterogeneous reactor theory. Since at the core and axial reflector boundaries the axial component of neutron flux density gradient value essentially exceeds the radial component, the latter can be neglected, and for the calculation of effective boundary conditions matrices a 1D model can be used. With the aim of reducing the number of axial harmonics, an improved heterogeneous method is proposed using effective conditions at the axial boundaries of the core. Heterogeneous reactor equations were obtained with the effective conditions at the core and axial reflector boundaries. The improved method of heterogeneous calculation has been implemented in the software module TREC, for the calculation of spatial distribution of neutron flux in the BARS code. For the purpose of improved method validation the test calculations of a model core with an intermediate neutron spectrum were performed. The calculation results were compared with the calculations using the basic version of the TREC code with the conditions of the flux at the reactor boundaries equal zero. Benchmark calculations with 40 axial harmonics by the basic and upgraded version of the TREC code have demonstrated high-precision agreement.
Light-water reactor, core, axial reflector, heterogeneous calculation method, neutron flux density, thermal neutrons, matrix of effective conditions
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