Multiplication factors of single pin hexagonal cells and VVER-1000 reactor assembly

Abstract The multiplication factor of an infinite hexagonal reactor lattice (a repetitive arrangement of unit hexagonal cells) depends on the pitch of the fuel pins. The length of the side of the unit hexagonal cell is related with this pitch and represents average moderator volume per fuel pin A mp . The aim of this paper is to study the trend of the multiplication factor, of these single pin hexagonal unit cells and VVER reactor assemblies, as a function of the parameter A mp . This study will provide us the use of an ‘effective pitch’ to be applied as a cost-effective replacement for more explicit assembly computations. A VVER-1000 reactor assembly containing 331 lattice locations has been studied. In this lattice, all the fuel pins are identical and are arranged in a hexagonal pitch. For computations, as this geometry is complicated, a multi-hexagonal approach has been used. Where the assembly contains 331 hexagonal unit cells with some extra moderator at the boundary of these cells. Some of the lattice locations are occupied by moderator (no fuel). Each of these moderator cells are surrounded by six fuel pin hexagonal cells. This type of 7 hexagon cells combinations are at many locations. To study the effect of extra moderator and structural material on the multiplication factor (K), it is assumed that the assembly contains 397 unit cells (one extra layer of cells contains only moderator). The multiplication factors for assemblies of 7, 331 and 397 hexagonal cells, with reflective boundary conditions on the outer boundaries, has been studied. It is observed that the results for the assemblies and for the unit hexagonal cells are in good agreement, in the range covered by these assemblies. For this study, the code DRAGON (Version-4) has been used. Monte Carlo results for unit cell and for complete assembly are also presented for comparison.