1974 Volume 11 Issue 10 Pages 422-433
The effect of neutron streaming upon the neutron multiplication factor in a fast critical assembly FCA-VI is estimated by considering the anisotropy of the diffusion coefficient. In this paper, the Benoist formula is adopted to obtain the diffusion coefficient. In his original ex-pression, many effects such as those of neutron angular distribution, multiple collision (correla-tion of different neutron passes) and finiteness of lattice system are included. In order to estimate these effects, the generalized first-flight collision probability method is adopted as in the previous paper, and an improvement is brought over the previous method so as to calcu-late the effective diffusion coefficient in a practical slab lattice cell which is asymmetric about the center of the cell.
Using the method of calculation described above, the anisotropic effect of neutron stream-ing in the lattices TA, TB, TC and TD used in the FCA VI-1 assembly is estimated. The effect of anisotropy of the diffusion coefficient upon the multiplication factor for the TA, TB, TC and TD lattices is found to be -0.156, -0.181, -0.242 and -0.330% Δk/k, respectively.
Usually, the effects of neutron distribution, multiple collision and anisotropic scattering are neglected in evaluating the diffusion coefficient when using the Benoist formula. Among these factors, the effect of multiple collision of neutrons has the largest effect upon the dif-fusion coefficient and upon the neutron multiplication factor. For the TA lattice, 30% of the overall effect of anisotropy is attributable to multiple collision.
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