Extrapolation distances for thermal neutrons have been obtained for slabs of beryllium and light water by minimising the difference between the steady-state leakage spectra calculated on the basis of diffusion and transport theories. Using the present values of extrapolation distance ensure that neutron spectra (both in beryllium and water) calculated on the basis of diffusion theory agree well with the corresponding transport theory spectra, even at distances close to the free surface. The authors find the extrapolation distance to be dependent of the thickness of the slab, increasing with increase in the thickness.
The calculated results of steady state, space- and angle-dependent fast neutron spectra inside various one dimensional slabs of natural uranium are reported. The effect of anisotropy in scattering on fast neutron spectra has been studied for three different orders of anisotropy, zero, first and third. It is shown that with a high energy, plane source of monoenergetic neutrons, spectral equilibrium is established in natural uranium slabs beyond 90 cm from the source plane, with an associated diffusion length of 19.9 cm. This is consistent with the lower bound for diffusion length in natural uranium. Further, the neutron flux is larger when anisotropy in scattering is included in the calculations.
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