Calculation of thermal neutron spectra in D2O lattices

A knowledge of the thermal neutron spectrum inside a lattice cell is important for calculating the thermal cross-sections, η and f. In the present work, three different methods for calculating the spectrum are described. The first of them generalizes the well-known Wilkins' theory in the case of heterogeneous media. The spatial dependence of the spectrum is taken into account with the help of collision probabilities. The second method is a multigroup P1 approximation. The third is a multigroup-cum-multizone approximation. In the last method, the multigroup procedure of the P1 approximation is combined with the multizone procedure of the generalized Wilkins' theory. With the help of a few examples, the three methods are compared both with one another and with other methods. Certain other examples are used to demonstrate the effect of the temperatures of the moderator, the coolant and the fuel on the neutron spectrum in a typical power reactor. (author). 12 refs, 6 figs, 5 tabs