Study of neutron wave propagation through a subcritical assembly

We report here the results of a series of calculations on the propagation of neutron waves in multiplying media using multigroup diffusion theory. For a heavy-water natural uranium subcritical assembly (taking into account the slowing down of fission neutrons), the variation of the attennuation constant α and phase lag ξ with frequency have been studied and compared with the available esperimental data. It is found that the multigroup theory gives satisfactory agreement with the experimental data. The striking feature of the multigroup theory is that it gives the correct frequency dependence of α and ξ not only at low frequencies but also at high frequencies. On the other hand, the two group theory results totally fail at high frequencies. The multigroup theory is also better than the Fermi-age theory because the latter, though gives a correct behaviour of the dispersion curve, shows a general disagreement with the experimental data for all frequencies.