THERMAL-NEUTRON TRANSPORT PROCESSES IN H2O-D2O MIXTURES IN TEMPERATURE-RANGE 253 TO 21K - POSSIBLE USE OF THESE MODERATORS AS COLD-NEUTRON SOURCES

The results of a detailed and systematic study of time-dependent, space-dependent, and steady-state neutron spectra in different assemblies of various H//2O-D//2O mixtures in the temperature range of 253 to 21 K are reported. By the matrix diagonalization method, the multigroup Boltzmann diffusion equation was solved to obtain asymptotic and transient spectra in assemblies at 253 K, with buckling values ranging from 0 to 0. 4 cm** minus **2. Mixtures of D//2O content of 20, 50, and 80 wt% are considered. The calculated values of the fundamental mode decay constant and the diffusion parameters are compared with the experimental values. Neutron spectra and the effective diffusion length L(x), at different distances from the source plane, were calculated at 253 K. The asymptotic values of L(x) compare fairly well with calculated values. For the steady-state problem, the multigroup inhomogeneous Boltzmann diffusion equation was solved by the matrix inversion method for assemblies at 77 and 21 K with buckling values ranging from 0 to 0. 1113 cm** minus **2. Mixtures with D//2O content of 0, 5, 10, 20,. . . 90, 95, and 100% are considered. At 21 K, the relative usefulness of different assemblies as cold-neutron sources is discussed. For some selected assembly sizes, the optimum H//2O-D//2O mixtures that would give maximum cold-neutron flux were determined. In mixtures, as in the cases of H//2O and D//2O ice, cooling an assembly below 21 K does not decrease the effective temperature of the neutron distribution below that obtained at 21 K.