Effect of ²³⁵U Concentration on Fast Neutron Space and Time Eigenvalues and Eigenfunctions in Uranium

The results of a detailed study of a fast neutron diffusion length and pulsed problem in depleted and enriched subcritical uranium assemblies (0.2 to 4% ²³⁵U) are reported. The multigroup space- and time-dependent equations are solved using the eigenfunction expansion method. The effect of ²³⁵U concentration on space (diffusion length problem) and time (pulsed problem) eigenvalues and eigenfunctions, particularly on the “discrete” eigenvalue and eigenfunction, is discussed. The approach to equilibrium (both in space and in time) of fast neutrons changes with changing ²³⁵U concentration.