Few Group Analysis of D₂O-U²³⁵ Assemblies

Few-group analysis is applied to a variety of D₂O-U²³⁵ critical assemblies. Use of relatively simple prescriptions for obtaining group constants is sufficient to give good values of the reactivity over a wide range of concentrations of U²³⁵ in D₂O. Among these simple prescriptions is one which attempts to take into account the spatial variation in the neutron spectrum in reflected systems. Use of this prescription improves the calculated reactivity by about 5% over that obtained with only a single thermal neutron spectrum characteristic of the core.