AbstractExperimental data to establish criticality control specifications for enriched uranium rods undergoing dissolution are extremely limited. A principal difficulty in treating the problem theoretically is that the resonance absorbing238U is admixed in the aqueous solution in which the rods are immersed. The“narrow resonance”and“infinite mass”approximations are applied; and from this application, expressions are developed for treating resonance capture by an absorbing lump embedded in a moderator admixed with the absorber. The computed change in the critical buckling of a heterogeneous array on replacing the water moderator by a uranyl nitrate solution is in good agreement with experiment. Results from survey calculations for 3 and 5 wt%238U rods latticed in uranium-water mixtures are given. It was concluded that for enrichments up to 5 wt% 235 U, dissolver vessels designed geometrically safe for water-moderated arrays of uranium rods will remain safe during the dissolution process.