Experimental 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 absorbing 238U 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.