The need for criticality control in the proposed reprocessing of slightly enriched non-production fuels at Hanford has led to the development of a geometrically "safe", vertical tube, recirculating dissolver. A study of the nitric acid dissolution of uranium dioxide in a pilot plant dissolver of this type is reported here. The study was pointed toward the comparison of uranium dioxide dissolution rates in a batch and a recirculating dissolver and the definition of hydraulic problems associated with the recirculation of nitric acid, by air lift, technique through beds of reacting uranium dioxide.