A description of a fluid-fuel-fluid-absorber thermal reactor for the production of U233 is presented herein, and the conditions for maximum efficiency as a breeder (shortest doubling time) are elaborated. With very little modification of operating conditions and no modifications in the structure, the machine shows promise as an efficient U235-U233 converter, with the possibility also of by-product power. Some of the development work required is indicated. In particular, achievement of high pumping rates for maximum heat transfer and fabrication of very thin-walled zirconium tubing pay large dividends.