Catalytic reactors based on the water gas shift reaction were designed, built and tested to provide data for the design of a prototype reactor as an alternative to the uranium hot metal beds in a Fusion Fuel Clean Up (FCU) system. The reactor was designed so it could be implemented into a FCU using only existing technology. A closed loop system was chosen so that safety and net efficiency would not be compromised during upset conditions. The system uses only pure reactants thus eliminating the requirement for a carrier gas. The prototype reactor contains only 10 g of catalyst and is expected to last at least five years. The reactor is small and operates at ∼490 K. It will convert water to hydrogen, at a CO/H2O ratio of 1.5 with greater than 98% efficiency and with an estimated 95% efficiency for a tritiated stream of 90 000 Ci/day. This small decrease in efficiency is not expected to be a concern in the closed loop system proposed. Results show that the reactor performance is very stable even during upset conditions, wide ranges of flow, CO/H2O variance upward from 1.3, and the presence of various contaminants. The maximum tritium inventory during operation in a FCU system is calculated to be 10.4 Ci. The expected waste disposal inventory of the reactor is 3.2 Ci.