A confined jet reactor (mixer) is presented as a novel solution for the scalable continuous hydrothermal flow synthesis (CHFS) of nanoceramics. In CHFS, nanoceramics are formed upon mixing of two streams consisting of an aqueous metal salt solution at room temperature with a flow of less dense supercritical water (at 240 bar and 450 degrees C). Upon mixing, hydrolysis and dehydration occurs, resulting in the particles being formed in a continuous manner. The confined jet mixer used herein overcomes previous designs of mixers that can accumulate material internally and block. A method for scaling up the jet mixer (reactor) is described, to determine the size of jet mixer (internal mixer diameter 13.5 mm) prior to its use in a newly commissioned pilot plant designed to process flow rates 40 times greater than the equivalent laboratory-scale process (internal mixer diameter 4.6 mm). It was confirmed that the pilot plant scale mixer allowed safe and continuous operation with no blockages at much higher concentrations (i.e., higher molarity) of metal salt precursor than laboratory scale because of the higher velocities and larger physical dimensions of the mixer. Consequently, the pilot plant was used to manufacture nanoparticles at a rate >400 times that of the laboratory-scale process. The synthesis of zinc oxide nanoparticles was used as a model to compare the properties of particles produced on different production scales. The same model system was also used to assess the limitations of a scale-up strategy based on mass (i.e., increasing the molarity of the metal salt).