NOVELTY - A catalytic reactor comprises a hydrogen source; an oxygen source; an inert gas source; a reaction chamber comprising pellets coated with catalyst; a gas panel having gas connections for metering hydrogen, oxygen and inert gas into the reaction chamber with an approximately stoichiometric ratio of hydrogen:oxygen; and an outlet for conducting water vapor away from the reaction chamber.    USE - For producing high-purity water-vapor.    ADVANTAGE - The catalytic reactor has lower thermal budget and does not require heating to initiate or sustain the reaction. It allows the reaction temperature to be more precisely controlled for improved safety and reliability. The operating temperature of catalytic reactor can be reliably maintained at less than 350 degreesC, while the prior art apparatus operates at approximately 460 degreesC. The design of catalytic reactor provides improved flexibility of gas flows to allow adjustability of the flow rate and concentration of water vapor, hydrogen, oxygen and inert gas. It also allows the ability to easily change and recharge the catalyst. Reliable operation and lower manufacturing cost are achieved.    DETAILED DESCRIPTION - A catalytic reactor consists of hydrogen source, oxygen source, inert gas source, reaction chamber (120), gas panel (150), and outlet (222). The reaction chamber comprises pellets coated with catalyst. The gas panel has gas connections (102, 104, 106) connected to respective gas sources. It supplies hydrogen, oxygen and inert gas into the reaction chamber with an approximately stoichiometric ratio of hydrogen:oxygen to produce water vapor at a temperature below the autoignition temperature without undergoing combustion at high temperature. The outlet is used to conduct water vapor away from the reaction chamber.    An INDEPENDENT CLAIM is included for a method of producing high-purity water vapor by reacting hydrogen and oxygen gas in a reaction chamber.    DESCRIPTION OF DRAWING(S) - The figure shows a simplified schematic diagram of the inventive catalytic reactor.    Gas connections (102, 104, 106)    Reaction chamber (120)    Gas panel (150)    Outlet (222)