NOVELTY - A catalytic reactor comprises a first inner chamber filled with hydrocarbon de-sulfurization catalyst; a second chamber filled with pre-reforming catalyst; a third inner chamber filled with reforming catalyst; a fourth chamber filled with high-temperature shift catalyst, low-temperature shift catalyst and optionally methanation catalyst, and having two ring-shaped cavities; and a fifth chamber having an opening for combustion exhausts gases exit.    USE - The reactor is used for endothermic reaction processes, e.g. for light hydrocarbons steam reforming. The reactor may be used both in stationary and in automotive applications.    ADVANTAGE - The reactor has a compact structure and allows the exploitation of more than 75% of the whole volume, making it available to be filled with catalysts necessary to carry out of the following reactions in series: de-sulfurization of hydrocarbons; pre-reforming; reforming; high temperature shift; low temperature shift; and optionally methanation. The reactor has a geometric configuration that allows insertion of burners to achieve different thermal levels required from the respective used catalysts in a way that they do not damage the used catalysts, carry out the scheduled reactions, and get an elevated conversion of reactants and hydrogen production. The reactor has increased thermal efficiency, low start up time (for plants with fuel cells) to reach the steady state, and elevated operational flexibility.    DETAILED DESCRIPTION - A catalytic reactor comprises five chambers. The first inner chamber is filled with hydrocarbon de-sulfurization catalyst. The second chamber is external to and connected with the first chamber and is filled with pre-reforming catalyst. The third inner chamber is integrated to and connected with the second chamber and is filled with reforming catalyst. The third chamber operates in connection with burner(s), each equipped with a couple of nozzles. The fourth chamber is external to and connected with the third chamber and is filled with, in sequence, high-temperature shift catalyst, low-temperature shift catalyst and optionally methanation catalyst. The fourth chamber has two ring-shaped cavities including an upper cavity connected to the third chamber that is connected with reactant water to control the temperature of gases coming from the third chamber, and a lower cavity (16) for collection and exit of reaction products. The inlet of reactants is in the first chamber. The wrap of the catalytic reactor, fifth chamber, has the opening for combustion exhausts gases exit. An INDEPENDENT CLAIM is included for a method of controlling the temperature of reformed gases incoming in shift catalyst filled in fourth chamber, by statically mixing cold reactant water with hot reformed gases in upper ring-shaped cavity.    DESCRIPTION OF DRAWING(S) - The figure is a vertical sectional side view of a catalytic reactor.    External wall (6)    Cover (7)    Shell case (8)    Inner wall (9)    Static mixers (10, 15)    Vaporized water pipe (12)    Water pipe (14)    Lower ring-shaped cavity (16)    Duct (17)    Radial pipe (18)