At the time of loss of coolant accident, when station blackout occurs, hydrogen, radioactive nuclides, and steam are discharged from a broken portion of a pipe connected to a reactor pressure vessel into the primary containment vessel (13). A passive autocatalytic hydrogen treatment apparatus installs a catalytic layer (22) and heat exchanger tubes of a heat exchanger in a casing. High-temperature steam including hydrogen and radioactive nuclides is supplied into the heat exchanger tubes (5) and heats gas supplied into the casing (3). The steam is condensed in the heat exchanger tubes and generates mists. The mists are removed by a mist separator together with the radioactive nuclides. The gas including hydrogen fed from the mist separator (6) in the casing is heated by the aforementioned steam and is introduced into the catalytic layer. Hydrogen is combined with oxygen in the catalytic layer to steam.