[en] Upon occurrence of reactor accident, temperature, pressure, relative humidity and radiation level in a reactor container are extremely elevated respectively, and they act as a force for partially opening a lower portion of a vessel containing hydrogen adsorbing alloys. As a result, the hydrogen adsorbing alloys therein are exposed from the lower portion of the opened vessel and gradually discharged to the outside, in which adsorbed and stored hydrogen and oxygen in the reactor container are put to combustion reaction to elevate the temperature of the alloys. Hydrogen storing ability of the alloys is lowered along with temperature elevation, and hydrogen is continuously discharged to maintain the combustion reaction on the surface of the alloys. Meanwhile, particles of the alloys continue to discharge and burn hydrogen while collapsed into more fine particles by volume change due to discharge or heating or hydrogen, and consume oxygen in the reactor container. Occurrence of abrupt explosion of hydrogen and oxygen can thus be prevented, thereby enabling to prevent destruction entrained by abrupt elevation of pressure in the reactor container. (N.H.)