An underground nuclear power reactor system has a hollow blast tunnel which extends from one end of a containment member. The system includes a nuclear reactor vessel and other components that may be positioned on a movable support member or on a bottom wall of the containment member. A blast tunnel, which defines a blast chamber, has a plurality of spaced-apart debris deflectors positioned therein. The blast chamber has an upper wall with a roof opening formed therein which is selectively closed by a roof portion. If the reactor needs to be repaired or replaced, the roof portion is opened so that the reactor vessel can pass through the roof opening. If the reactor vessel explodes, a blast therefrom drives debris therefrom through a blast door and into the blast chamber where the deflectors reduce blast force as the debris passes through the blast chamber.