A nuclear power plant installation comprises a nuclear reactor that is disposed in an underground reactor room and connected to power generating equipment on the ground surface by pipes which pass between the underground reactor room and the ground surface. A plurality of underground cavities are disposed above the underground reactor room, each of said cavities comprising steel bridgework that defines the sides and tops of the cavity and an unreinforced concrete floor which supports a mass of particulate shielding material within the cavity. An explosive charge embedded within each such mass may be detonated in the event of a malfunction of the reactor to fracture the floor structure in the cavity if the floor has not been fractured by the reactor malfunction, thereby to cause the masses of mixed sand and boron powder to fall from said cavities into the reactor room and bury the malfunctioning reactor. The installation also includes one or more underground personnel rooms which are connected to the underground reactor room by horizontal tunnels that contain normally open doors which are automatically closed in the event of an explosion in the reactor room.
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