Safety design for nuclear power plants

A nuclear power plant installation comprising a nuclear reactor disposed in an underground reactor room that is separated from the ground surface by an intervening body of earth which shields aboveground personnel and equipment from radiation emanating from said reactor, said installation including power generating equipment that is located on the ground surface and connected to said reactor by pipes which pass from the ground surface to said underground reactor room, supplemental shielding means that is automatically operable immediately following occurrence of an emergency in said reactor room, said supplemental shielding means comprising a plurality of underground cavities disposed above the underground room in which said reactor is located, each of said cavities having a frangible floor structure which supports a mass of inert particulate shielding material within said cavity, and an explosive charge adjacent each such cavity which may be detonated upon occurrence of said emergency to fracture said floor structure if said floor structure has not been fractured by the events that have given rise to said emergency, the fracturing of said floor structures being operable to cause the inert particulate material within those cavities whose floors have been fractured to fall under the influence of gravity into said underground reactor room from said cavities above said reactor room so as to bury said reactor in a mass of said particulate material.