Abstract: A system to prevent supercriticality in nuclear power plants in the unlikely event of a core destructive accident terminating in the nuclear core meltdown. The system dilutes and poisons the molten core to maintain subcriticality, and is specially useful in mobile nuclear power plants, or in nuclear plants subject to seismic disturbances, where the orientation of the nuclear reactor after the accident is unknown. It is also applicable to alleviate the consequences of loss of coolant flow accidents from any cause. Aside from preventing supercriticality, the system serves the dual purpose of acting as a biological shield and/or structural member that reduces the deleterious effects of accidental core impaction, thus achieving its primary goal without compromising power plant weight and size constrants.A borated material, with a melting point greater than the fuel melting point, is inserted in the pressure vessel behind an inner wall.