A modular pressurized water reactor having a primary circuit including a reactive core, an upper internals, a steam generator heat exchanger and pressurizer housed within a reactor pressure vessel (10, 12) which is enclosed within a substantially close fitting containment, including a primary coolant hot leg between a coolant flow exit from the core and an upstream side of the steam generator heat exchanger and a coolant cold leg between a downstream side of the steam generator heat exchanger and a coolant flow entrance to the core, the hot leg and cold leg being housed within the reactor pressure vessel, the modular pressurized water reactor further including a combined passive heat removal system and high-head water injection system comprising: a core makeup tank including: a heat exchange assembly located within the core makeup tank, the heat exchange assembly having a primary side (44, 46) and a secondary side, the primary side having an interior flow path within the heat exchange assembly with a primary side inlet and a primary side outlet, the primary side (44, 46) being pressurized to at least the same pressure as the core; a primary side inlet plenum that is in fluid communication with the inlet of the interior flow path of the heat exchange assembly and the hot leg exiting the core; a primary side outlet plenum that is in fluid communication with the outlet of the interior flow path of the heat exchange assembly and the cold leg between the downstream side of the steam generator heat exchanger and the coolant flow entrance to the core; and a secondary side plenum within the secondary side of the heat exchange assembly having an inlet end and an outlet end and a secondary side flow path over an exterior of the heat exchange assembly interior flow path, connecting the inlet end to the outlet end of the secondary side plenum; an ultimate heat sink heat exchanger being connected to the core makeup tank between the inlet end and the outlet end of the secondary side plenum; and means for isolating the primary side (44, 46) of the heat exchange assembly from the core; characterized in that the secondary side plenum and a connection with the ultimate heat sink heat exchanger are configured to be pressurized to an extent to prevent boiling under accident conditions. a core makeup tank including: a heat exchange assembly located within the core makeup tank, the heat exchange assembly having a primary side (44, 46) and a secondary side, the primary side having an interior flow path within the heat exchange assembly with a primary side inlet and a primary side outlet, the primary side (44, 46) being pressurized to at least the same pressure as the core; a primary side inlet plenum that is in fluid communication with the inlet of the interior flow path of the heat exchange assembly and the hot leg exiting the core; a primary side outlet plenum that is in fluid communication with the outlet of the interior flow path of the heat exchange assembly and the cold leg between the downstream side of the steam generator heat exchanger and the coolant flow entrance to the core; and a secondary side plenum within the secondary side of the heat exchange assembly having an inlet end and an outlet end and a secondary side flow path over an exterior of the heat exchange assembly interior flow path, connecting the inlet end to the outlet end of the secondary side plenum; an ultimate heat sink heat exchanger being connected to the core makeup tank between the inlet end and the outlet end of the secondary side plenum; and means for isolating the primary side (44, 46) of the heat exchange assembly from the core; a heat exchange assembly located within the core makeup tank, the heat exchange assembly having a primary side (44, 46) and a secondary side, the primary side having an interior flow path within the heat exchange assembly with a primary side inlet and a primary side outlet, the primary side (44, 46) being pressurized to at least the same pressure as the core; a primary side inlet plenum that is in fluid communication with the inlet of the interior flow path of the heat exchange assembly and the hot leg exiting the core; a primary side outlet plenum that is in fluid communication with the outlet of the interior flow path of the heat exchange assembly and the cold leg between the downstream side of the steam generator heat exchanger and the coolant flow entrance to the core; and a secondary side plenum within the secondary side of the heat exchange assembly having an inlet end and an outlet end and a secondary side flow path over an exterior of the heat exchange assembly interior flow path, connecting the inlet end to the outlet end of the secondary side plenum; an ultimate heat sink heat exchanger being connected to the core makeup tank between the inlet end and the outlet end of the secondary side plenum; and means for isolating the primary side (44, 46) of the heat exchange assembly from the core;