Patent Number: 
Section: claims

1. A nuclear power generation system comprising:a reactor pressure vessel having a reactor core;a pressuriser in fluid communication with the reactor pressure vessel; anda cooling injection system including a make-up tank having a tank inlet and a tank outlet, the injection system configured to switch between an operating condition and a fault response condition when the coolant level in the pressuriser drops below a threshold level,wherein:in the operating condition, the tank outlet is isolated from the reactor pressure vessel such that coolant is retained in the make-up tank, and the tank inlet is in fluid communication with the reactor pressure vessel and the pressuriser; andin the fault response condition, the tank inlet is isolated from the reactor pressure vessel, the tank outlet is in fluid communication with the reactor pressure vessel such that coolant from the make-up tank can flow into the reactor pressure vessel to provide cooling of the reactor core, and the tank inlet is in fluid communication with the pressuriser such that residual pressure in the pressuriser can drive coolant from the make-up tank into the reactor pressure vessel. 2. The coolant injection system according to claim 1, wherein the makeup tank is arranged to supply boronated water. 3. The coolant injection system according to claim 1, comprising a valve arrangement operable in response to the coolant level in the pressuriser to switch the injection system between the operating condition and the fault response condition. 4. The coolant injection system according to claim 3, wherein the valve arrangement comprises a first valve located intermediate the tank outlet and the reactor pressure vessel, and a second valve located intermediate the reactor pressure vessel and the pressuriser. 5. The coolant injection system according to claim 1, wherein the make-up tank inlet is connected to the pressuriser via a pressure balance line for maintenance of the make-up tank at system pressure. 6. The coolant injection system according to claim 5, wherein the pressuriser is connected to the reactor vessel via a surge line. 7. The coolant injection system according to claim 6, wherein the pressure balance line and make-up tank form part of an auxiliary circuit branching from the surge line. 8. The coolant injection system according to claim 7, wherein the auxiliary circuit further comprises an outlet line which connects the make-up tank outlet to the surge line. 9. The coolant injection system according to claim 1, wherein the pressuriser contains one or more heaters, and the threshold level is a level at which the heater(s) are at least partially uncovered. 10. The coolant injection system according to claim 1, wherein the make-up tank is positioned vertically above the reactor vessel. 11. The coolant injection system according to claim 1, further comprising an isolation valve for isolating the pressuriser from the make-up tank inlet. 12. The coolant injection system according to claim 11, further comprising a gas inlet line arranged to supply gas to the make-up tank to maintain pressure in the nuclear power generation system when the pressure balance line isolation valve is closed.