Patent Number: 
Section: claims

1. A nuclear reactor system having a pressurized coolant circuit including a pressure vessel, heat exchanger, core makeup tank and connecting piping, the connecting piping including a main coolant piping loop connecting the heat exchanger to the pressure vessel in a closed loop configuration and a makeup water connection connecting the core makeup tank to the pressure vessel, the nuclear reactor system being housed within a containment shell and further comprising;an in-containment water reservoir that is maintained vented to an atmosphere of the containment shell;a depressurization system for automatically depressurizing the pressurized coolant circuit in the event of a design basis accident and connecting the water reservoir to the pressure vessel; anda blocking device is connected to the depressurization system for preventing the depressurization system from activating when coolant within the core makeup tank is above a preselected level. 2. The nuclear reactor system of claim 1 wherein the blocking device fails in a failsafe condition wherein the failure of substantially any component within the blocking device will stop the blocking device from preventing the depressurization system from activating. 3. The nuclear reactor system of claim 1 wherein the preselected level is a level at which the core makeup tank is considered substantially full. 4. The nuclear reactor system of claim 1 wherein the core makeup tank comprises at least two water tanks connected to the pressure vessel and wherein the blocking device prevents the depressurization system from activating when coolant within each of the water tanks is above the preselected level. 5. The nuclear reactor system of claim 4 wherein the blocking device does not prevent the depressurization system from activating when coolant within any of the water tanks is below the preselected level. 6. The nuclear reactor system of claim 4 wherein the blocking device does not prevent the depressurization system from activating when disabled by a remote manual override switch or other interlocking condition. 7. The nuclear reactor system of claim 1 wherein the blocking device is connected to a priority command input of a component interface module controlling a valve of the depressurization system.