Patent Number: 
Section: claims

1. A passive safety system for removing decay heat from a nuclear power system, comprising:a reactor vessel;a containment structure surrounding the reactor vessel;a divided flow channel surrounding a perimeter of the containment structure, wherein the divided flow channel includes a coolant downcomer opening at a coolant source inlet, and a coolant riser between the coolant downcomer and the containment structure, wherein the coolant downcomer and coolant riser are in fluid communication at a bottom of the divided flow channel; anda damper in the divided flow channel, wherein the damper is configured to move between an open position permitting coolant flow through the divided flow channel and a closed position restricting coolant flow through the divided flow channel, and wherein the damper is positioned in the divided flow channel to move in the direction of gravity into the open position. 2. The system of claim 1, wherein the reactor vessel includes a molten salt, wherein the coolant riser and the coolant downcomer are in fluid communication only at the bottom of the divided flow channel, and wherein air from the atmosphere is configured to flow through the divided flow channel to cool the containment structure and the reactor vessel when the damper is in the open position. 3. The system of claim 1, wherein the divided flow channel is horizontal where the damper is installed, wherein the damper in the closed position extends at least partially vertically upward to substantially close the divided flow channel. 4. The system of claim 1, further comprising:a hinge at a first end of the damper, wherein the hinge rotatably connects the damper to the divided flow channel; andan attachment connecting to a second end of the damper and configured to rotate the damper about the hinge. 5. The system of claim 1, further comprisingan attachment removably joining to the damper and configured to hold the damper in the open position and position the damper in the closed position. 6. The system of claim 5, wherein the attachment includes an electromagnet, and wherein the damper includes a magnetic material such that the damper is securely joined to the attachment only when the electromagnet is powered. 7. The system of claim 5, further comprising:a winch coupled to the attachment and configured to move the attachment and the damper in the divided flow channel. 8. The system of claim 5, further comprising:a power source connected to the attachment, wherein the attachment is configured to hold the damper in the open position under power provided by the power source; anda temperature-dependent switch controlling the power provided by the power source. 9. The system of claim 8, wherein the temperature-dependent switch is configured to remove the power provided by the power source at a temperature associated with a transient of the nuclear power system so that the damper is positioned in the open position at the temperature. 10. The system of claim 9, wherein the temperature-dependent switch includes a conductor that melts at the temperature so as to open a circuit at the temperature and remove the power. 11. The system of claim 9, wherein the temperature-dependent switch is positioned in the coolant riser. 12. A passive safety system for removing decay heat from a nuclear power system, comprising:a coolant flowpath about a nuclear power reactor configured to carry a coolant flow to remove heat from the reactor; anda damper system in the coolant flowpath, wherein the damper system includes a damper configured to move, at a threshold temperature, from a closed position restricting the coolant flow to an open position permitting the coolant flow, and wherein the damper system is positioned in the coolant flowpath to move in the direction of gravity into the open position. 13. The system of claim 12, wherein the damper system further includes power source and a temperature-dependent switch in the coolant flow, wherein the temperature-dependent switch is configured to disconnect the power source at the threshold temperature. 14. The system of claim 13, wherein the damper system further includes an attachment holding the damper in the closed position only when connected to the power source. 15. The system of claim 14, wherein the attachment includes an electromagnet powered by the power source, and wherein the damper includes a magnetic material. 16. The system of claim 13, wherein the temperature-dependent switch includes a material completing a circuit, wherein the material is configured to melt at the threshold temperature so as to open the circuit. 17. The system of claim 12, wherein the coolant flowpath is a divided flow channel surrounding the reactor, wherein the divided flow channel includes a coolant downcomer opening at a coolant source inlet, and a coolant riser between the coolant downcomer and a containment structure, wherein the coolant downcomer and coolant riser are in fluid communication at a bottom of the divided flow channel. 18. The system of claim 17, wherein the coolant riser and the coolant downcomer are in fluid communication only at the bottom of the divided flow channel, and wherein air from the atmosphere is configured to flow through the divided flow channel to cool the reactor when the damper is in the open position.