Patent Number: 052020835
Section: claims

1. A nuclear reactor system comprising a reactor core and a main heat transport path containing a first heat removal component, at least one main coolant pump and coolant wherein, during normal operation, the coolant is pumped in the main heat transport path by said main coolant pump through a core of the reactor to said heat removal component and back to the reactor core to transport heat generated in the reactor core to the heat removal component; the system comprising a further decay heat removal path connected in parallel with the heat removal component and main coolant pump, the further decay heat removal path including a heat exchange component located at an elevation such that a natural convection flow will occur in the decay heat removal path from a high temperature outlet for coolant from the reactor core through the heat exchange component and to a low temperature inlet of the reactor core when said main coolant pump is shutdown; the further decay heat removal path includes a means to prevent flow in a direction opposite to the intended natural convection flow and a means to maintain a small flow of coolant from said outlet and through the heat exchange component during normal operation of the main coolant pump. 2. A nuclear reactor system as defined in claim 1, wherein the means to maintain the small flow of coolant is a first high-head low-flow pump and the means to prevent flow in the opposite direction is a check valve in the decay heat removal path. 3. A nuclear reactor system as defined in claim 2, wherein the first heat removal component is a steam generator. 4. A nuclear reactor system as defined in claim 2 wherein a second high-head low-flow pump is connected in parallel to said first high-head low-flow pump. 5. A nuclear reactor system as defined in claim 4, wherein a control valve is located in the decay heat removal path to control said small flow to a suitable value. 6. A nuclear reactor system as defined in claim 4, wherein a speed control for the high-head low-flow pumps provides a control to maintain said small flow at a suitable value. 7. A nuclear reactor system as defined in claim 1, wherein the means to prevent flow in the opposite direction is a check valve in the decay heat removal path and the means to maintain said small flow is a bleed located between an outlet of the heat exchange component and the check valve. 8. A nuclear reactor system as defined in claim 1, wherein the heat exchange component is a heat exchanger located in a large tank of coolant. 9. A nuclear reactor system as defined in claim 7, wherein the heat exchange component is a heat exchanger located in a large tank of coolant. 10. A nuclear reactor system as defined in claim 7, wherein the heat removal component is a steam generator. 11. A nuclear reactor system as defined in claim 10, wherein the bleed from the decay heat removal path is connected to a purification unit. 12. A nuclear reactor system as defined in claim 1, wherein the first heat removal component is connected to a high temperature outlet header of the reactor and said at least one main coolant pump is connected to a low temperature inlet header of the reactor. 13. A nuclear reactor system as defined in claim 12, wherein the heat removal component is a steam generator. 14. A nuclear reactor system as defined in claim 13, wherein the means to maintain the small flow of coolant is a first high-head low-flow pump and the means to prevent flow in the opposite direction is a check valve in the decay heat removal path. 15. A nuclear reactor system as defined in claim 14, wherein a further high-head low-flow pump is connected in parallel to said first high-head low-flow pump. 16. A nuclear reactor system as defined in claim 15, wherein a speed control for the high-head low-flow pumps provides a control to maintain said small flow at a suitable valve. 17. A nuclear reactor system as defined in claim 16, wherein the heat exchange component is a heat exchanger located in a large tank of water. 18. A nuclear reactor system comprising a reactor core through which a coolant flows from a low temperature inlet header to a high temperature outlet header with a primary cooling circulation path being located between the high temperature outlet header and the low temperature inlet header including main circulation pumps and a heat removal component, wherein a decay heat removal path is connected between the high temperature outlet header and low temperature inlet header and includes a heat exchange component located at an elevation such that a natural convection circulation flow can start in the decay heat cooling path from the high temperature outlet header to the low temperature inlet header when the main circulation pumps are shutdown, a high-head low-flow pump being located in the decay heat cooling path for maintaining a small flow of coolant in the decay heat cooling path in the same direction as the natural convection circulation flow during normal operation of the main circulation pumps. 19. A nuclear reactor system as defined in claim 18 wherein a check valve in the decay heat removal path prevents flow in a direction opposite to the intended natural convection circulation flow. 20. A nuclear reactor system comprising a reactor core through which a coolant flows from a low temperature inlet header to a high temperature outlet header with a primary cooling circulation path being located between the high temperature outlet header and the low temperature inlet header including main circulation pumps and a heat removal component, wherein a decay heat removal path is connected between the high temperature outlet header and low temperature inlet header and includes a heat exchange component located at an elevation such that a natural convection circulation flow can start in the decay heat cooling path from the high temperature outlet header to the low temperature inlet header when the main circulation pumps are shutdown, a coolant bleed line being located in the decay heat cooling path for maintaining a small flow of coolant in the decay heat cooling path in the same direction as the natural convection circulation flow during normal operation of the main circulation pumps.