Patent Number: 
Section: claims

1. A nuclear fission reactor, the reactor comprising a core, a pool of coolant liquid, and a heat exchanger for extracting heat from the coolant liquid, wherein:the core comprises an array of hollow fuel tubes, each containing a molten salt fuel comprising one or more fissile isotopes, the fuel tube array being at least partly immersed in the pool of coolant liquid, the fuel tube array comprising a region where the density of the fissile isotopes during operation of the reactor is sufficient to cause a self-sustaining fission reaction;wherein heat transfer from the molten salt fuel in each fuel tube to the exterior of that tube is achieved bynatural convection of the molten salt fuel;and wherein the molten salt fuel comprising fissile isotopes is unpumped and contained entirely within the fuel tubes during operation of the reactor. 2. A reactor according to claim 1, wherein each fuel tube comprise internal baffles configured to divide at least a portion of the tube into segments. 3. A reactor according to claim 2, wherein each segment has a height and diameter that are the same order of magnitude so as to facilitate convection within the segment. 4. A reactor according to claim 1, wherein each fuel tube has a diameter of at least 5 mm. 5. A reactor according to claim 1, wherein one or more of the fuel tubes is formed as a helix. 6. A reactor according to claim 1, wherein an upper portion of each fuel tube is non-linear so as to prevent neutrons passing directly up the fuel tube and out of the reactor. 7. A reactor according to claim 1, wherein each fuel tube comprises a temperature sensor configured to determine the temperature of the molten salt fuel by any one of:properties of light emitted from the molten salt fuel;an amount of expansion of the fissile material;a resonant acoustic frequency of a gas column in the tube. 8. A reactor according to claim 1 wherein the coolant liquid is a molten metal salt contained within a single tank, and the circulation of the coolant liquid is driven by natural convection only. 9. A reactor according to claim 1 wherein the coolant liquid contains a fertile isotope which produces a bred fissile isotope under neutron flux. 10. A reactor according to claim 9, wherein the reactor comprises a layer of molten metal in contact with the coolant liquid, the molten metal being such that the bred fissile isotope is soluble in the molten metal, the reactor further comprising a system for extracting the molten metal. 11. A reactor according to claim 10, and comprising a system for circulating the molten metal such that it is drawn from the layer of molten metal and reintroduced to the coolant liquid as a spray or a plurality of columns, wherein the molten metal is passed to the heat exchanger prior to being reintroduced. 12. A reactor according to claim 1, wherein the heat exchanger comprises an array of tubes within the coolant liquid, the tubes containing a liquid and/or gas which circulates through the array of tubes, wherein the liquid and/or gas is any of:water;steam;water and steam;helium;carbon dioxide;air;nitrogen;a molten metal or metal salt. 13. A reactor according to claim 1, wherein the fuel tubes comprise any of:a molybdenum alloy;pure molybdenum;a carbon composite; andsilicon carbide. 14. A reactor according to claim 1, wherein the core further comprises a neutron moderating material. 15. A reactor according to claim 14, wherein said neutron moderating material is provided in moderator tubes within the array of fuel tubes. 16. A reactor according to claim 1, wherein the number density of fuel tubes is reduced towards the centre of the array of fuel tubes. 17. A reactor according to claim 1, wherein the concentration of fissile and/or fertile isotopes in the molten salt fuel within each fuel tube is increased for fuel tubes at the outside of the array of fuel tubes. 18. A reactor according to claim 1, wherein the fuel tubes comprise or contain a metal having a reactivity with halogens intermediate between the fuel tube material and the fissile isotopes. 19. A reactor according to claim 1, wherein the molten salt fuel comprises trihalides of the fissile isotopes. 20. A method of operating a nuclear fission reactor comprising a core, a pool of coolant liquid and a heat exchanger, the core comprising an array of hollow fuel tubes, each containing a molten salt fuel comprising one or more fissile isotopes, the fuel tube array being at least partly immersed in the pool of coolant liquid and comprising a region where the density of the fissile isotopes during operation of the reactor is sufficient to cause a self-sustaining fission reaction, the method comprising:containing the molten salt fuel entirely within the fuel tubes, the molten salt fuel being unpumped;transferring heat from the molten salt fuel in each fuel tube to the exterior of that tube and thus to the coolant usingnatural convection of the molten salt fuel;andextracting heat from the coolant using the heat exchanger.