Patent Number: 
Section: claims

1. A method of controlling the reactivity of a molten salt fission reactor, wherein the molten salt fission reactor comprises a core and a coolant tank, the core comprising fuel tubes containing a molten salt fissile fuel, and the coolant tank containing a molten salt coolant, wherein the fuel tubes are immersed in the coolant tank, the method comprising:dissolving a neutron absorbing compound in the molten salt coolant, the neutron absorbing compound comprising a halogen and a neutron absorbing element; andreducing the neutron absorbing compound to a salt of the halogen and an insoluble substance comprising the neutron absorbing element, the halogen being fluorine or chlorine, wherein the insoluble substance is insoluble in the molten salt coolant and not volatile at a temperature of the coolant during operation of the reactor,wherein the neutron absorbing element has a Pauling electronegativity of greater than 1.5 and is a neutron absorber. 2. The method of claim 1, wherein the step of reducing the neutron absorbing compound comprises adding a reducing agent to the molten salt coolant. 3. The method of claim 1, wherein the step of reducing the neutron absorbing compound comprises electrochemical reduction of the neutron absorbing compound. 4. The method of claim 1, further comprising extracting the insoluble substance from the coolant. 5. The method of claim 4, wherein extracting the insoluble substance comprises any one or more of:withdrawing the insoluble substance from the top or bottom of the coolant;filtration;sedimentation;centrifugation;sparging with inert gas. 6. The method of claim 3, further comprising extracting the insoluble substance from the coolant, wherein the step of removing the insoluble substance comprises collection of the insoluble substance on an electrode. 7. The method of claim 1, wherein the neutron absorbing compound is a metal halide, the neutron absorbing element being a metal, and the insoluble substance being the pure metal or an insoluble salt of the metal. 8. The method of claim 7, wherein the metal is any one of indium and silver. 9. The method of claim 1, wherein the neutron absorbing compound is a haloborate salt, the haloborate being fluoroborate or chloroborate, the neutron absorbing element is boron, and the insoluble substance is an insoluble boride. 10. The method of claim 2, wherein the neutron absorbing compound is a haloborate salt, the haloborate being fluoroborate or chloroborate, the neutron absorbing element is boron, the insoluble substance is an insoluble boride and the reducing agent is thorium, zirconium or zirconium difluoride. 11. A nuclear fission reactor comprising:a core comprising fuel tubes containing a molten salt fissile fuel;a coolant tank containing a molten salt coolant, wherein a neutron absorbing compound is dissolved in the molten salt coolant, the neutron absorbing compound comprising a halogen, the halogen being fluorine or chlorine, and a neutron absorbing element;wherein the fuel tubes are immersed in the coolant; anda reduction unit configured to reduce the neutron absorbing compound to a salt of the halogen and an insoluble substance comprising the neutron absorbing element wherein the insoluble substance is insoluble in the molten salt coolant and not volatile at a temperature of the coolant during operation of the reactor;wherein the neutron absorbing element has a Pauling electronegativity of greater than 1.5. 12. A nuclear fission reactor according to claim 11, and comprising a neutron absorber addition unit configured to dissolve a neutron absorbing compound in the molten salt coolant, the neutron absorbing compound comprising a halogen and a neutron absorbing element. 13. A nuclear fission reactor according to claim 11 wherein the reduction unit comprises:an inlet configured to be immersed in a pool of coolant salt of the nuclear fission reactor;a mixing chamber configured to mix coolant drawn through the inlet with a reducing agent in order to reduce a neutron absorbing compound within the coolant salt into an insoluble substance containing a neutron absorbing element of the neutron absorbing compound, and a salt;a filtration unit configured to filter the insoluble substance from the coolant salt;an outlet configured to return the filtered coolant salt to the pool of coolant salt; anda pump configured to cause a flow of coolant salt from the pool through the outlet, then into the mixing chamber, then into the filtration unit, then out of the outlet. 14. A nuclear fission reactor according to claim 11, wherein the reduction unit comprises:an anode and a cathode, each of which is immersed in the coolant; anda voltage regulator configured to supply a voltage between the anode and cathode sufficient to electrolyse the neutron absorbing compound and insufficient to electrolyse other components of the molten salt coolant.