Patent Number: 
Section: claims

1. A method of controlling reactivity of a nuclear fission reaction in a fast spectrum, molten salt reactor comprising:sustaining a nuclear fission reaction with fast neutrons in an unmoderated nuclear reactor core, the fast neutrons generated by a molten fuel salt within the unmoderated nuclear reactor core, thereby generating heated molten fuel salt in the unmoderated nuclear reactor core;removing heat from the molten fuel salt by circulating the molten fuel salt between the unmoderated nuclear reactor core and one or more heat exchangers in which heated molten fuel salt is cooled;monitoring one or more reactivity parameters indicative of reactivity of the molten fuel salt within the nuclear reactor core, the one or more reactivity parameters including a first parameter; andwhen the first parameter indicative of reactivity indicates that the reactivity has increased above an upper threshold of reactivity, reducing the reactivity by replacing a first volume of the molten fuel salt with a second volume of a feed material that does not contain any fissile material. 2. The method of claim 1 wherein the first volume and the second volume are the same. 3. The method of claim 1 wherein the feed material consists of a mixture of a selected fertile material and salt. 4. The method of claim 1 wherein the replacing comprises:inserting one or more volumetric displacement bodies into the molten fuel salt within the molten salt reactor. 5. The method of claim 1 further comprising:determining the second volume of the feed material to be added to the nuclear reactor core necessary to bring the first parameter within the selected range. 6. The method of claim 1 wherein the first parameter indicative of reactivity of the molten fuel salt within the nuclear reactor core is keff and the selected range of nominal reactivity is from 1.0 to 1.035. 7. The method of claim 1 wherein the first parameter indicative of reactivity of the molten fuel salt within the nuclear reactor core is keff and the selected range of nominal reactivity is from 1.001 to 1.005. 8. The method of claim 1 wherein the first parameter indicative of reactivity of the molten fuel salt within the nuclear reactor core is keff and the selected range of nominal reactivity is from 1.0 to 1.01. 9. The method of claim 1 further comprising:monitoring the one or more reactivity parameters indicative of reactivity of the nuclear reactor core; andcontrolling exchange of the first volume of the molten fuel salt with the second volume of a feed material, wherein the feed material consists of a mixture of a selected fertile material and salt based on the one or more reactivity parameters. 10. The method of claim 1 wherein the feed material consists of UCl3 and one or more of UCl4, NaCl, MgCl2, CaCl2, BaCl2, KCl, SrCl2, VCl3, CrCl3, TiCl4, ZrCl4, ThCl4, AcCl3, NpCl4, PuCl3, AmCl3, LaCl3, CeCl3, PrCl3, and/or NdCl3.