Patent Number: 
Section: claims

1. A fusion-fission engine comprising:a chamber having a fusion source of neutrons;a fuel region for containing fertile or fissile fuel surrounding at least a portion of the chamber, wherein neutrons from the fusion source are introduced into the fertile or fissile fuel to convert it into different atomic weight elements and produce heat;a coolant which includes lithium circulating through the fertile or fissile fuel to extract heat therefrom for use in another process; anda coolant replenishing system for replenishing the lithium based coolant, the coolant replenishing system providing both 6Li and 7Li coolant in a specified ratio, wherein changing the ratio enables control of the heat produced by the engine. 2. A fusion-fission engine as in claim 1 wherein the fusion source of neutrons comprises neutrons produced by laser inertial confinement fusion. 3. A fusion-fission engine as in claim 2 wherein the neutrons from the fusion source convert some of the lithium into tritium. 4. A fusion-fission engine as in claim 3 wherein by absorption of the neutrons, production of the tritium reduces the electrical power output of the engine. 5. A fusion-fission engine as in claim 4 wherein during an initial phase of operation the engine is controlled to produce excess tritium by the coolant having a higher ratio of 6Li to 7Li, and during a later phase of operation the engine is controlled to produce less tritium by the coolant having a lower ratio of 6Li to 7Li. 6. A fusion-fission engine as in claim 1 wherein the neutrons are produced by laser light applied to targets containing tritium. 7. A fusion-fission engine as in claim 6 wherein tritium for use in the targets is manufactured by the fusion-fission engine. 8. A fusion-fission engine as in claim 1 wherein the fertile or fissile fuel comprises discrete units of fissile material and the engine further comprises:a safety system including a dump tank coupled to the chamber to receive the discrete units of fissile material from the chamber if the coolant ceases circulating through the fuel region;a convection cooling system coupled to the dump tank for cooling the discrete units of fissile material while in the dump tank; anda connection to the dump tank for removing the discrete units of fissile material from the dump tank.