Patent Number: 
Section: claims

1. A nuclear fission reactor system comprising:a core region containing a fuel structure, the fuel structure having a first region and a second region contiguous with the first region, each one of the first and second regions being capable of criticality independent of the other of the first and second regions, the fuel structure being configured to permit propagation of a propagating nuclear fission deflagration wave along a propagation path from the first region to the second region, the propagation path including at least one portion of retracement from the second region to the first region, the fuel structure including a first nuclear fission deflagration wave igniter disposed in the first region and a second nuclear fission deflagration wave igniter disposed in the second region. 2. The nuclear fission reactor system of claim 1, wherein the propagation path includes a first portion along a first direction and wherein the at least one portion of retracement follows the first direction. 3. The nuclear fission reactor system of claim 1, wherein the propagation path includes a first portion along a first direction and wherein the at least one portion of retracement opposes the first direction. 4. The system of claim 1, wherein:the fuel structure includes a first neutron environment ahead of the propagating nuclear fission deflagration wave; anda second neutron environment that is different from the first neutron environment along the at least one portion of retracement. 5. The system of claim 4, wherein:the first neutron environment includes a first amount of fissile isotope; andthe second neutron environment includes a second amount of fissile isotope that is different from the first amount of fissile isotope. 6. The system of claim 4, wherein:the first neutron environment includes a first amount of fission products; andthe second neutron environment includes a second amount of fission products that is different from the first amount of fission products. 7. The system of claim 4, wherein the second neutron environment is different from the first neutron environment due to changes in control of neutron modifying structures. 8. The system of claim 4, wherein the second neutron environment is different from the first neutron environment due to changes in thermal heat extraction levels. 9. The system of claim 1, wherein the propagating nuclear fission deflagration wave and the fuel structure move relative to each other. 10. The system of claim 9, wherein:the fuel structure is stationary; andthe propagating nuclear fission deflagration wave propagates through the fuel structure. 11. The system of claim 9, wherein:the propagating nuclear fission deflagration wave remains substantially spatially fixed; andthe fuel structure is moved relative to the propagating nuclear fission deflagration wave.