Patent Number: 
Section: claims

1. A nuclear fission reactor comprising:a burning wavefront heat generating region of a nuclear fission reactor, the burning wavefront heat generating region including a nuclear fission igniter centrally located within a nuclear fission reactor core and configured to initiate a propagating nuclear fission deflagration wave;a condensed phase density fluid flowable in thermal contact with the burning wavefront heat generating region and with a heat extraction region substantially out of thermal contact with the burning wavefront heat generating region;one or more operating condition detectors positioned to detect an operating condition in the burning wavefront heat generating region of the nuclear fission reactor core and to generate a control signal indicating the detected operating condition;a plurality of neutron modifying structures configured to direct the propagating nuclear fission deflagration wave within the burning wavefront heat generating region of the nuclear fission reactor core according to a selected propagation parameter, the control signal selectively controlling placement of the neutron modifying structures within the burning wavefront heat generating region of the nuclear fission reactor core when the detected operating condition satisfies a predetermined criterion,wherein the selected propagation parameter is a selected propagation rate and the plurality of neutron modifying structures are configured to speed up a propagation rate by being inserted behind a burnfront of the propagating nuclear fission deflagration wave. 2. The nuclear fission reactor of claim 1, wherein the condensed phase density fluid includes at least one condensed phase density fluid chosen from liquid metals, terphenyls, polyphenyls, fluorocarbons, and FLIBE. 3. The nuclear fission reactor of claim 1, wherein the condensed phase density fluid includes a nuclear inert material. 4. The nuclear fission reactor of claim 3, wherein the nuclear inert material includes He4. 5. The nuclear fission reactor of claim 1, wherein the neutron modifying structures include at least one of neutron absorbing material and neutron moderating material. 6. The nuclear fission reactor of claim 1, wherein the plurality of neutron modifying structures are further configured to direct the propagating nuclear fission deflagration wave within the burning wavefront heat generating region of the nuclear fission reactor core by selected ones of the neutron modifying structures being inserted into and removed from the burning wavefront heat generating region. 7. The nuclear fission reactor of claim 1, wherein the detected operating condition includes at least one local temperature in the nuclear fission reactor core, wherein the one or more operating condition detectors include a plurality of temperature detectors positioned to detect local temperature in the nuclear fission reactor core, and wherein the control signal includes a temperature profile of the nuclear fission reactor core. 8. The nuclear fission reactor of claim 1, wherein the detected operating condition includes at least one local temperature in the nuclear fission reactor core, wherein the one or more operating condition detectors include a plurality of temperature detectors positioned to detect local temperature in the nuclear fission reactor core, wherein the control signal includes a temperature profile of the nuclear fission reactor core, and wherein the plurality of neutron modifying structures are selectively removed from the nuclear fission reactor core when the at least one local temperature is below a predetermined temperature threshold. 9. The nuclear fission reactor of claim 1, wherein the control signal indicates at least one member of a group comprising: a power level, neutron level, neutron spectrum, neutron absorption, and fuel burnup level. 10. A nuclear fission reactor comprising:a burning wavefront heat generating region of a nuclear fission reactor, the burning wavefront heat generating region including a nuclear fission igniter centrally located within a nuclear fission reactor core and configured to initiate a propagating nuclear fission deflagration wave;a condensed phase density fluid flowable in thermal contact with the burning wavefront heat generating region and with a heat extraction region substantially out of thermal contact with the burning wavefront heat generating region;one or more operating condition detectors positioned to detect an operating condition in the burning wavefront heat generating region of the nuclear fission reactor core and to generate a control signal indicating the detected operating condition;a plurality of neutron modifying structures configured to direct the propagating nuclear fission deflagration wave within the burning wavefront heat generating region of the nuclear fission reactor core according to a selected propagation parameter, the control signal selectively controlling placement of the neutron modifying structures within the burning wavefront heat generating region of the nuclear fission reactor core when the detected operating condition satisfies a predetermined criterion,wherein the selected propagation parameter is a selected propagation rate and the plurality of neutron modifying structures are configured to slow down a propagation rate by being inserted ahead of a burnfront of the propagating nuclear fission deflagration wave. 11. The nuclear fission reactor of claim 10, wherein the condensed phase density fluid includes at least one condensed phase density fluid chosen from liquid metals, terphenyls, polyphenyls, fluorocarbons, and FLIBE. 12. The nuclear fission reactor of claim 10, wherein the condensed phase density fluid includes a nuclear inert material. 13. The nuclear fission reactor of claim 12, wherein the nuclear inert material includes He4. 14. The nuclear fission reactor of claim 10, wherein the neutron modifying structures include at least one of neutron absorbing material and neutron moderating material. 15. The nuclear fission reactor of claim 10, wherein the plurality of neutron modifying structures are further configured to direct the propagating nuclear fission deflagration wave within the burning wavefront heat generating region of the nuclear fission reactor core by inserting and removing selected ones of the neutron modifying structures into the burning wavefront heat generating region. 16. The nuclear fission reactor of claim 10, wherein the detected operating condition includes at least one local temperature in the nuclear fission reactor core, wherein the one or more operating condition detectors include a plurality of temperature detectors positioned to detect local temperature in the nuclear fission reactor core, and wherein the control signal includes a temperature profile of the nuclear fission reactor core. 17. The nuclear fission reactor of claim 10, wherein the detected operating condition includes at least one local temperature in the nuclear fission reactor core, wherein the one or more operating condition detectors include a plurality of temperature detectors positioned to detect local temperature in the nuclear fission reactor core, wherein the control signal includes a temperature profile of the nuclear fission reactor core, and wherein the plurality of neutron modifying structures are selectively removed from the nuclear fission reactor core when the at least one local temperature is below a predetermined temperature threshold. 18. The nuclear fission reactor of claim 10, wherein the control signal indicates at least one member of a group comprising: a power level, neutron level, neutron spectrum, neutron absorption, and fuel burnup level.