Patent Number: 
Section: claims

1. A mobile, graphite-moderated fission reactor, comprising:a pressure vessel defining an interior volume,an active core region located within the interior volume of the pressure vessel, the active core region including a fuel assembly and a reflector; andat least one control system including a plurality of control rod drive mechanisms, wherein at least a portion of the at least one control system is located in a control region within the interior volume of the pressure vessel, wherein the fuel assembly includes a plurality of fuel unit cells and a plurality of control unit cells, each unit cell including a longitudinally extending graphite body with a longitudinally extending channel with a cladding,wherein a fuel rod is positioned in the channel of each of the plurality of fuel unit cells and forms a fuel rod flow annulus between an outer surface of the fuel rod and an inner surface of the cladding of the channel of the fuel unit cell,wherein a plurality of fuel rod flow annulus features are attached to the inner surface of the cladding of the channel of the fuel unit cell,wherein the plurality of fuel rod flow annulus features are positioned at longitudinally separated locations in an axial direction of the fuel rod flow annulus,wherein, at each longitudinally separated location, the plurality of fuel rod flow annulus features are circumferentially distributed at radially equal intervals such that an angular separation of the plurality of fuel rod flow annulus features at each longitudinally separated location satisfies the relationship 360/N, where N is the number of fuel rod flow annulus features at the longitudinally separated location and N≥3,wherein the fuel rod flow annulus features at a first longitudinally separated location are rotationally offset relative to the fuel rod flow annulus features at an adjacent longitudinally separated location, andwherein the pressure vessel is sized for mobile transport using a ship, train or truck. 2. The mobile, graphite-moderated fission reactor according to claim 1, wherein an amount of the rotational offset of the fuel rod flow annulus features is half the angular separation of the fuel rod flow annulus features. 3. The mobile, graphite-moderated fission reactor according to claim 1, wherein the longitudinally separated locations are equally spaced in the axial direction of the annulus. 4. The mobile, graphite-moderated fission reactor according to claim 1, further including a shipping container, wherein the pressure vessel is contained within the shipping container. 5. The mobile, graphite-moderated fission reactor according to claim 1, wherein the plurality of fuel rod flow annulus features attached to the inner surface of the cladding of the channel of the fuel unit cell are a first plurality of fuel rod flow annulus features, andwherein a second plurality of fuel rod flow annulus features are attached to the outer surface of the fuel rod. 6. The mobile, graphite-moderated fission reactor according to claim 1, wherein a control rod is positioned in the channel of each of the plurality of control unit cells and forms a control rod flow annulus between an outer surface of the control rod and an inner surface of the cladding of the channel of the control unit cell, andwherein a plurality of control rod flow annulus features are attached to the inner surface of the cladding of the channel of the control unit cell. 7. The mobile, graphite-moderated fission reactor according to claim 6, wherein the plurality of control rod flow annulus features are circumferentially distributed at radially equal intervals. 8. The mobile, graphite-moderated fission reactor according to claim 6, wherein the plurality of control rod flow annulus features are equally spaced at longitudinally separated locations. 9. The mobile, graphite-moderated fission reactor according to claim 6, wherein the plurality of control rod flow annulus features are equally spaced at longitudinally separated locations, and wherein, at each longitudinally separated location, the plurality of control rod flow annulus features are circumferentially distributed at radially equal intervals. 10. The mobile, graphite-moderated fission reactor according to claim 9, wherein the control rod flow annulus features at successive, longitudinally separated locations are rotationally offset relative to each other. 11. The mobile, graphite-moderated fission reactor according to claim 9, wherein, at each longitudinally separated location, there are three or more control rod flow annulus features. 12. The mobile, graphite-moderated fission reactor according to claim 11, wherein the control rod flow annulus features at successive, longitudinally separated locations are rotationally offset relative to each other. 13. The mobile, graphite-moderated fission reactor according to claim 6, further including a shipping container, wherein the pressure vessel is contained within the shipping container. 14. The mobile, graphite-moderated fission reactor according to claim 6, wherein the plurality of control rod flow annulus features are positioned at longitudinally separated locations in an axial direction of the control rod flow annulus, wherein, at each longitudinally separated location, the plurality of control rod flow annulus features are circumferentially distributed at radially equal intervals such that an angular separation of the plurality of control rod flow annulus features at each longitudinally separated location satisfies the relationship 360/N, where N is the number of control rod flow annulus features at the longitudinally separated location and N≥3, andwherein the control rod flow annulus features at a first longitudinally separated location are rotationally offset relative to the control rod flow annulus features at an adjacent longitudinally separated location. 15. The mobile, graphite-moderated fission reactor according to claim 14, further including a shipping container, wherein the pressure vessel is contained within the shipping container. 16. The mobile, graphite-moderated fission reactor according to claim 14, wherein an amount of the rotational offset of the control rod flow annulus features is half the angular separation of the control rod flow annulus features. 17. The mobile, graphite-moderated fission reactor according to claim 6, wherein the plurality of control rod flow annulus features attached to the inner surface of the cladding of the channel of the control unit cell are a first plurality of control rod flow annulus features, andwherein a second plurality of control rod flow annulus features are attached to the outer surface of the control rod. 18. The mobile, graphite-moderated fission reactor according to claim 17, wherein the control rods are translatable relative to a guide structure, and wherein at least three longitudinally separated locations of the second plurality of control rod flow annulus features are present in the guide structure during translation of the control rods.