What is claimed is:1. A nuclear reactor comprising:a nuclear reactor core volume configured to hold molten fuel salt during a sustained nuclear fission reaction;a neutron reflector assembly of a plurality of reflector elements separated by an interstitial space within a reflector volume surrounding the nuclear reactor core volume, the interstitial space configured to hold molten fuel salt;a fixed core barrel between the nuclear reactor core volume and the neutron reflector assembly; andthe neutron reflector assembly being further configured to adjust fast neutron flux and thermal neutron flux within the nuclear reactor core volume by altering reflectivity characteristics of reflector material in the reflector elements;wherein each reflector element is a reflector tube containing at least some neutron reflecting material and the interstitial space is separated from the nuclear reactor core by the fixed core barrel; andwherein the fixed core barrel is arranged to allow molten fuel salt to pass between the reactor core volume and the interstitial space between the reflector tubes in the reflector volume.2. The nuclear reactor of claim 1 wherein the interstitial space contains a chloride salt.3. The nuclear reactor of claim 2 wherein the chloride salt has an enriched amount of the Cl isotope.4. The nuclear reactor of claim 1 wherein at least one of the reflector tubes is rotatable.5. The nuclear reactor of claim 1 wherein at least one of the reflector tubes includes a neutron absorbing element in addition to the reflecting material.6. The nuclear reactor of claim 5 wherein the neutron absorbing element is a partial liner inside a casing of the reflector tube.7. The nuclear reactor of claim 1 further comprising:at least one insertable core barrel sized to fit within and adjacent to the fixed core barrel and, thereby, reducing the nuclear reactor core volume.8. The nuclear reactor of claim 1 wherein at least two of the reflector tubes in the plurality of reflector elements have different radius values.9. A nuclear reactor comprising:a nuclear reactor core volume configured to hold molten fuel salt during a sustained nuclear fission reaction;a neutron reflector assembly of a plurality of reflector elements separated by an interstitial space within a reflector volume surrounding the nuclear reactor core volume, the interstitial space configured to hold molten fuel salt;a fixed core barrel between the nuclear reactor core volume and the neutron reflector assembly; andthe neutron reflector assembly being further configured to adjust fast neutron flux and thermal neutron flux within the nuclear reactor core volume by altering reflectivity characteristics of reflector material in the reflector elements;wherein each reflector element is a reflector tube containing at least some neutron reflecting material and the interstitial space is separated from the nuclear reactor core by the fixed core barrel; andwherein at least one of the reflector tubes is rotatable.10. The nuclear reactor of claim 9 wherein the interstitial space contains a chloride salt.11. The nuclear reactor of claim 10 wherein the chloride salt has an enriched amount of the Cl isotope.12. The nuclear reactor of claim 9 wherein the fixed core barrel is arranged to allow molten fuel salt to pass between the reactor core volume and the interstitial space between the reflector tubes in the reflector volume.13. The nuclear reactor of claim 9 wherein at least one of the reflector tubes includes a neutron absorbing element in addition to the reflecting material.14. The nuclear reactor of claim 13 wherein the neutron absorbing element is a partial liner inside a casing of the reflector tube.15. The nuclear reactor of claim 9 further comprising:at least one insertable core barrel sized to fit within and adjacent to the fixed core barrel and, thereby, reducing the nuclear reactor core volume.16. The nuclear reactor of claim 9 wherein at least two of the reflector tubes in the plurality of reflector elements have different radius values.17. A nuclear reactor comprising:a nuclear reactor core volume configured to hold molten fuel salt during a sustained nuclear fission reaction;a neutron reflector assembly of a plurality of reflector elements separated by an interstitial space within a reflector volume surrounding the nuclear reactor core volume, the interstitial space configured to hold molten fuel salt;a fixed core barrel between the nuclear reactor core volume and the neutron reflector assembly; andthe neutron reflector assembly being further configured to adjust fast neutron flux and thermal neutron flux within the nuclear reactor core volume by altering reflectivity characteristics of reflector material in the reflector elements;wherein each reflector element is a reflector tube containing at least some neutron reflecting material and the interstitial space is separated from the nuclear reactor core by the fixed core barrel; andwherein at least two of the reflector tubes in the plurality of reflector elements have different radius values.18. The nuclear reactor of claim 17 wherein the interstitial space contains a chloride salt.19. The nuclear reactor of claim 18 wherein the chloride salt has an enriched amount of the Cl isotope.20. The nuclear reactor of claim 17 wherein at least one of the reflector tubes is rotatable.21. The nuclear reactor of claim 17 wherein at least one of the reflector tubes includes a neutron absorbing element in addition to the reflecting material.22. The nuclear reactor of claim 21 wherein the neutron absorbing element is a partial liner inside a casing of the reflector tube.23. The nuclear reactor of claim 17 further comprising:at least one insertable core barrel sized to fit within and adjacent to the fixed core barrel and, thereby, reducing the nuclear reactor core volume.24. The nuclear reactor of claim 17 wherein the fixed core barrel is arranged to allow molten fuel salt to pass between the reactor core volume and the interstitial space between the reflector tubes in the reflector volume.25. A nuclear reactor comprising:a nuclear reactor core volume configured to hold molten fuel salt during a sustained nuclear fission reaction;a neutron reflector assembly of a plurality of reflector elements separated by an interstitial space within a reflector volume surrounding the nuclear reactor core volume, the interstitial space configured to hold molten fuel salt;a fixed core barrel between the nuclear reactor core volume and the neutron reflector assembly; andthe neutron reflector assembly being further configured to adjust fast neutron flux and thermal neutron flux within the nuclear reactor core volume by altering reflectivity characteristics of reflector material in the reflector elements;wherein the core barrel is arranged to allow molten fuel salt to pass between the reactor core volume and the interstitial space between the reflector elements in the reflector volume.26. The nuclear reactor of claim 25 wherein the interstitial space contains a chloride salt.27. The nuclear reactor of claim 26 wherein the chloride salt has an enriched amount of the Cl isotope.28. The nuclear reactor of claim 25 wherein each reflector element is an annular element containing at least some neutron reflecting material.29. The nuclear reactor of claim 25 wherein each annular element is a channel containing neutron reflecting material.30. The nuclear reactor of claim 25 wherein each reflector element is an annular element containing at least some neutron reflecting material.31. The nuclear reactor of claim 25 wherein each reflector element is a reflector tube containing at least some neutron reflecting material.32. The nuclear reactor of claim 31 wherein at least two of the reflector tubes in the plurality of reflector elements have different radius values.33. The nuclear reactor of claim 31 wherein at least one of the reflector tubes is rotatable.34. The nuclear reactor of claim 31 wherein at least one of the reflector tubes includes a neutron absorbing element in addition to the reflecting material.35. The nuclear reactor of claim 34 wherein the neutron absorbing element is a partial liner inside a casing of the reflector tube.36. The nuclear reactor of claim 25 further comprising:at least one insertable core barrel sized to fit within and adjacent to the fixed core barrel and, thereby, reducing the nuclear reactor core volume.