Patent Number: 
Section: claims

1. Neutron shielding for the central column of a tokamak nuclear fusion reactor, the neutron shielding comprising an electrically conductive neutron absorbing material, wherein the neutron shielding is arranged such that the electrically conductive neutron absorbing material forms a solenoid for the initiation of plasma within the tokamak. 2. Neutron shielding according to claim 1, wherein the neutron shielding comprises an electrically insulating material arranged to separate turns of the solenoid formed by the electrically conductive neutron absorbing material. 3. Neutron shielding according to claim 1, wherein the neutron shielding is arranged such that any straight path between the location of the plasma during operation of the nuclear fusion reactor and the central column passes through the neutron absorbing material. 4. Neutron shielding according to claim 3, wherein the neutron shielding is formed from a plurality of radially arranged layers, each layer including neutron absorbing material arranged to cover gaps in the neutron absorbing material in other layers of the plurality of layers. 5. Neutron shielding according to claim 4, wherein each layer is constructed so as to form a solenoid, with alternate layers having opposite sense of winding, and wherein adjacent layers are connected in series at the top or bottom of the layers. 6. Neutron shielding according to claim 1, wherein the neutron shielding is constructed from a plurality of arcuate segments of the electrically conductive neutron absorbing material, wherein said segments are connected to provide a substantially helical current path. 7. Neutron shielding according to claim 6, wherein each segment comprises a first interlocking feature at one end of one face of the segment, and a second complementary interlocking feature at the opposite end of the opposite face of the segment, wherein the segments are connected by connecting the first interlocking feature of a first segment with the second interlocking feature of a subsequent segment. 8. Neutron shielding according to claim 7, wherein one end of a top arcuate face of the first segment is connected by the interlocking features to an opposite end of a bottom arcuate face of the subsequent segment. 9. Neutron shielding according to claim 7, wherein the first interlocking feature is a protrusion, and the second interlocking feature is a complementary recess. 10. Neutron shielding according to claim 7, wherein the first interlocking feature is a dowel, and the second interlocking feature is a complementary bore. 11. Neutron shielding according to claim 7, wherein the first interlocking feature is formed from a material having a greater shear strength than the electrically conductive neutron absorbing material. 12. Neutron shielding according to claim 7, wherein the first and/or second interlocking feature is formed from a material having a greater conductivity than the electrically conductive neutron absorbing material. 13. Neutron shielding according to claim 7, wherein each segment comprises an electrical connection region which is in contact with the respective subsequent segment, and a layer of insulating material arranged such that the only connection between segments is via the electrical connection region of each segment. 14. Neutron shielding according to claim 13, wherein the electrical connection region comprises a material having a greater conductivity than the electrically conductive neutron absorbing material. 15. Neutron shielding according to claim 14, wherein the electrical connection region comprises a patch made of a metal. 16. Neutron shielding according to claim 6, wherein each segment has top and bottom surfaces defined by two circular arcs and the lines connecting their ends, where the circular arcs have common angle and centre, and each segment has sides which extend vertically between the top and bottom surfaces. 17. Neutron shielding according to claim 16, wherein the angle of each circular arc is less than 180 degrees, and the segments are arranged to provide two or more helical current paths. 18. Neutron shielding according to claim 6, wherein the segments are arranged in two layers, the second layer being arranged radially outward of the first layer, and displaced axially and rotationally from the first layer such that any straight path between the location of the plasma during operation of the nuclear fission reactor and the central column passes through neutron shielding. 19. Neutron shielding according to claim 1, wherein the electrically conductive neutron absorbing material is a cemented carbide and/or boride comprising a binder and an aggregate, the aggregate comprising particles of a carbide and/or boride compound, and the binder comprising a metal. 20. Neutron shielding according to claim 19, wherein the aggregate comprises one or more of:tungsten carbide;tungsten boride;a ternary tungsten borocarbide. 21. Neutron shielding according to claim 1, wherein the electrically conductive neutron absorbing material has an electrical conductivity at 300K greater than 106 S/m. 22. Neutron shielding according to claim 1, wherein the electrically conductive neutron absorbing material comprises a metal of the sixth period of the periodic table. 23. Neutron shielding according to claim 15, wherein the metal is copper.