Patent Number: 
Section: claims

1. A method for treating nuclear sludge comprising subjecting the nuclear sludge to a plasma treatment in a plasma chamber, in the presence of an oxidant, to melt at least some of the inorganic components of the sludge, wherein the plasma chamber comprises a crucible having a cooled inner surface, this surface cooled sufficiently such that the inorganic components in contact with the inner surface are in a solid state and form a barrier between the part of surface of the crucible with which they are in contact and the molten inorganic components of the sludge;wherein the plasma chamber comprises two graphite electrodes; andwherein the electrodes are operated in one or both of:(i) a first mode in which an electric arc is passed between the electrodes above the level of the nuclear sludge (remotely coupled); or(ii) a second mode in which an electric arc is passed between the electrodes through the inorganic components of the sludge (transferred). 2. A method according to claim 1, wherein the plasma is generated by DC electricity. 3. A method according to claim 1, wherein the inner surface of the crucible comprises copper. 4. A method according to claim 1, wherein during the plasma treatment the internal surface of the crucible is maintained at a temperature below the solidus temperature of the inorganic components of sludge. 5. A method according to claim 4, wherein the inner surface of the crucible is at a temperature of 50° C. or below. 6. A method according to claim 1, wherein the crucible is water-cooled. 7. A method according to claim 1, wherein the method further comprising transferring the molten components of the sludge to a container for the storage of nuclear waste. 8. A method according to claim 1, wherein the sludge contains one or more materials selected from magnesium hydroxide, silicon dioxide, uranium oxide, magnesium carbonate, aluminium oxide, sodium oxide and magnesium oxide. 9. A method according to claim 1, wherein the method produces a solid product that contains one or more materials selected from forsterite, cordierite, albite and clinoptilolite and other zeolites. 10. A method according to claim 1, wherein the plasma treatment is carried out at a temperature of 1000° C. or more. 11. A method according to claim 1, wherein the plasma treatment is carried out at a temperature of 1800° C. or less. 12. A method according to claim 1, wherein the oxidant present within the plasma chamber comprises air. 13. A method according to claim 1, wherein gases selected from nitrogen, argon and air are fed to the plasma chamber. 14. A method according to claim 1, wherein the sludge is mechanically agitated during the plasma treatment. 15. A method according to claim 1, wherein at least one of the two graphite electrodes has a coating comprising alumina. 16. A method according claim 1, wherein the plasma chamber is further provided with:(i) a water-cooling system for cooling at least part of the inner surface of the crucible, wherein water can be passed between an outer wall and an inner wall of the crucible in order to cool the inner wall;(ii) an inlet for an oxidant adapted such that the oxidant and waste are mixed before or upon entry into the interior of the plasma chamber;(iii) an upper chamber and a lower chamber, the upper chamber being adapted to allow molten material in the upper chamber to flow by gravity into the lower chamber; and/or(iv) one or more electrodes having a coating comprising alumina.