Patent Number: 
Section: claims

1. An overpack for a storage container for radioactive waste, the overpack being assembled from metal plates each forming a wall of the overpack, the metal plates each being substantially flat and comprising an inner plate and an outer plate of laminate construction, and the overpack providing sufficient shielding for the radioactive waste, wherein the overpack is provided with spacers that are smaller than the metal plates to separate the metal plates from a storage container during use, and wherein the outer plate projects beyond an edge of the inner plate, so that mating edges of the metal plates are of stepped shape, so ensuring there is no risk of a shine-through path at the mating edges and the corners of the overpack. 2. An overpack as claimed in claim 1 wherein the metal plates are of steel or cast iron. 3. An overpack as claimed in claim 1 wherein each wall of the overpack consists of a single metal plate. 4. An overpack as claimed in claim 3 wherein each wall of the overpack comprises a mild steel sheet. 5. An overpack as claimed in claim 1 wherein each metal plate is of thickness between 75 mm and 300 mm. 6. An overpack as claimed in claim 1 also comprising bolts connecting the plates together. 7. An overpack as claimed in claim 1 also comprising a sintered metal filter arranged so that hydrogen gas can escape from the overpack. 8. An overpack as claimed in claim 1 wherein the spacers are of stainless steel. 9. An overpack as claimed in claim 1 where the spacers are of a non-metallic material, wherein the non-metallic material is a ceramic. 10. An overpack as claimed in claim 1 wherein the spacers comprise stainless steel spacer plates. 11. An overpack as claimed in claim 1 wherein the spacers are attached to each wall formed by one of the metal plates. 12. A kit for making an overpack as claimed in claim 1, the kit comprising the metal plates, connectors to join them together, and spacers that are smaller than the metal plates.