Patent Number: 
Section: claims

1. A waste immobilising medium having a sodium silicate based glass matrix in which there is contained radioactive waste at a waste loading from about 80 weight % to about 90 weight % wherein the waste comprises at least 90% of a first metals containing component wherein the metals include iron, nickel and chromium, and up to 10% of a second component containing one or more fission products, these % being calculated using the masses of the oxides of the metals of the first component and of the fission products respectively. 2. A waste immobilising medium according to claim 1 wherein at least a portion of the first component is dissolved in the glass matrix. 3. A waste immobilising medium according to claim 2 wherein the metals of the first component are dissolved in the glass matrix up to their solubility limits. 4. A waste immobilising medium according to claim 1 wherein at least 90% of the waste calculated as above is comprised of iron, nickel, chromium and zinc. 5. A waste immobilising medium according to claim 1 wherein at least 90% of the waste calculated as above is comprised of iron, nickel and chromium. 6. A waste immobilising medium according to claim 1 wherein the glass comprises a weight ratio of silica to sodium oxide of between about 4.5–2.5:1. 7. A waste immobilising medium according to claim 6 wherein the weight ratio is about 4:1. 8. A waste immobilising medium according to claim 1 wherein there is a monazite phase. 9. A method of preparing a waste immobilising medium including the steps offorming a mixture comprising radioactive waste, a sodium containing precursor and silica, wherein the waste comprises at least 90% of a first metals containing component wherein the metals include iron, nickel and chromium, and unto 10% of a second component containing one or more fission products, these % being calculated using the masses of the oxides of the metals of the first component and of the fission products respectively;drying the mixture;calcining the dried mixture; andpressing and sintering the calcined mixture so that the resulting medium contains from about 80 weight % to about 90 weight % of radioactive waste. 10. A method according to claim 9 wherein the sodium containing precursor is sodium oxide (Na2O) or sodium silicate. 11. A method according to claim 9 wherein the mixture is formed between the waste and a composition which comprises a glass frit of about 20 weight % sodium oxide (Na2O) and about 80 weight % silica (SiO2). 12. A method according to claim 9 wherein a rare earth element is included in the mixture. 13. A method according to claim 9 wherein the waste is denitrated before or whilst forming the mixture. 14. A method according to claim 9 wherein the calcination is carried out in a neutral or reducing atmosphere. 15. A method according to claim 9 wherein the calcination is carried out between 650–800° C., preferably about 750° C. 16. A method according to claim 9 wherein the compaction and sintering is carried out by hot uniaxial pressing or hot isostatic pressing. 17. A method according to claim 16 wherein the temperature for hot isostatic pressing is 1000–1400° C. 18. A method of treating radioactive waste streams from the decontamination of plants, said streams comprising at least 90% of oxides of iron, nickel and chromium as well as one or more fission products, the method including the steps offorming a mixture comprising the radioactive waste, a sodium containing precursor, andsilica;drying the mixture;calcining the dried mixture; andpressing and sintering the calcined mixture to provide a sodium silicate glass based matrix. 19. A method according to claim 18 wherein the sodium containing precursor is sodium oxide (Na2O) or sodium silicate. 20. A method according to claim 18 wherein the mixture is formed between the waste and a composition which comprises a glass frit of about 20 weight % sodium oxide (Na2O) and about 80 weight % silica (SiO2). 21. A method according to claim 18 wherein a rare earth element is included in the mixture. 22. A method according to claim 18 wherein the waste is denitrated before or whilst forming the mixture. 23. A method according to claim 18 wherein the calcination is carried out in a neutral or reducing atmosphere. 24. A method according to claim 18 wherein the calcination is carried out between 650–800° C., preferably about 750° C. 25. A method according to claim 18 wherein the compaction and sintering is carried out by hot uniaxial pressing or hot isostatic pressing. 26. A method according to claim 25 wherein the temperature for hot isostatic pressing is 1000–1400° C. 27. A waste immobilising medium having a sodium silicate based glass matrix in which there is contained radioactive waste wherein the waste comprises at least 90% of a first metals containing component and up to 10% of a second component including one or more fission products calculated using the masses of the oxides of the fission products and of the metals of the first component, wherein the metals include iron, nickel and chromium, and, optionally, zinc, the metals of the first component being dissolved in the glass matrix up to their solubility limits, and wherein the glass comprises a weight ratio of silica to sodium oxide of between about 4.5–2.5:1. 28. A waste immobilising medium according to claim 27 wherein there is a monazite phase. 29. A method of treating radioactive waste streams from the decontamination of plants, said streams comprising at least 90% of oxides of iron, nickel and chromium as well as one or more fission products, the method including the steps offorming a mixture comprising the radioactive waste and a glass flit of about 20 weight % sodium oxide (Na2O) and about 80 weight % silica (SiO2), optionally with the inclusion of a rare earth element;drying the mixture;calcining the dried mixture between 650–800° C.; and pressing and sintering the calcined mixture by hot uniaxial pressing or hot isostatic pressing to provide a sodium silicate glass based matrix.