Glasses and Glass-Ceramics for Nuclear Waste Immobilization

Effect of P2O5 and Na2O on the Solubility of Molybdenum and Structural Features in Borosilicate Glass

“…Some countries, such as China, France, and the UK, have adopted the closed nuclear fuel cycle technology route, which features the reprocessing of spent nuclear fuel (SNF) to recover transuranium elements (TRUs) such as uranium (U) and plutonium (Pu) [ 1 , 2 , 3 , 4 , 5 ]. The current most mature plutonium uranium redox extraction (Purex) process is used in commercial nuclear fuel recycling, and a highly level waste (HLW) from this step is rich in many types of fission products, corrosion products, remaining TRUs, and the solvent that is added during reprocessing [ 4 , 5 , 6 , 7 , 8 ].…”

“…Furthermore, vitrification is the only industrial technology to immobilize HLW [ 9 ]. Borosilicate glass is a favorable matrix because of its good durability, irradiation resistance, and high capacity of most nonvolatile elements in HLW [ 1 , 2 , 9 , 10 ]. However, it also faces complex problems in practical applications.…”

“…Borosilicate glass has a low solubility for some key waste components in HLW [ 1 , 11 ]. For instance, the molybdate (Mo) loading limit in conventional borosilicate glasses is less than 1.5 mol% [ 2 , 12 , 13 ].…”

“…This can be explained by the high field strength of Mo 6+ (1.89–1.935 Å −2 ) [ 14 ]. Many studies indicate that Mo 6+ induces short-range ordering of surrounding oxygens, which leads to Mo predominantly occurring as isolated [MoO 4 ] 2− tetrahedra in alkali and alkaline earth-rich depolymerized regions of the borosilicate glass structure and are not linked directly to the glass network [ 1 , 2 , 15 , 16 , 17 ]. Thus, excess Mo tends to experience phase separation in the form of crystalline molybdates (yellow phase) from waste-immobilized glass [ 2 , 18 , 19 ].…”

“…Recent studies indicate that the addition of phosphorus also enhances S Mo in silicate and borosilicate glass [ 5 , 18 , 27 ]. Unlike the effect of doping with normal cations, the successful integration of Mo in the right proportion of the phosphate network occurs through P–O–Mo bonds in silicate–phosphate glass, instead of only forming isolated [MoO 4 ] 2− ; in other words, the formation of P–O–Mo bonds probably is also one of the keys to improving S Mo in borosilicate glass by doping with P [ 1 , 18 , 27 , 28 ]. However, P can be used as both network formers and phosphate species in borosilicate glass, the ratio of which depends strongly on the glass composition.…”