Improving Molybdenum and Sulfur Vitrification in Borosilicate Nuclear Waste Glasses Using Phosphorus: Structural Insights from NMR

Abstract

Reclaiming residual fissile materials from spent nuclear fuel rods results in highly radioactive waste that must be immobilized in durable materials to protect the biosphere from harmful environmental exposure. The glasses typically used for this purpose are less effective at incorporating highly charged cations than cations in lower oxidation states. Molybdenum and sulfur solubilities in model aluminoborosilicate nuclear glasses were studied using solid-state nuclear magnetic resonance (NMR) spectroscopy. The extent of molybdenum and sulfur incorporation was significantly improved by doping the glasses with phosphorus, a high-field-strength cation capable of competing with Mo6+ and S6+ for oxygen, resulting in more oxygen sharing and a higher degree of Mo and S integration into the glass network. The addition of 3 mol % each of molybdenum and sulfur to phosphorus-free glasses led to the separation of crystalline sodium molybdates and sodium sulfates, whereas the replacement of 5 mol % SiO2 by P2O5 resulted in complete Mo and S incorporation into the glass, enhancing their respective loading limits by several times. NMR spectroscopy indicates that this improvement originates from the availability of chain phosphates as binding sites. This work demonstrates the value of using phosphate additives to improve Mo and S integration in nuclear glasses, providing insight into its specific structural role and foundational understanding for further development as a material design principle.

Cation field‐strength effects on ion irradiation‐induced mechanical property changes of borosilicate glass structures

“…To gain more quantitative insight into the evolution of the 11 B [4] NMR spectral regions for the glasses with variable M z + CFS, each spectrum was deconvoluted into two 11 B [3] contributions and three counterparts from 11 B [4] ( m Si) environments with m = {2, 3, 4}, 80,84,86 each centered at the respective 11 B [4] shift, $$\delta _{\rm{B}}^{[ 4 ]}( {m{\rm{Si}}} )$$, and exhibiting a fractional population $$x_{\rm{B}}^{[ 4 ]}( {m{\rm{Si}}} )$$. This resulted in near‐constant shifts of {$$\delta _{\rm{B}}^{[ 4 ]}( {4{\rm{Si}}} )$$, $$\delta _{\rm{B}}^{[ 4 ]}( {3{\rm{Si}}} )$$, $$\delta _{\rm{B}}^{[ 4 ]}( {2{\rm{Si}}} )$$} ≈ {−1.7, −0.3, 1.2} ppm separated by approximately 1.5 ppm (Table 2), in very good agreement with those reported very recently by Krishnamurthy and Kroeker 87 . The present shifts are also close to the values of Du and Stebbins, 84 and intermediate of those reported in refs.…”

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

“…Three main resonances were present in these spectra based on documented 31 P chemical shifts, including a broad resonance centered near 1.5 ppm, a resonance appearing around 4–6 ppm, and a broad resonance between −7 and −4 ppm. The resonance near 5 ppm and 1.5 ppm were from isolated orthophosphate species (P 0 ) and diphosphate species (P 1 ) within the glass structure [ 18 , 31 ], respectively.…”

“…This phenomenon was not obvious due to the low phosphorus content in the Mx series and because the difference in Al content in the samples was not significant except for M34-1. The chemical shift between −7 and −4 ppm in Table 4 fell between a pyrophosphate (P–O–B/P–O–Al) anchored to a borate or an aluminate unit; however, it was not possible to identify contributions to 31 P signals from P–O–B or P–O–Al bonds based on the chemical shift alone [ 18 , 27 ].…”

“…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 ]. Furthermore, the yellow phase may contain radioactive elements ( 137 Cs or 90 Sr) and dissolve easily in water [ 17 , 20 ].…”

“…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.…”