Compositional Dependence of Solubility/Retention of Molybdenum Oxides in Aluminoborosilicate-Based Model Nuclear Waste Glasses

Abstract

Molybdenum oxides are an integral component of the high-level waste streams being generated from the nuclear reactors in several countries. Although borosilicate glass has been chosen as the baseline waste form by most of the countries to immobilize these waste streams, molybdate oxyanions (MoO) exhibit very low solubility (∼1 mol %) in these glass matrices. In the past three to four decades, several studies describing the compositional and structural dependence of molybdate anions in borosilicate and aluminoborosilicate glasses have been reported in the literature, providing a basis for our understanding of fundamental science that governs the solubility and retention of these species in the nuclear waste glasses. However, there are still several open questions that need to be answered to gain an in-depth understanding of the mechanisms that control the solubility and retention of these oxyanions in glassy waste forms. This article is focused on finding answers to two such questions: (1) What are the solubility and retention limits of MoO in aluminoborosilicate glasses as a function of chemical composition? (2) Why is there a considerable increase in the solubility of MoO with incorporation of rare-earth oxides (for example, NdO) in aluminoborosilicate glasses? Accordingly, three different series of aluminoborosilicate glasses (compositional complexity being added in a tiered approach) with varying MoO concentrations have been synthesized and characterized for their ability to accommodate molybdate ions in their structure (solubility) and as a glass-ceramic (retention). The contradictory viewpoints (between different research groups) pertaining to the impact of rare-earth cations on the structure of aluminoborosilicate glasses are discussed, and their implications on the solubility of MoO in these glasses are evaluated. A novel hypothesis explaining the mechanism governing the solubility of MoO in rare-earth containing aluminoborosilicate glasses has been proposed.

Impact of rare earth ion size on the phase evolution of MoO3-containing aluminoborosilicate glass-ceramics

“…In order to design a borosilicate-based glassy waste form, it is imperative to understand the compositional and structural drivers governing the solubility of molybdenum oxides in the relevant glass chemistries. Based on the existing literature [9][10][11], the solubility of MoO 3 in alkali borosilicate glasses is limited to ~1 mass% when no rare earth ions are present (see below) [12].…”

Effect of V2O5 on crystallization tendency and chemical durability of Mo-bearing aluminoborosilicate glass

“…Mo cations primarily exist as a hexavalent state Mo 6+ in borosilicate glasses prepared in oxidizing or neutral conditions and take the form of [MoO 4 ] 2− tetrahedra, which locate in the alkali and alkaline earth enriched depolymerized regions of the glass network [13][14][15][16]. In order to improve the solubility of MoO 3 via suppressing the formation of yellow phase in borosilicate glass, a number of studies have been conducted by developing new or modified glass formulation [17][18][19][20][21]. The strategies include adding proper amount of rare earth (e.g., Nd and La) [13,[17][18][19] or high field strength modifiers such as Li [20], Mg [21] to the components, adjusting the ratio of Ca to Na and Na to B of the components [22][23][24][25][26], and increasing the B 2 O 3 level [27] in borosilicate glass.…”

Swift heavy ion-irradiated multi-phase calcium borosilicates: implications to molybdenum incorporation, microstructure, and network topology

“…This phase can act as a carrier for radioactive cesium, strontium, and minor actinides [11,12], thus affecting the safety case for storage in a geological repository. In order to prevent yellow phase precipitation, increased incorporation of molybdenum into an alternative glass composition or selective and controlled formation of water-durable CaMoO 4 [13] is currently of industrial interest [9,[14][15][16].…”

“…These tools can also be used to increase the solubility of molybdenum. A rapid quench rate of 10 4°C min -1 has been found to increase incorporation up to 2.5 mol% MoO 3 [28], while the inclusion of rare earths such as Nd 2 O 3 has had a similar effect by increasing the disorder in the depolymerized region of the glass [16,27]. Compositionally, the preferential charge balancing of BO 4 and MoO 4 2anionic entities by available cations is an important factor in initiating liquid-liquid phase separation and determining molybdate speciation.…”