Single phase melt processed powellite (Ba,Ca)MoO4 for the immobilization of Mo-rich nuclear waste

Abstract

Crystalline and glass composite materials are currently being investigated for the immobilization of combined High Level Waste (HLW) streams resulting from potential commercial fuel reprocessing scenarios. Several of these potential waste streams contain elevated levels of transition metal elements such as molybdenum (Mo). Molybdenum has limited solubility in typical silicate glasses used for nuclear waste immobilization.Under certain chemical and controlled cooling conditions, a powellite (Ba,Ca)MoO 4 crystalline structure can be formed by reaction with alkaline earth elements. In this study, single phase BaMoO 4 and CaMoO 4 were formed from carbonate and oxide precursors demonstrating the viability of Mo incorporation into glass, crystalline or glass composite materials by a melt and crystallization process. X-ray diffraction, photoluminescence, and Raman spectroscopy indicated a long range ordered crystalline structure. In-situ electron irradiation studies indicated that both CaMoO 4 and BaMoO 4 powellite phases exhibit radiation stability up to 1000 years at anticipated doses with a crystalline to *Manuscript Click here to view linked References amorphous transition observed after 1 X 10 13 Gy. Aqueous durability determined from product consistency tests (PCT) showed low normalized release rates for Ba, Ca, and Mo (<0.05 g/m 2 ).

Impacts of composition and beta irradiation on phase separation in multiphase amorphous calcium borosilicates

“…As previously mentioned, the fabrication process causes the cell parameters to be higher than single crystals, which irradiation aids in remediating. CaMoO 4 as a single crystal has been previously observed stable against amorphization following Ar-irradiation, which created 5 dpa of structural modifications [48] and low energy electron irradiation produced through in-situ TEM [70]. This study proves that CaMoO 4 is also stable with minimal alteration in a calcium borosilicate for up to 1.34 GGy of β-irradiation, though crystals may migrate through the surrounding amorphous network.…”

Phase separation and crystallization of complex borosilicate melts for glass-ceramic waste forms

“…z(αn) = ϕT 2 (non-isothermal test methods) (8) and ϕ is the normalized specific heat flow per sample mass [32]. The maxima of the z(an) function vs (an) was found to be within the range predicted using the JMA model, shown in…”

“…Peterson measured quench rates from the melt to the molten tin to be ~65°C/s and from the molten tin to the water bath at room temperature to be ~30°C/s. [23,[27][28][29][30][31][32]. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) model can be used to characterize the time and temperature dependence of transformation processes, commonly but not limited to characterizing the time and temperature dependence on crystallization.…”

“…The applicability of the JMAK model to describe crystallization kinetics for glasses was investigated by Malek for both non-isothermal and isothermal conditions [32]. To understand the validity of the JMAK model it is important to understand the assumptions about the model.…”

An Interdisciplinary View of Interfaces: Perspectives Regarding Emergent Phase Formation

“…For instance, Smith et al multiple other phases such as intermetallic alloy particles, grains of calcium aluminum titanate, and titanium aluminate [76]. Formulations containing Mo waste elements often form well-known alkali molybdates such as Cs 2 MoO 4 with poor aqueous durability [77]. In current SYNROC formulations targeting waste streams from potential commercial nuclear fuel recycling in the United States, the Cs containing hollandite and pyrochlore phase containing the most prevalent lanthanide Nd as Nd 2 Ti 2 O 7 constitute a majority of the phase assemblage (combined $80%), therefore understanding interfacial interactions are of prime interest.…”