2.1 Sample preparation

The designated non-radioactive simulated waste contains cesium, strontium, zirconium, cerium, and samarium fluorides (Table 2). In this paper, MF_n represents the mixed fluorides in the simulated waste.

Samples of NaAlP(-Fe) glass (compositions listed in Table 3) were prepared by melting a mixture of ammonium dihydrogen phosphate (NH₄H₂PO₄), sodium hydroxide (NaOH), aluminum oxide (Al₂O₃), iron oxide (Fe₂O₃), and the raw material of the simulated wastes (Table 2). Well-mixed powders for each batch were first preheated to 300 °C (with a heating rate of 10 °C/min) in alumina crucibles and maintained at this temperature for 3 h to evaporate ammonia and water. The batches were then melted in air at 950–1200 °C (according to their components) for 30 min. The fused glass masses were poured into a preheated steel mold, held at 450–475 °C for 1 h to relieve internal stresses, and then cooled to room temperature.

The experiments aimed to establish the maximum amount of fluoride that could be incorporated into these glasses. The melting schedules were adjusted according to the viscosity of the melt and the observed volatility. The Fe₂O₃ was added to improve the thermal stability and resistance to crystallization of NaAlP glass. For comparison, IP glasses containing mixed fluorides (the simulated waste) were also prepared.

2.2 Characterization of samples

The XRD was used to identify the crystalline phases present in the glass samples (D8 advance, Bruker, Germany). Raman spectra on glass powder were recorded using a 532 nm laser and a spectrometer coupled with an optical microscope (HR800, Renishaw, England). The surfaces of the glass powder were investigated by SEM (LEO 1530VP).

2.3 Chemical durability test

The product of any competent radioactive waste treatment process should be highly resistant to corrosion by water (leaching) and not readily dispersible (i.e., monolithic, not ‘‘dusty’’). Experiments were performed to evaluate the leaching resistance of phosphate glasses and were conducted according to the product consistency test A (PCT-A) [18], which has become the DOE’s de facto standard leaching test. The glass samples were crushed manually in an agate mortar and sieved to 75–150 µm. Powders were washed following the PCT operating procedure, mixed with distilled water at a ratio of 10 ml/g, and held at 90 °C for 7 days in a stainless-steel vessel. Upon completion of each testing period, the normalized elemental mass loss was determined by chemical analysis of the leachate using ICP-AES (Na, Al, P, Fe) and ICP-MS (Cs, Sr, Zr, Ce, Sm). The data were corrected by subtracting the normalized values for accompanying blank solutions. A fluorine-ion electrode was used to analyze the F-ion concentration. A typical density of 3.0 g/cm³ was assumed for glasses, and the surface area was then calculated to be 1.79 m²/g. The leaching data were converted to leachability units (g/m²) using the formula:

Here, C_i is the concentration of species i in the leachate in g/mL, V is the volume of the leachate in mL, f_i is the initial fraction of species i in the solid, and A is the surface area of the solid in m².

After the PCT test, the powder from the leaching container was washed again with pure water, transferred to a clean watch glass, and dried at 90 °C. At this temperature, the adsorbed moisture is removed but not the water of hydration. The surface of the glass powder was investigated after drying.

3.1 X-ray diffraction

Fig. 1(a) presents the XRD patterns of the xMF_n-(100-x)NaAlP glass system with x = 4 mol%, 7 mol%, 12 mol%, 20 mol% and 28 mol%. No crystalline phases were detected by XRD for compositions containing up to 14 mol% of MF_n in the glasses. For samples with MF_n ratios ≥20 mol%, the crystalline phase was clearly detected as indicated by sharp peaks. The major crystalline phase is identified as monazite (Sm, Ce, Zr)PO₄. The results indicate that devitrification increases with increasing fluoride content. This feature is related to the atomic percentage of the modifier cation Mⁿ⁺ (Sm, Ce, Zr) [19]. Previous studies have shown that the aluminum atom appears in four-, five-, and six-fold coordination environments [16]. While the (Sm, Ce, Zr) atom coordination number is six or higher [20, 21], a strong polymerization effect may result.

Fig. 1.Full-screen View

XRD patterns: (a) NaAlP glasses (xMF_n- (100-x)NaAlP), and (b) NaAlP-Fe glasses (xMF_n-(100-x)NaAlP-Fe).

Fig. 1(b) presents XRD patterns of the xMF_n-(100-x) (Fe-NaAlP) glass systems with x = 4.4 mol%, 6.4 mol% and 8.5 mol%. The XRD patterns of the Fe₂O₃-modified NaAlP glasses show that the structure of this glass is more susceptible to fluoride addition: for samples with MF_n ratios ≥ 8.5 mol%, numerous additional crystalline phases including FePO₄, AlPO₄, and Na₃PO₄ are observed.

3.2 Raman spectroscopy

The short range order (SRO) of phosphate glasses is best described by considering the dependence of the fractions of different tetrahedral phosphate species Qⁿ on composition (the superscript 'n' represents the number of bridging oxygens on a PO₄ tetrahedron). Raman spectra of the fluoride-containing glass samples NaAlP(1-5) and NaAlP-Fe(1,3) are shown in Fig. 2.

Fig. 2Full-screen View

Raman spectra of NaAlP and NaAlP-Fe glasses containing MFn.

Versus the spectra of the crystalline materials, the vibrational modes in amorphous materials can be expected to become broader because of the thermally excited distribution of local geometries and vibrational coupling. The peaks in the spectra can be assigned to their vibrational mode according to known frequencies from iron [22], sodium [23], sodium aluminum [16], and calcium [24] phosphate glasses (Table 4).

The most intense peak in each spectrum is near 1050 cm^-1 and can be assigned to the symmetric stretch of a non-bridging oxygen on a Q¹-tetrahedron in the (PO₃)_sym mode. The higher-frequency shoulders evident in each spectrum could result from asymmetric PO₃ modes associated with Q¹-tetrahedra, but could also be from symmetric PO₂-stretching modes associated with Q²-tetrahedra. Likewise, the lower-frequency shoulders could be assigned to other PO₃ modes associated with Q¹-tetrahedra or to PO₄ modes associated with Q⁰-tetrahedra units in the glasses.

The bands between 700 cm^-1 and 800 cm^-1 are related to the symmetric P–O–P-stretching modes associated with linkages to Q¹-tetrahedra. Indeed, the Raman spectra reveal the existence of Q¹ and Q⁰ units in the glasses with a trace of Q². With increasing fluoride content, the Qⁿ units shift slightly in relative amplitude and frequency. It is interesting that the PO₄ symmetric stretch (Q⁰) mode exhibits a greater relative intensity in the NaAlP5 glass than in the other glasses. Distinct Raman line structure appears in the NaAlP₅, in which crystallization is observed by XRD.

In contrast to other phosphate glasses (e.g., Refs [16, 22, 23]), the Raman spectra in Fig. 2 show that the NAlP(-Fe) glasses containing fluorides have no long chain phosphate structures, which is similar to the results observed in CaO-P₂0₅ by Kasuga [24]. In Refs [25, 26], it is argued that a fluorine atom can replace an oxygen atom in a Qⁿ unit to slightly reduce network connectivity in the glass. This may be indirect evidence of fluorine’s ability to break the phosphate network into smaller parts that are less constrained by neighboring units.

3.3 Chemical durability

The leaching rates (g/m²) were calculated based on the normalized elemental mass loss (Table 5). The leaching data for the samples indicate that all releases values are ≤0.2 g/m² in the test period—this meets the requirements of EJ 1186-2005 (<1 g/(m²d)) [27]. The release rates of Sr²⁺, Ce^3+/4+, Zr⁴⁺, and Sm³⁺ are an order of magnitude lower than Na⁺, P⁵⁺, Fe³⁺, Al³⁺, F^-, and Cs⁺. In addition, the quantity released did not change substantially when the waste form crystallized (NaAlP₅).

The good chemical durability of NaAlP glasses is primarily due to the presence of strong and hydration-resistant Al–O–P bonds [12], which replace unstable P–O–P bonds. The latter become depolymerized upon hydration through the formation of terminal -OH groups [8, 17]. This result is consistent with the Raman spectra results, which indicate that the NaAlP glasses contain few Q² units but Q³ units are not evident. It is known that the addition of Fe₂O₃ can also improve durability. This oxide acts as both an intermediate and a glass-modifying oxide and can be present in the glass network in both Fe²⁺ and Fe³⁺ states to replace P–O–P bonds with the more durable P–O–Fe²⁺ or P–O–Fe³⁺ bonds [28, 29]. This is consistent with the leaching results of the NaAlP-Fe samples. However, the addition of Fe₂O₃ to NaAlP glasses decreases the fluoride loading markedly. It might therefore be prudent to limit the addition of Fe₂O₃ in NaAlP glasses.

In practice, the performance of any candidate HLW form on the PCT test relative to the DOE’s benchmark has come to define whether or not it is “satisfactory”. Data from FZ-1 [6], ARM-1 [30, 31] and CVS-IS [6] leaching tests conducted under similar conditions were also converted to leachability units for comparison. As shown in Table 6, the release rates of sodium, phosphorus, aluminum, strontium, and iron are quite low—typically less than the quantity released from ARM-1 and CVS-IS borosilicate glasses. The dissolution rate of these phosphate glasses in distilled water is lower than that of these borosilicate glasses and is as low as that of the IP glass FZ-1 in distilled water.

The SEM images of glass particles of samples NaAlP₂ and NaAlP-Fe1 are shown in Fig. 3. The NaAlP glass has high surface area (Fig. 3a and 3b), and the treated NaAlP glass particles are covered with a thin layer of a honeycomb structure consisting of nm-sized spherules and pores (Fig. 3b). In contrast, the NaAlP-Fe glass tested for 7 days at 90 °C (Fig. 3d) did not show obvious signs of corrosion (Fig. 3c). The absence of sharp scratches in Fig. 3c and the absence of a detectable corrosion layer on the surface in Fig. 3d provides clear evidence of the excellent chemical durability of these waste forms as detailed in Table 5.

Fig. 3.Full-screen View

SEM micrographs of surface: (a) pristine NaALP₂, (b) NaALP₂ after a 7-day PCT test, (c) pristine NaALP-Fe1, and (d) NaALP-Fe1 after a 7-day PCT test.