Development of monazite glass-ceramic wasteforms for the immobilisation of pyroprocessing wastes

Pyrochemical reprocessing is a potential route for the reprocessing of fuels from next-generation reactors. Lanthanide fission products may be separated from the reprocessing salt by a number of methods; however, salts may be entrained in the resultant product. This work demonstrates conceptual monazite glass-ceramic wasteforms based on the quarternary (100−x)(10Na2O–36Fe2O3–54P2O5)–xLa2O3 (x = 5, 10, 15), with La2O3 as a surrogate for the lanthanide oxide waste stream. Samples were produced by the melting of La2O3 with a sodium-iron-phosphate glass frit and characterised by XRD and SEM–EDX. The monazite phase was successfully formed at all waste loadings with clear segregation of La to the crystalline phase; however, high La2O3 loading (>10 mol%) was found to destabilise the glass system resulting in gross crystallisation. These initial results indicate that monazite glass-ceramics are promising wasteforms for this waste stream.


Introduction
There is a resurgence of interest in the pyrochemical reprocessing (pyroprocessing) of spent nuclear fuels in the context of Generation IV reactor systems, which may not be compatible with aqueous reprocessing flowsheets. In this approach, the spent fuel is dissolved in a LiCl-KCl eutectic salt (LKE, Li 0.48 K 0.52 Cl), with electrochemical separation of uranium and plutonium from the fission products [1,2]. The LKE salt is resistant to radiolysis and thus better suited to high burn up and short cooled nuclear fuels, compared to aqueous solvent extraction; additionally, co-extraction of uranium and plutonium offers enhanced proliferation resistance [1,2]. Periodically, the LKE salt will require decontamination of fission products to maintain efficiency. One approach for removal of the lanthanide fission products is oxidative precipitation, to yield Ln 2 O 3 , LnOCl, or a mixture thereof, depending on the reaction conditions, which may be achieved by oxygen sparging of the LKE molten salt [3][4][5] according to the following reactions [6]: Assuming reaction completion to yield precipitated lanthanide oxides, the waste stream may be reasonably expected to carry entrained LKE salt and other potential fission products. Consequently, a glass-ceramic wasteform may be appropriate for this waste stream, with a monazite LnPO 4 phase to immobilise the lanthanide, with entrained fission products and LKE salt immobilised in the glass phase. Accordingly, our interest is focussed on the potential of iron phosphate glass-monazite ceramic systems for this application, given the expected chemical compatibility of the monazite and phosphate glass components. Indeed, Asuvathraman et al. reported a monazite glass composite wasteform for the immobilisation of fission product HLW arising from aqueous reprocessing of fast breeder reactor fuel [7][8][9][10][11]. A monazite Ca 0.8 Ce 0.2 PO 4 host phase was first synthesised by a sol gel route, with the equivalent of 20 wt% of HLW fission products and minor actinides targeted for substitution on the Ca/Ce site. The powder material was mixed with 20 wt% 40Fe 2 O 3 -60P 2 O 5 , to form an apparently durable glass composite. This approach is of interest since it demonstrates the apparent flexibility of the monazite phase towards incorporation of fission product elements. However, it would be preferable to produce such a glass-ceramic wasteform in a single one-pot process.
T h e q u a t e r n a r y s y s t e m s ( 1 0 0 − x ) (36Fe 2 O 3 -10B 2 O 3 -54P 2 O 5 )-xLn 2 O 3 were reported to afford iron phosphate glass-monazite ceramic materials, according to the systematic studies of Wang et al. [8][9][10][11]. The (100 − x)(36Fe 2 O 3 -10B 2 O 3 -54P 2 O 5 )-xCeO 2 system was reported to be fully amorphous up to 9 mol% CeO 2 incorporation, above which monazite CePO 4 was crystallised, and additionally FePO 4 above 18 mol% [8]. Wang et al. also studied the (100 − x) (36Fe 2 O 3 -10B 2 O 3 -54P 2 O 5 )-xLn 2 O 3 system, with Ln = Nd and La, which is described by a similar phase diagram with a fully amorphous material produced up to 4 mol% Ln 2 O 3 incorporation, above which monazite LnPO 4 was crystallised [9,10]. Wang et al. further reported the application of this conceptual wasteform to the immobilisation of a simulant high level waste (HLW) produced in China, rich in actinides, transuranics and molybdenum [11]. The formulation design was (100 − x) (36Fe 2 O 3 -10B 2 O 3 -54P 2 O 5 )-xHLW; an amorphous product was obtained up to 5 mol% incorporation, above which monazite (Ce,Nd,La)PO 4 was crystallised. Incorporation of CeO 2 and Ln 2 O 3 in the quaternary systems did not markedly affect the glass dissolution rate relative to the base glass, which was of the order of 10 -2 g m −2 day −1 as measured by total weight loss of monoliths in a batch dissolution experiment at 90 °C in deionised water. However, incorporation of the HLW simulant reduced the dissolution rate to the order of 10 −3 -10 -4 g m −2 day −1 , likely due to the formation of hydration-resistant Zr-O-P bonds. Deng

Materials synthesis
An iron phosphate base glass (IPG), of composition 10Na 2 O-36Fe 2 O 3 -54P 2 O 5 (mol%), was produced by melting stoichiometric quantities of Na 2 CO 3 , Fe 2 O 3 and NH 4 H 2 PO 4 . Batched material was melted for 4 h at 1100 °C and quenched into water to produce a frit of mm dimensions. The frit was mixed with La 2 O 3 to produce a batches with 5, 10 and 15 mol% La 2 O 3 . Batched material was transferred to a recrystallized alumina crucible and melted for 4 h at 1100 °C. The melts were cast onto a steel plate, transferred to a furnace and annealed at 500 °C for 1 h before cooling at 1 °C min −1 to room temperature.

Materials characterisation
Synthesised glasses were characterised by powder X-ray diffraction (XRD). XRD was performed using a Bruker D2 Phaser diffractometer utilising Cu Kα radiation, utilising a Ni foil Kβ filter and a point step of 0.02 from 10° to 70° 2θ; the energy discriminator window of the position-sensitive Lynxeye detector was set to maximise the rejection of the fluorescent background.
Scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDX) was used to evaluate the microstructure and average composition of the synthesised glass-ceramics. Scanning electron microscopy (SEM) using a Hitachi TM3030 SEM equipped with a Bruker Quantax EDX. An accelerating voltage of 15 kV was used for imaging. Glass-ceramics were prepared for SEM analysis by mounting in cold setting resin and polishing with SiC paper and progressively finer diamond pastes to an optical finish (1 mm). Figure 1 shows the results of powder X-ray diffraction of the synthesised glass-ceramic. As can be seen, the target monazite phase was successfully synthesised for the 5 and  4 and FePO 4 at incorporation rates as low as 4 mol% [12]. It would, therefore, appear that the addition of Na 2 O in this study (and B 2 O 3 in previous studies [9,10]) stabilises the iron phosphate glass matrix against crystallisation of FePO 4 . This is of significance, since the crystallisation of iron phosphate glass phases is known to adversely affect the durability of iron phosphate glasses, through loss of hydration-resistant Fe-O-P bonds [13]. The 15 mol% La 2 O 3 sample was found to be highly crystalline, the target monazite phase was successfully formed alongside FePO 4 , NaFeP 2 O 7 and retained La 2 O 3 . It is hypothesised that the formation of a larger fraction of monazite fraction removes phosphorus from the glass network, destabilising it, resulting in gross crystallisation.

Results
Asuvathraman et al. noted that the addition of 0.2 f.u. (formula units) Ca to CePO 4 resulted in a change in unit cell size and a measurable change in the lattice parameters of the target phase relative to stoichiometric CePO 4 [7]. The refined lattice parameters of the monazite phase (10 mol% sample) produced in this study are in excellent agreement with those previously published in the literature for stoichiometric LaPO 4 ( Table 1) [14]. It is, therefore, unlikely that significant substitution of other ions into the monazite structure has occurred and the precipitated monazite is stoichiometric LaPO 4 .
SEM-EDX observation of the synthesised samples was in agreement with XRD analysis, see Fig. 2. Micron-scale crystallites enriched in La and P were clearly visible in all three samples and were concluded to be monazite, LaPO 4 . For the successful glass-ceramics (i.e. 5 and 10 mol% La 2 O 3 ), observed monazite crystallites were in the size range of 1-10 µm, and it was also possible to observe a core of La 2 O 3 oxide in some crystallites. EDX analysis of the crystallites and the matrix confirmed the presence of La in the crystalline phase and Na and Fe in the glass with P observed in both, as determined by X-ray emission lines. Point EDX measurements taken from the glass matrix and crystallites found distinct segregation of La from the matrix and little incorporation of Fe or Na in the monazite crystals, and the absence of substituent ions in the monazite phase was in agreement with the results of Rietveld refinement of XRD data.
The EDX data demonstrate partitioning of La from the glass to crystalline monazite LaPO 4 phase (Fig. 3), similar to that of monazite GdPO 4 and mixed (Ce, La, Nd)PO 4 in iron borophosphate glasses by Wang et al. [8,10]. The substitution of Gd 2 O 3 for Fe 2 O 3 in the borophosphate base glass has been observed to result in an increase in the volume fraction of the monazite phase, as a result of greater lanthanide concentration, and also, it leads to a greater tendency for the glass matrix to crystallise [10]. Considering that the observed crystallisation behaviour and segregation of La from the glass phase, as determined by EDX (Fig. 3), it would appear likely that direct substitution of La 2 O 3 for Fe 2 O 3 would improve the relative yield of monazite but may increase the tendency of the glass matrix to crystallise due to the poorer glass-forming ability of La relative to Fe.

Conclusions
This study has found that it is possible to produce monazite-alkali iron phosphate glass-ceramics via incorporation of La 2 O 3 into a sodium iron phosphate base glass melting and annealing at 500 °C for 1 h. The synthesised glass-ceramic was found to contain distinct crystallites of 1-10 µm in size with clear segregation of La to the monazite phase. The formulation developed here demonstrated an Ln 2 O 3 incorporation rate which marginally exceeds that previously achieved in iron borophosphate glasses. Partial crystallisation of the glass matrix, as observed at high Ln 2 O 3 incorporation rates in iron borophosphate glasses, which is known to be detrimental to durability, was found to occur at higher substitution levels than observed previously. These data are very encouraging for further optimisation, through a more extensive systematic study of La 2 O 3 incorporation rate and the extended phase diagram. The behaviour of more complex simulant Ln 2 O 3 waste streams should also be investigated to understand the potential for fractionation of different Ln species between glass and ceramic phases.
Data availability The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Conflict of interest
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