Review
Structure study of new uranium loaded borosilicate glasses
M. Fábián
a,b,⁎ , E. Sváb
b, M.v. Zimmermann
caCentre for Energy Research, H-1525 Budapest, P.O.B. 49, Hungary
bWigner Research Centre for Physics, H-1525 Budapest, P.O.B. 49, Hungary
cHasylab at Desy, Notkestrasse 85, D-22603 Hamburg, Germany
a b s t r a c t a r t i c l e i n f o
Article history:
Received 6 June 2013
Received in revised form 1 September 2013 Available online xxxx
Keywords:
Borosilicate glasses;
Radioactive waste disposal;
Neutron diffraction;
X-ray diffraction;
Reverse Monte Carlo simulation
The structure of multi-component borosilicate SiO2–B2O3–Na2O–BaO–ZrO2glass loaded with 30 wt.% UO3was investigated by neutron diffraction and high-energy X-ray diffraction. Reverse Monte Carlo modelling was applied to obtain a possible 3-dimensional atomic configuration consistent with the experimental data. It was established that the glassy network consists of tetrahedral SiO4and of mixed tetrahedral BO4and trigonal BO3
units. With increasing boron content the relative number of BO3/BO4increases and thefirst neighbour rB\O
distance decreases. For the U\O correlations two distinctfirst neighbour distances were determined at 1.8 Å with 1.9 oxygen atoms and at 2.2 Å with 3.7 oxygen atoms. Significant second neighbour correlations have been established between uranium and the network former (Si, B), the modifier (Na) and the stabilizer (Zr) atoms. From these observations we may conclude that uranium ions take part in the network forming. This may be the reason of the observed good glassy stability and hydrolytic properties.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
High-level radioactive waste (HLW) produced by spent fuel reprocessing of civil nuclear reactors currently is incorporated into an inert host material. HLW contains actinides (fission and activation prod- ucts), primarily U, and Pu that is thefission byproduct of the UO2burning process. The vitrification technology for radioactive waste management commonly favours the use of borosilicate glasses. Borosilicates are gen- erally accepted as proper HLW isolating media[1,2], as they have a unique blend of processing and product characteristics, which make them nearly ideal for this application. They are satisfying the following major requirements: the radioactive elements become immobilized as part of the host material structure, the leaching rate of radioactive ele- ments is acceptably low, and the encapsulation cost is acceptable. Struc- tural characterization of these glasses is essential for the understanding of glass durability. However, relatively few structural investigations have been performed on uranium-loaded glasses[3,4]because of the large number of constituent elements.
Recently, we have prepared 5-component sodium borosilicate host (matrix) glasses SiO2–B2O3–Na2O–BaO–ZrO2, which proved to be stable and capable of hosting uranium[5–8]. Network formers SiO2and B2O3
have strong covalent bonds involving tetrahedral SiO4and mixed tetra- hedral BO4and BO3triangles. These network formers are located in the centre of oxygen polyhedral configuration. The polyhedra are then tied together by sharing corners, thereafter various elements of HLW occupy
positions in this three dimensional network depending on the electro- negativity, ionic size etc. Na2O serves as network modifier, while BaO and ZrO2are glass and hydrolytic stabilizers. Modifiers form weaker bond with oxygen atoms than the network formers and, act to balance the negatively charged borosilicate network. These modifiers are useful in formulating glass with potential to incorporate high molecular weight compounds.
In this work we present the structure determination of a new 6-component glass system, as far as we know not studied yet, [SiO2· B2O3· Na2O · BaO · ZrO2] + UO3using both neutron diffrac- tion (ND) and X-ray diffraction (XRD) experiments. Simultaneous RMC simulation of ND and XRD data was applied to generate a reliable 3-dimensional atomic configuration to get information on the short and medium-range order. The partial atomic pair-correlation functions, the characteristic distances and coordination-numbers are calculated with special interest on the uranium surrounding. The results for one concentration of this series, we have reported earlier[9].
2. Experimental details 2.1. Samples
The glassy samples were prepared by melt-quench technique. A high temperature electrical furnace was used with a platinum crucible under atmospheric conditions. The glasses of composition 70 wt.%[(65−x) SiO2· xB2O3· 25Na2O · 5BaO · 5ZrO2] + 30wt.%UO3with x = 5, 10, 15, 20 mol% (hereafter referred to as UB5, UB10, UB15, UB20) were prepared from raw materials of p.a. grade, SiO2, Na2CO3supplied by Reactivul (Bucuresti), BaO and ZrO2 by Merck (Darmstadt), UO3
⁎ Corresponding author at: Centre for Energy Research, H-1525 Budapest, P.O.B. 49, Hungary.
E-mail address:fabian.margit@energia.mta.hu(M. Fábián).
0022-3093/$–see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jnoncrysol.2013.09.004
Contents lists available atScienceDirect
Journal of Non-Crystalline Solids
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j n o n c r y s o l
the quenched glasses in an agate mill.
The density of the samples was measured by chemical method, using a hydrostatic balance based on Archimedes' principle. The mea- sured density of the glasses was 3.75 ± 0.04, 3.68 ± 0.04, 3.62 ± 0.03 and 3.53 ± 0.03 g·cm−3for UB5, UB10, UB15 and UB20, respec- tively. The number density, which is an important input parameter for the RMC modelling (see later inSection 3) was calculated from these experimental data, and resulted in an average number density of ρ0= 0.08 atoms · Å−3, being the same value for all compositions within limit of error.
The elemental composition was verified by Prompt Gamma Activa- tion Analysis (PGAA)[11,12], the measured values were used in data evaluation procedure, as they are tabulated inTable 1. The samples proved to be fully amorphous, no crystalline phase was detected. For bo- rosilicate glasses it is an often problem, that the glass is hydrolytic, and with time it becomes humid. In our previous studies we have prepared and studied several compositions based on silicate and borosilicate networks of 4-components (Si, B, Na, Ba–oxide glasses), and it was established that the amorphous samples partly crystallize and absorb hydrogen[5]. By adding Zr and U these properties could be improved [7–9]. The amorphous and the hydrolytic states of these new samples were periodically checked by ND measurements at ambient conditions.
Anyhow, we have handled the samples with the greatest care by keep- ing them in a desiccator.
2.2. Neutron and X-ray diffraction experiments
Neutron diffraction measurements have been performed at the 10 MW Budapest research reactor using the‘PSD’neutron powder diffrac- tometer[13]. Monochromatic wavelength ofλ0= 1.068 Å was used.
The diffraction spectrum was measured in the momentum transfer range of Q = 0.95–9.8 Å−1. The powder specimens of about 3–4 g werefilled in cylindrical vanadium sample holder of 8 mm diameter, 50 mm height and 0.07 mm wall thickness. The specimens had to be handled with a special care due to their radioactivity. Correction and normalization procedures utilized to obtain the total structure factor S(Q) from the measured pattern was described in our previous work [8].Fig. 1/a shows the ND experimentalS(Q) data for the investigated samples together with the results of RMC simulation (details of the RMC modelling will be discussed in the next section).
The high energy X-ray diffraction measurements were carried out at the BW5 experimental station[14]at HASYLAB, DESY. The powdered samples werefilled into quartz capillary of 2 mm in diameter (wall
the overlappingQ-range for UB5 composition inFig. 3. to see the details better. The intensities or/and the positions of the oscillations posses unlike values: for the neutronS(Q) the peak positions are at 1.3, 2.0, 2.9, 5.3, 8.2 Å, while for the X-rayS(Q) at 1.3, 2.1, 3.7, 5.5, 7.4, 9.3 Å.
The twofirst peak positions are at similar values, but their intensities are significantly different. This is due to the different weighting factors, wij, of the partial structure factors,Sij(Q) defined as:
S Qð Þ ¼Xk
i;j
wijSijð Þ;Q ð1Þ
wijND¼ cicjbibj
Xk
i;j
cibj 2 4
3 5
2; ð2Þ
wij
XRDð Þ ¼Q cicjfið ÞQ fjð ÞQ Xk
i;j
cifið ÞQ 2
4
3 5
2; ð3Þ
whereci,cjare the molar fractions of the components,bi,bjare the co- herent neutron- andfi(Q),fj(Q) are the X-ray scattering amplitudes, and k is the number of elements in the sample. Note, that the neutron scattering amplitude is constant[15], while the X-ray scattering ampli- tude isQ-dependent[16]and for each atom in a somewhat different way. In order to see the weight of the different atom pairs in the total S(Q) functions for the two radiations,Table 2presentswijND
andwijXRD
(Q= 0.8 Å−1) for several atom pairs. It can be seen that the Si\O atom pairs have a significant contribution for both radiations, however, for ND experiment it is more dominant than for XRD, while the B\O plays a significant contribution only in the neutron experiment. The O\O contribution has a dominant weight in the neutron experiment, in contrast to the X-ray case, where it is much weaker. On the other hand the uranium surrounding, as a heavy element, appears with a sig- nificant weight in the XRD experiment.
The experimental data were Fourier transformed to characterize the average atomic distributions. The atomic total pair-correlation function, g(r) was calculated (for more details see our recent work[8]):
g rð Þ ¼1þ 1 2πρ0r
Z
Qmax
o
Q S Q½ ð Þ−1sinQr dQ ð4Þ
whereρ0is the average number density, andQmaxis the integration limit determined by the actual upper limit of the experiment. The value ofQmaxdetermines the r-space resolutionΔr¼Q2πmax. In the ND experiment Qmax= 9.8 Å−1, which provides a rather low resolution
~0.6 Å, while in the XRD experiment Qmax= 25 Å−1 leading to a much higher resolution ~0.25 Å. The results are shown inFigs. 1/b and2/b for the ND and XRD experiments, respectively.
Some qualitativefindings for the local structure can be made from the totalg(r) functions. In the neutron case a broadfirst peak appears at 1.6 Å peak position for UB5, and it slightly shifts to lower values, Table 1
Elemental composition (at.%) of uranium loaded multi-component waste glasses measured by PGAA method. The error is ± 0.1 at.%.
Samples Elemental composition (at.%)
Si B Na O Ba Zr U
UB5 15.2 3.4 12.7 61.0 1.3 2.8 3.1
UB10 14.4 5.9 12.7 59.8 1.4 2.4 3.1
UB15 15.9 9.1 11.8 55.5 1.2 2.9 3.3
UB20 11.1 13.7 8.1 61.1 0.5 2.3 3.1
1.5 Å for UB20 composition (seeFig. 1/b). Due to the lowr-space reso- lution this peak is the convolution of B\O and Si\O atomic pairs, for other atomic pair contributions we cannot make any conclusions. The second broad peak is centred at 2.6 Å, this can be attributed to the O\O contributions. The X-rayg(r) is more informative due to the rela- tively highr-space resolution. Thefirst coordination sphere splits into well-resolved peaks centred at 1.6, 1.8 and 2.2 Å, and an asymmetry ap- pears at low r-side which becomes even a small resolved peak at 1.4 Å for UB20 sample. The peaks may be attributed to B\O (1.4 Å), Si\O (1.6 Å), U\O1(1.8 Å) and U\O2(2.2 Å); the vertical lines inFig. 2/b are guide for the eye. The small peaks in the 2.5–3.3 Å interval cannot be identified; they contain the Na\O, Zr\O, Ba\O and O\O distribu- tions. It is noteworthy, that at higher r-values a relatively high intensity broad peak appears at around 3.7 Å. As far as in the neutrong(r) the intensity of this peak is considerably weaker, we may suppose that the uranium second neighbour correlations play a significant role in its formation.
3. Results from Reverse Monte Carlo modelling
The investigated uranium loaded multi-component borosilicate glasses containk= 7 different atoms, resulting in 28gij(r) functions, according to the formulak(k+ 1)/2. Several atomic pair-distributions overlap with each other, thus from the totalg(r) functions it is not a re- alistic expectation to obtain the characteristic structural parameters. In such cases the reverse Monte Carlo simulation method[17]proved to be a very effective and widely used computer modelling procedure, however, because of the very high number of simulated parameters the obtained results of an actual RMC run have to be handled very care- fully. The RMC minimizes the squared difference between the experi- mental S(Q) and the calculated one from a 3-dimensional atomic configuration. The RMC algorithm calculates the one-dimensional
partial atomic pair correlation functionsgij(r), and they are inverse Fourier transformed to calculate the partial structure factors,Sij(Q):
Sijð Þ ¼Q 1þ4πρ0
Q Z
rmax
0
r gh ijð Þ−r 1i
sinQr dr; ð5Þ
wherermaxis the half edge-length of the RMC simulation box, andρ0
is the number density. The structure of the computer configuration is modified by moving the atoms randomly until the calculatedS(Q) (see Eqs.(1)–(5)) agrees with the experimental one within the experi- mental error. Moves are only accepted if they are in accordance with certain constraints (see below the ones applied in this work).
In the present work we have utilized our previous experiences obtained on a systematic study of glassy materials starting from the sim- ple B2O3[18]and SiO2[19], through the two component B2O3–Na2O [18] and SiO2–Na2O [19], followed by the three component SiO2– B2O3–Na2O and the five component borosilicate SiO2–B2O3–Na2O– BaO–ZrO2host glass systems[6–8]. For all samples we have applied RMC modelling to obtain atomic structure parameters, and the results have indicated that the RMC simulation is a successful method to obtain stable and reproducible structural data for the investigated amorphous systems.
In this study we have used the RMC software package developed by the group of Pusztai[20]. For the RMC starting model a disordered atomic configuration was built up with a simulation box containing 5000 atoms withρ0= 0.08 atoms · Å−3and rmax= 19.84 Å for all investigated specimens.
In the RMC simulation procedure two types of constraints were used; the minimum interatomic distances between atom pairs (cut-off distances) and connectivity constraints. We have used connectivity con- straints for the two basic network formers, for the Si\O and B\O atom Fig. 1.Neutron diffraction total structure factors (a) and atomic pair-correlation function (b) of uranium loaded borosilicate waste glasses. Experimental data (symbols) and RMC simu- lation (solid line). The difference between the experimental and calculatedS(Q) data is shown at the bottom of thefigure.
Fig. 2.X-ray diffraction total structure factors (a) and atomic pair-correlation function (b) of uranium loaded borosilicate waste glasses. Experimental data (symbols) and RMC simulation (solid line). The difference between the experimental and calculatedS(Q) data is shown at the bottom of thefigure.
pairs, and also for the U\O atom pairs. The latter will be discussed in the next section. It is widely accepted in the literature[21,22]and our pre- vious results on similar compositions [7–9 and references therein] have concluded that silicon has tetrahedral 4-fold oxygen coordination (SiO4), while boron is surrounded both by 3-fold (BO3) and 4-fold (BO4) coordinated oxygen atoms. Thus, in the RMC calculations each Si atom was forced to be surrounded by 4 oxygen neighbours between 1.5–2.0 Å, and B atoms were forced to have either 3-fold or 4-fold oxy- gen neighbours between 0.9–1.7 Å.
In the RMC simulation procedure we have used as starting cut-off distances, the characteristic values obtained in our previous work for UB10 composition[9]. Several RMC runs have been performed by mod- ifying the cut-off distances in the way, that the results of each run have been carefully checked to obtain reasonable data for eachgij(r) and coordination number distributions,Nij(CN).
The RMC calculation converged well and thefinalS(Q) matched the experimental one, as it is displayed inFig. 1/a for the ND and inFig. 2/a for the XRD case. As far as the experimental and the calculated data practically fully overlap, we have calculated and illustrated the devia- tion curves in the same figures. The actual set of average cut-off distances used in thefinal RMC runs are the following: Si\O 1.53 Å, B\O 1.1 Å, O\O 2.47 Å, Na\O 2.05 Å, Zr\O 1.98 Å, Ba\O 2.55 Å, U\O 1.75 Å, U\B 2.8 Å, U\Si 3.3 Å, U\Na 3.3 Å, U–Zr 3.4 Å and U\Ba 3.4 Å. It should be noted, that the cut-off values were calculated independently for each composition by analyzing the bestfit obtained for the experimental and calculatedS(Q) curves. The actual cut-off values were very similar for each composition, therefore here the aver- age values are given.
The most informative atomic pair correlation functions and coordi- nation number distributions obtained for the investigated samples are shown inFigs. 4 and 5. The nearest neighbour distances for several atom pairs are tabulated inTable 3, while the averagefirst neighbour coordination numbers are summarized inTable 4.
In order to characterize the network structure of the RMC generated atomic configuration, the atomic partial pair-correlation functions and the coordination number distributions have been analyzed.
changes are within limit of error.
The B\O bond length slightly decreases from 1.55 Å to 1.35 Å (see Table 3) and the average coordination number also decreases from 3.4 to 3.2 atoms (seeTable 4) from the UB5 to UB20 glasses with increasing B2O3content. In our previous high resolution neutron diffraction study of the multi-component matrix glasses[8]we have established two pos- sible local network configurations with a larger (1.60 Å) and with a shorter (1.40 Å) B\O bond distances corresponding to the 4-fold and to the 3-fold oxygen coordinated boron atoms. Taking into consider- ation the relatively low resolution of the present neutron diffraction ex- periment, the agreement is reasonable, especially for the UB5 and UB10 compositions. For all samples studied in this work 3- and 4- fold oxygen coordinated boron atoms were obtained and with increasing boron con- tent the number of 3-fold coordinated boron atoms is increasing (see Fig. 5/b), which leads to the decrease of B\O nearest neighbour dis- tance to 1.35 Å, in accordance with our earlier results reported for the multi-component borosilicate glasses[8]. In our earlier study on B2O3– Na2O glass we have discussed [18and references therein], that Na+ ions convert the trigonal BO3units into tetrahedral BO4units, and the linkage of BO3and BO4form superstructural units as proposed originally by Krogh-Moe [24] and later by NMR studies[22,25]and also by model calculations based on empirical potentials[26]. In the present work the ratio of the B2O3/Na2O concentration increases from UB5 to UB20 sam- ples, thus the sodium charge compensation effect for BO4decreases, which leads to the increase of the number of BO3with respect to BO4
units.
The U\O pair distribution functions were obtained with a very good reproducibility from the RMC simulations, due to their relatively high
~17% weighting factor in the XRD experiment (seeTable 2).Fig. 4/h dis- playsgU\O(r), where two well-resolved peaks appear at relatively short distances, centred at U\O1= 1.8 ± 0.05 Å and U\O2= 2.2 ± 0.05 Å for all investigated samples. Thefirst neighbour peak positions could be determined with a relatively high accuracy, because they do not overlap with other atomic pair-distributions.
4. Discussion
Here we focus our interest on the analysis of the atomic surrounding of uranium ions and on the role of uranium in the network forming. The experimentalg(r) data (seeFig. 2/b) and RMC modelling, without any Fig. 3.Comparison of neutron (open square) and X-ray (solid square) experimental data
for UB5 sample.
Table 2
Several weighting factors,wij(%) of uranium loaded waste glasses. ThewijXRD
(Q) values are given atQ= 0.8 Å−1, whilewijND
doesn't depend onQ.
Samples Weighting factors,wij
Si\O B\O Na\O Zr\O Ba\O O\O U\O U\Si U\B U\Na U\Zr Si\Si
UB5 ND 15.4 5.5 11.3 4.9 1.6 43.1 6.3 1.1 0.4 0.8 0.3 1.4
XRD 7.5 0.5 4.7 5.6 4.0 7.8 17.6 8.5 0.5 5.3 6.5 1.8
UB10 ND 14.0 9.3 10.8 4.1 1.6 40.7 6.2 1.1 0.7 0.8 0.3 1.2
XRD 7.5 0.5 4.7 5.6 4.0 7.8 17.6 8.5 0.5 5.3 6.5 1.8
UB15 ND 14.2 13.0 9.2 4.5 1.3 34.6 6.0 1.2 1.1 0.8 0.3 1.4
XRD 7.1 1.1 3.9 5.0 3.3 6.4 16.9 9.4 1.5 5.2 6.9 2.0
UB20 ND 10.1 20.0 6.5 3.6 0.6 38.9 5.6 0.7 1.4 0.4 0.3 0.6
XRD 7.0 2.4 3.9 5.6 1.9 10.0 21.9 7.7 2.6 4.2 6.5 1.2
connectivity constraint for the U\O neighbours, have shown that the first neighbour U\O distribution shows two well resolved distances at 1.8 and 2.2 Å. The average U\O coordination number is 5.5 atoms calculated for the 1.6–2.4 Å interval, which covers both U\O1 and U\O2distributions. As far as, RMC modelling leads to a“most random” configuration without application of constraints, a relatively broad and symmetric coordination number distributions have been obtained, as we have reported in our previous work on UB10 sample[9].
In this work we have applied connectivity constraints for both U\O1
and U\O2distributions in the RMC modelling, taking into consideration the overall good agreement of ourfindings for the twofirst neighbour distances at 1.8 and 2.2 Å and the average CNU\O= 5.5 atoms with the corresponding data obtained from the literature[27–31]on uranium borosilicate crystalline phases and glasses. Based on the above works, UO22+uranyl groups can be characterized with two axial oxygen atoms between 1.77–1.85 Å and with four tofive equatorial oxygen atoms in in- terval 2.21–2.25 Å. These bond length values show that the uranyl ions have a well-defined local structure that includes two oxygen atoms in axial position and four additional oxygen atoms in equatorial plane and, forms UO6cluster, with mixed U(V) and U(VI) coordination numbers.
Based on the similarity of the two characteristicfirst neighbour U\O distances determined in this work and in the above listed references and in the similar value of the averagefirst neighbour U\O coordina- tion number for the entire interval (1.6–2.4 Å), it is reasonable to sup- pose, that in our system the local uranium coordination is similar to the above systems. Thus, we applied connectivity constraints for the shorter and for the longerfirst neighbour U\O distances. In the RMC calculations each uranium atom of the 3-dimensional atomic configura- tion was forced to be surrounded by 2 oxygen neighbours at ~1.8 Å and by 4 oxygen neighbours at ~2.2 Å. The actual intervals of the connectiv- ity constraints are indicated inTable 4in brackets. The corresponding U\O1and U\O2coordination number distributions generated by the RMC algorithm are shown inFig. 5/c,d. The connectivity constraints are fulfilled excellently, and the distributions do not show differences for the four specimens. The calculated average coordination numbers are CNU\O1= 1.9 ± 0.2 atoms and CNU–O2= 3.7 ± 0.2 atoms and the totalfirst neighbour U\O coordination number is ~5.6 atoms.
Significant correlations may be observed at higherr-values between uranium and the network former Si and B atoms, and with the modifier cations Na and Zr, as well, as it is displayed inFig. 6. It should be noted, Fig. 4.Partial atomic correlations for the uranium loaded borosilicate glasses for UB5 (square), UB10 (open circle), UB15 (star), UB20 (triangle): a) Si\O, b) Si\Si, c) B\O, d) O\O, e) Na\O, f) Ba\O, g) Zr\O and h) U\O atom pairs.
that usually the second neighbour partial correlation functions calculat- ed from RMC simulation do not show such characteristic correlations.
Although, these correlation functions are rather noisy because of the rel- atively low number of the contributing atoms in the RMC simulation box, clear correlations may be observed for U\Si at 3.4 ± 0.1 Å, for U\B at a somewhat shorter distance 3.0 ± 0.1 Å, and for the modifier cations at higher distances, U\Na at 3.8 ± 0.1 Å and U\Zr at 3.7 ± 0.1 Å. This indicates that uranium atoms are connected through an ox- ygen atom with the network former and modifier atoms, which build up the network structure. The basic glassy network consists of mixed
tetrahedral SiO4, tetrahedral BO4and trigonal BO3units, which are part- ly connected by uranium atoms. This is afingerprint that uranium ions take part as network former or partly as network former, which leads to the well-defined glassy structure with a high chemical and hydrolytic stability.
5. Conclusions
We have performed ND and high-energy XRD measurements on multi-component borosilicate host glasses loaded with 30 wt.% UO3in order to get structural information on the basic network structure and on the uranium surrounding. The S(Q) data sets were simulated by RMC modelling to obtain a possible 3-dimensional atomic configuration consistent with the experimental data.
Fig. 5.Coordination number distributions for UB5 (black), UB10 (stripe), UB15 (cubes), UB20 (grey) glasses from RMC modelling: a) Si\O, b) B\O, c) U\O1, d) U\O2, e) Na\O and f) O\O atom pairs.
Table 3
Nearest neighbour atomic distances,rij(Å) in uranium loaded borosilicate waste glasses.
The error is ± 0.03Ǻfor Si\O; ± 0.05Ǻfor Si\Si, B\O and U\O; while it is about
±0.1Ǻfor other atom pairs, as estimated from the data obtained from different RMC runs.
Atom pairs Interatomic distances,rij(Å) Samples
UB5 UB10 UB15 UB20
Si\O 1.6 1.55 1.55 1.6
Si\Si 3.0 3.0 3.0 3.0
B\O 1.55 1.5 1.35 1.35
O\O 2.5 2.5 2.5 2.5
Na\O 2.05 2.10 2.10 2.05
Ba\O 2.7 2.7 2.7 2.7
Zr\O 2.0/2.25 2.0/2.25 2.0/2.25 2.0/2.3
U\O 1.8/2.24 1.84/2.24 1.75/2.20 1.8/2.25
U\Si 3.4 3.4 3.4 3.6
U\B 2.95 2.95 2.9 2.85/3.15
U\Na 3.7 3.8 3.8 3.9
U\Zr 3.7 3.7 3.8 3.8
Table 4
The averagefirst neighbour coordination numbers, CNij(atom) for the uranium loaded borosilicate glasses. The actual intervals (Å) are indicated in brackets. The error is estimated from different RMC runs: ±0.05 atom for Si\O; ±0.1 atom for B\O; ±0.3 atom for Na\O and O\O and ±0.2 atom for U\O atom pairs.
Atom pairs Coordination numbers, CNij
Samples
UB5 UB10 UB15 UB20
Si\O 3.95 (1.45–1.85) 3.92 (1.45–2.0) 3.80 (1.45–2.0) 3.87 (1.4–2.0) B\O 3.4 (1.3–1.7) 3.4 (1.3–1.75) 3.1 (1.1–1.95) 3.2 (1.3–1.75) Na\O 3.6 (2.0–2.45) 3.7 (2.0–2.45) 3.6 (1.95–2.5) 3.6 (2.0–2.37) O\O 5.9 (2.4–2.95) 5.6 (2.4–3.0) 5.2 (2.4–2.9) 5.2 (2.35–2.95) U\O1 1.90 (1.75–1.95) 1.97 (1.66–1.9) 1.89 (1.68–1.85) 1.92 (1.7–1.9) U\O2 3.69 (2.1–2.4) 3.71 (2.1–2.35) 3.80 (2.15–2.32) 3.60 (2.05–2.4)
It was established that the basic network structure consists of tetra- hedral SiO4units and of mixed tetrahedral BO4and trigonal BO3units, being rather similar to the network structure of the corresponding host glass as reported in our previous study[8]. With increasing boron content the relative number of BO3/BO4increases and thefirst neigh- bourrB\Odistance decreases.
For the U\O correlations two distinctfirst neighbour distances were determined at 1.8 ± 0.05 Å with 1.9 ± 0.2 oxygen atoms and at 2.2 ± 0.05 Å with 3.7 ± 0.2 oxygen atoms. Furthermore, significant second neighbour atomic pair correlations have been established between uranium and the network former (Si, B), the modifier (Na) and the stabilizer (Zr) atoms. From these observations we may conclude that uranium ions take part in the network forming. This may be the reason of the observed good glassy stability and hydrolytic properties.
Acknowledgement
The research was supported by the European Community's Sev- enth Framework Programme under grant agreement No.226716 and No.283883-NMI3, and by the Hungarian research fund OTKA-PD- 109384.
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Fig. 6.Second neighbour correlations for the uranium loaded borosilicate glasses for UB5 (square), UB10 (open circle), UB15 (star), UB20 (triangle): a) U\Si, b) U\B, c) U\Na and d) U\Zr.
