Neutron and high-energy x-ray diffraction measurements have been performed on multi-component 55SiO(2)·10B(2)O(3)·25Na(2)O·5BaO·ZrO(2) borosilicate host glass loaded with 30 wt% UO(3). Both the traditional Fourier transformation technique and the reverse Monte Carlo simulation of the experimental data have been applied to get structural information. It was established that the basic network structure consists of tetrahedral SiO(4) units and of mixed tetrahedral BO(4) and trigonal BO(3) units, similar to the corresponding host glass. Slight changes have been observed in the oxygen surroundings of the Na and Zr modifier cations; both the Na-O and Zr-O distances decrease and a more compact short-range structure has been obtained compared to the host glass. For the U-O correlations two distinct peaks were resolved at 1.84 and 2.24 Å, and for higher distances intermediate-range correlations were observed. Significant correlations have been revealed between U and the network former Si and B atoms. Uranium ions take part in the network forming, which may be the reason for the observed good glassy stability and hydrolytic properties.

A neutron diffraction structure study has been performed on multi-component borosilicate glasses with compositions (65 − x)SiO2 · xB2O3 · 25Na2O · 5BaO · 5ZrO2, x = 5–15 mol%. The structure factor has been measured up to a rather high momentum transfer value of 30 Å−1, which made high r-space resolution available for real space analyses. Reverse Monte Carlo simulation was applied to calculate the partial atomic pair correlation functions, nearest neighbor atomic distances and coordination number distributions. The Si–O network consists of tetrahedral SiO4 units with characteristic first neighbor distances at rSi–O = 1.60 Å and rSi–Si = 3.0 Å. The boron environment contains two well-resolved B–O distances at 1.40 and 1.60 Å and both 3- and 4-fold coordinated B atoms are present. A chemically mixed network structure is proposed including [4]B–O–[4]Si and [3]B–O–[4]Si chain segments. The O–O and Na–O distributions suggest partial segregation of silicon and boron rich regions. The highly effective ability of Zr to stabilize glassy and hydrolytic properties of sodium-borosilicate materials is interpreted by the network-forming role of Zr ions.

Neutron diffraction structure study has been performed on multi-component sodium borosilicate based waste glasses with the composition of (65−x)SiO2.·xB2O3·25Na2O·5BaO·5ZrO2, x=5–15mol%. The maximum momentum transfer of the experimental structure factor was 30Å−1, which made available to determine the distribution function with high r-space resolution. Reverse Monte Carlo modelling was applied to calculate several partial atomic pair correlation functions, nearest neighbor distances and coordination numbers have been revealed. The characteristic features of Si–O and Si–Si distributions are similar for all glassy samples, suggesting that the Si–O network consisting of tetrahedral SiO4 units is highly stable even in the multi-component glasses. The B–O correlations proved to be fairly complex, two distinct first neighbor distances are present at 1.40Å and 1.60Å, the latter equals the Si–O distance. Coordination number distribution analyzes has revealed 3 and four-coordinated boron atoms. The O–O distribution suggests a network configuration consisting of boron rich and silicon rich regions. Our findings are consistent with a structure model where the boron rich network contains mostly trigonal BO3 units, and the silicon rich network is formed by a mixed continuous network of [4]Si–O–Si[4] with several different [4]B–O–Si[4] and [3]B–O–Si[4] linkages.

The structure of 0.7SiO(2)-0.3Na(2)O glass was investigated by means of neutron and high-energy x-ray diffraction. The maximum momentum transfer was 35 and 23.5 angstrom(-1) for the two experiments. The two datasets were modelled simultaneously by the reverse Monte Carlo simulation technique. By using reasonable constraints it was possible to separate the six partial pair correlation functions. Nearest neighbour distances, coordination numbers and bond angle distributions have been revealed. It was found that 63% of the O atoms are in the bridging position. The Na-O distance is 2.29 angstrom and the coordination number is 2.5. The Na-Na nearest neighbour distance is 2.6 angstrom, a value significantly smaller than previously reported. Neighbouring sodium ions tend to be located at the same oxygen atom. The average Si-O ring size is 7.6.

One of the key issues in predicting the suitability of cementitious waste forms for safe disposal of radioactive wastes is the long-term stability of the strength dominating cement hydrate, the calcium silicate hydrate gel (C-S-H).Its microstructure not only determines physical characteristics like mechanical strength and microporosity, but also heavily influences chemical parameters, especially the pH, which is essential in preventing corrosion of reinforced materials [1].The C-S-H structure and the structural changes caused e.g. by carbonation in air are still not well known.The principal reason is the poor crystallinity, the difficulties in preparing homogeneous samples and the poor stability during analysis.Raman spectra of a series of mechanochemically prepared C-S-H samples with C/S ratios from 0.2 to 1.5 reveal changes in structure with changes in the C/S ratio.Samples with C/S ratios from 0.66 to 1 are dominated by Q 2 (chain elements).At C/S > 1 dimers are the main building unit [2].Exposure to air results in very fast surface carbonation.Amorphous calcium carbonate is formed within minutes.Type and extent of carbonation is very sensitive to the C/S ratio of the primary phase.In experiments, which lasted up to 6 month, most spectra showed broad bands of amorphous silica.However, C-S-H with a C/S ratio of 0.66 and 0.75 showed only minor carbonation.Amorphous carbonate crystallizes over time to give primarily vaterite at C/S > 0.67 and aragonite at C/S<= 0.5.Calcite was not observed as a principal carbonation product [3].With respect to carbonation the results suggest the addition of silica fume to cementitious waste forms.

The effect of uranium oxide on the structure of sodium borosilicate host glasses has been studied by neutron diffraction. The samples were prepared by quenching the melted mixtures of composition 70 wt% [(65 - x)SiO2 - xB(2)O(3) - 25Na(2)O - 5BaO - 5ZrO(2)]+ 30 wt% UO3 with x = 5, 10 and 15 mol%. It was found, that the U-Ioaded glasses posses good glass and hydrolytic stability. An enhanced probability for inter-mediate atomic correlations at around 4.8 angstrom has been established. The RMC simulation of the neutron diffraction data is consistent with a model where the uranium ions are incorporated into interstitial voids in the essentially unmodified network structure of the starting host glass. The U-O atomic pair correlation functions show a sharp peak at around 1.7 angstrom, and several farther distinct peaks are at 2.8, 3.6 and 4.1 angstrom. The uranium ions are coordinated by six oxygen atoms in the 1.6-3.4 angstrom interval. (c) 2007 Elsevier B.V. All rights reserved.

We have performed structure study on a newly synthesized sodium borosilicate matrix glass system with general composition of 65SiO(2)(*)5B(2)O(3)(*)25Na(2)O (*)5BaO (mole%) doped with CeO2 and ZrO2 with the aim to model radioactive PuO2 and also, to increase the hydrolytic and network stability of the glasses. Neutron diffraction experiments have been performed and reverse Monte Carlo modelling was applied to obtain the partial atomic pair correlation functions. We have established that ZrO2 is an effective glass stabilizer, and the short-range structure of the CeO2 doped samples preserves the basic network configuration of the matrix glasses, making them suitable for radioactive waste material storage.

Borosilicate glasses are of significant current interest as suitable materials for isolating host media for radioactive waste materials [1,2].We have undertaken a systematic structure investigation on a newly synthesized borosilicate based matrix glass system of general formula of mole% (65-x)SiO 2 *25 Na 2 O*5BaO*5B 2 O 3 *xZrO 2 (0£x£5) and CeO 2 for simulation of radioactive PuO 2 .As Ce and Pu coordination in complex oxide environments is similar, we expect that the Pu coordination will be properly simulated by CeO 2 addition in the host glasses.The samples were prepared by melting in platinum crucible at 1300-1450 °C, working in atmospheric conditions.Neutron diffraction measurements were performed at the 10 MW Budapest research reactor using the 'PSD' and 'MTEST' neutron diffractometers [3].Despite of the great hydrolytic stability of the samples, the first few experiments revealed their tendency to superficially adsorb H 2 O. Atmospheric humidity caused a surface swelling of the air-kept samples, and the hydrogen contained by the hydrolysed layer produced great incoherent scattering.The samples were dried at 120 C for 4 hours under vacuum conditions, which proved to be completely sufficient to obtain neutron diffraction pattern adequate for data treatment.For data evaluation both the traditional direct Fourier-transformation, and the reverse Monte Carlo (RMC) simulation methods were applied.As a result, we have obtained the partial atomic pair correlation functions for these multi-component glasses, making possible to determine first neighbour atomic distances and coordination numbers.For RMC starting model a disordered atomic configuration was build up.The convergence of the RMC calculation was good inspite of the extremely high number of simulated parameters, and the final fit matched very well the experimental structure factors.We could successfully calculate the different atomic distances of the glass network, i.e.Si-O and B-O distances at 1.4 and 1.7 Å, respectively, the discrete values of Si-B, Si-Na, B-Na, O-O, O-Na and Ba-Na distances around 2.6 Å, and the further neighbours up to 4.5 Å. The work was