Neutron-and high-energy synchrotron X-ray diffraction experiments have been performed on the (75-x)SiO2-xB2O3-25Na2O x=5, 10, 15 and 20 mol% glasses. The structure factor has been measured over a broad momentum transfer range, between 0.4-22 Å -1 . For data analyses and modeling the Fourier transformation and the RMC simulation techniques have been applied. The partial atomic pair correlation functions, the nearest neighbour distances, coordination number distributions and average coordination number values and three-particle bond angle distributions have been revealed. The Si-O network proved to be highly stable consisting of SiO4 tetrahedral units with characteristic distances at rSi-O=1.60 Å and rSi-Si=3.0(5) Å. The behaviour of network forming boron atoms proved to be more complex. The first neighbour B-O distances show two distinct values at 1.30 Å and a characteristic peak at 1.5(5) Å and, both trigonal BO3 and tetrahedral BO4 units are present. The relative abundance of BO4 and BO3 units depend on the boron content, and with increasing boron content the number of BO4 is decreasing, while BO3 is increasing.Keyword: borosilicate glasses, neutron and X-ray diffraction, RMC simulation IntroductionRecently, several experiments have been reported on the study of structure and properties of sodium borosilicate glasses from fundamental and industrial point of view, due to the potential applicability for immobilizing of high-level radioactive wastes, like U-, Pu-, Th-oxides [1-7 and therein]. It has been found that the modifier effect of Na ions influences the ratio of nature of glass network formers Si, B and it is possible to archive a mixed glass network former effect. This effect is believed to have a structural origin, yet a precise understanding of it is still lacking because of the structural complexity of the sodium borosilicate glasses. The structure of borate glasses with alkali oxides has been extensively studied. A nuclear magnetic resonance (NMR) spectroscopy measurements show that the fraction of boron atoms tetrahedral coordinated to the total number of boron atoms varied with the modifier compositions [8][9][10]. By Raman spectroscopy, typical borate groups, such as boroxol, trigonal and tetrahedral units were found to exist in several borate compounds [11,12]. Recently, we have started to examined the atomic structure of a newly prepared three-component sodium-borosilicate system, using a combination of neutron diffraction (ND), high-energy X-ray diffraction (XRD) and reverse Monte Carlo (RMC) modelling that are capable of building threedimensional structure models and so yield a more detailed description of the atomic-scale glass structure. In our previous works we studied the binary sodium silicate glasses (70SiO2-30Na2O) [13], the binary sodium borate glasses (75B2O3-25Na2O) [14] and a five-component sodium borosilicate glass (55SiO2-25Na2O-10B2O3-5BaO-5ZrO2) [15]. Here, we apply the same approach to a ternary

Abstract. Understanding of the incorporation of actinides in borosilicate matrix used for nuclear waste storage is of a great importance for radioactive waste immobilization. This study carried out on matrix glasses doped respectively with 30wt% UO3 and CeO2, Nd2O3 used for chemical modelling of the actinides. Neutron and X-ray diffraction measurements and Reverse Monte Carlo (RMC) simulations were performed. For several glasses, it was found that the basic network structure consists of tetrahedral SiO4 units and of mixed trigonal BO3 and tetrahedral BO4. The BO3 and BO4 units are linked to SiO4, forming mixed [4] Si-O- [3] B and [4] Si-O- [4] B bond-linkages. From significant second neighbour atomic pair correlations have been revealed that U, Ce, Nd accommodates in both silicate and borate site. IntroductionThe final and safe storage of radioactive waste materials is nowadays an existing and urgently to be solved problem. The most feasible and accepted way for storage of high-level radioactive waste (HLW) is the vitrification process, where the active elements are melted and poured into glass form, and thereafter deposited in the deep geological formation, where the objective is to retain the radionuclides in the host rocks, and to block access to the biosphere [1,2]. Borosilicate glass is a preferred and accepted waste form [3][4][5][6]. Vitrification plants are expected to be operational for the next 50 years, and a great number of relevant research to further enhance effectiveness and reduce cost being conducted in this area [7,8]. Future plans should reflect the best application of processing technologies and a knowledge of waste forms that reduces risk to the public and the environment while also avoiding unnecessary cost and delay. Essentially the most important task in this area is to find a simple and well controllable process where we can immobilize the HLW's. The main problems are the high number of components, high melting temperature, crystallization, unknown basic processes and a high preparation costs.Step by step we have made successful attempt to prepare borosilicate matrix glasses with the general composition of SiO2-B2O3-Na2O-BaO-ZrO2. In this study we intend to explore basic network structure on the glasses by performing neutron-and X-ray diffraction experiments, which are complementary each other, X-ray diffraction data are dominated by contributions from heavier elements, while neutron diffraction gives information mainly on the light elements. Our aim is to develop, optimize and compare the basic network structure of this glasses especially regarding the matrix glass with that of the uranium and lanthanides containing glassy network. In order to do this, simultaneous RMC simulation of the two data sets is applied to generate reliable 3-dimensional atomic configurations and to calculate the atomic parameters.