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Corium Lavas: Structure and Properties of Molten UO2-Zro2 Under Meltdown Conditions Received: 21 November 2017 O

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Corium Lavas: Structure and Properties of Molten UO2-Zro2 Under Meltdown Conditions Received: 21 November 2017 O www.nature.com/scientificreports OPEN Corium lavas: structure and properties of molten UO2-ZrO2 under meltdown conditions Received: 21 November 2017 O. L. G. Alderman1,2, C. J. Benmore2, J. K. R. Weber1,2, L. B. Skinner 2,3, A. J. Tamalonis1, Accepted: 22 January 2018 S. Sendelbach1, A. Hebden4 & M. A. Williamson4 Published: xx xx xxxx In the exceedingly rare event of nuclear reactor core meltdown, uranium dioxide fuel reacts with Zircaloy cladding to produce eutectic melts which can subsequently be oxidized by coolant/moderator water. Oxidized corium liquids in the xUO2·(100 − x)ZrO2 system were produced via laser melting of UO2-ZrO2 mixtures to temperatures in excess of 3000 K. Contamination was avoided by foating the droplets on a gas stream within an aerodynamic levitator and in-situ high-energy x-ray difraction experiments allowed structural details to be elucidated. Molecular dynamics simulations well reproduced difraction and density data, and show less compositional variation in thermal expansion and viscosity than suggested by existing measurements. As such, corium liquids maintain their highly penetrating nature irrespective of the amount of oxidized cladding dissolved in the molten fuel. Metal- oxygen coordination numbers vary with both composition and temperature. The former is due to mismatch in native values, nUO(x = 100) ≈ 7 and nZrO(x = 0) ≈ 6, and the requirement for oxygen site stabilization. The latter provides a thermal expansion mechanism. Multiscale modelling of material processes is essential for both forensic investigation of historically rare fssion reactor core meltdowns, as well as development and design of future power reactors with improved safety and performance1–3. At the smallest, atomic length scales, molecular dynamics based on semi-empirical interatomic potentials remains a popular and powerful tool4–8, allowing access to wider length and time scales as compared to full quantum mechanical treatments of the electronic structure. Experimental validation of such simulations is of paramount importance1, but data at the extreme temperatures reached during core meltdown is scarce owing to the inherent practical challenges. Recently measurements pertaining to the atomistic structure of molten fuel and fuel-bearing phases have become possible9–11. Herein we seek to apply such techniques to the molten corium phase, obtained during extensive fuel-clad interaction, and to exploit these measurements by evaluating promis- ing interatomic potentials which, in turn, are used to predict key structural and bulk physical properties including thermal expansion and viscosity. Melt viscosity in particular is an important property governing the spread- ing, and thereby the cooling, of corium in core-catchers12. Te potentials and properties derived are suitable for larger scale simulations, including solid-melt interfaces between core materials, or to study the incorporation and behaviour of fssion products. Moreover the predicted physical properties can be used to inform higher level fnite element codes as part of multiscale reactor models. A severe accident scenario involving melting of the reactor core leads to the formation of corium lavas which are initially composed primarily of fuel and cladding materials. Te most common such materials are based on UO2 and zirconium respectively. Subsequent interaction with water present as coolant and/or neutron moderator leads to oxidation of U-Zr-O liquids and H2 generation. In this paper, we study the structure and properties of oxidized molten corium in the UO2-ZrO2 pseudobinary obtained by laser heating ~100 mg quantities of ceramic material during levitation on a gas jet13,14 within a hermetically sealed chamber9,10, Fig. 1B. In-situ measure- ments using high-energy synchrotron x-rays yield the melt’s structure factors, S(Q), which provide atomistic level structural information used to verify the efcacy of molecular dynamics (MD) models, themselves based on 1Materials Development, Inc, 3090 Daniels Court, Arlington Heights, IL, 60004, USA. 2X-ray Science Division, Argonne National Laboratory, Argonne, IL, 60439, USA. 3Mineral Physics Institute, Stony Brook University, Stony Brook, NY, 11794-2100, USA. 4Nuclear Engineering, Argonne National Laboratory, Argonne, IL, 60439, USA. Correspondence and requests for materials should be addressed to O.L.G.A. (email: [email protected]) or C.J.B. (email: [email protected]) SCIENTIFIC REPORTS | (2018) 8:2434 | DOI:10.1038/s41598-018-20817-z 1 www.nature.com/scientificreports/ Figure 1. Comparison of measured (black circles) and MD modelled (red curves) x-ray difraction data for liquid 27UO2·73ZrO2 at 3070 K. (A) Interference functions Q(S(Q) – 1), (B) schematic sample environment showing levitator within sealed chamber10, (C) close-up schematic of corium sample showing x-rays impinging on the top few hundred microns, (D) structure factors, S(Q) – 1, (E) close-up of two-dimensional data before integration of the above-plane scattering, (F) Fourier transforms of the data in (A) – the total correlation −1 functions, T(r), as well as the weighted partial pair contributions wij(r)⊗tij(r). A Qmax = 17 Å was used, without any Q-dependent window or modifcation function. Te various reciprocal- and real-space functions are defned in22,40. 11,15 interatomic potentials refned to similar measurements on the molten endmember dioxides . Te UO2 poten- tials employed herein are derived from those of Yakub, et al.4. Results and Discussion Te agreement obtained between modelled and experimental x-ray S(Q) and real-space Fourier transforms T(r) is excellent, Figs 1 and 2. Te real-space goodness-of-ft parameters Rχ16 are reported in Table S1. Rχ is actually smallest for the binary 27 mol% UO2 melt, even compared to the pure endmembers (Figs S2 and S3), which may be attributed to the increased structural and chemical disordering arising from the presence of two diferent cati- onic species. Te slightly larger Rχ for the 4, 20 and 21 mol% UO2 corium melts (Fig. 2) is likely due to a margin- ally higher mean UO2 content being represented in the x-ray measurements, as compared to the MD simulations. Since UO2 is volatilized throughout these short measurements, even integrating over only the last few seconds (Table S2) results in a time averaged UO2 content that is higher than that of the recovered material, at which MD simulations were run. Tis interpretation is supported by the excess intensity in the measured T(r) circa 2.55 Å due to U-O bonds (Fig. 2), and the fact that this increases in magnitude at earlier times. Te data in Fig. 2 further SCIENTIFIC REPORTS | (2018) 8:2434 | DOI:10.1038/s41598-018-20817-z 2 www.nature.com/scientificreports/ Figure 2. Stack plot comparisons of all six difraction measurements (open points) and corresponding MD modelled functions (red curves), with measured compositions marked. (A) Q(S(Q) – 1), (B) T(r) with −1 Qmax = 11.77 Å and without any Q-dependent window or modifcation function. Te inset shows the full detector panel with masked regions in red (mainly beamstop). In both parts A and B, the blue points correspond to the nominally 30 mol% UO2 sample measured in Ar:5%H2 which overlay those of the sample of the same nominal composition measured in Ar, demonstrating that the data are essentially indistinguishable. indicate negligible diferences between measurements made in Ar and highly reducing Ar:5%H2 gases (cf. 20 and 21 mol% UO2 samples). It is important to bear in mind that at such high temperatures, and at low oxygen poten- tials, the formation of low valence states of uranium, initially U3+, can be signifcant. However, previous work 9 on UO2-x melts revealed only subtle changes in the liquid difraction patterns under diferent oxygen fugacities, 3+ 4+ 9,17 from which a U content of ≲ 27% was estimated in Ar:5%H2. Since U is favored thermodynamically in Ar (owing to ppm levels of oxygen) and, based on the previous work, can be maintained kinetically even at lower oxygen potentials, we neglect the presence of lower valence states in our analyses. Simulated melt densities were also obtained within reasonably good agreement with existing measure- 18–21 ments , Fig. 3, despite considerable uncertainty in the experimental density of molten ZrO2. Te experimental verifcation of the MD models by x-ray difraction and comparison of their densities to the literature, as well as 4,11 their basis in the well-tested Yakub potentials for UO2 , encourages further interrogation of their statistical structure and properties. Composition Dependent Coordination Numbers. Several interesting phenomena are revealed in Fig. 4A, which shows that the cation-oxygen coordination numbers, nZrO and nUO, as well as their mean, nRO, depend on both composition and temperature (see also Fig. S4). SCIENTIFIC REPORTS | (2018) 8:2434 | DOI:10.1038/s41598-018-20817-z 3 www.nature.com/scientificreports/ Figure 3. Densities of liquids in the xUO2·(100 − x)ZrO2 system. Symbols correspond to selected MD 19 models of the present work. Molten UO2 density is that measured by Breitung and Reil (Pu bearing) in close agreement with Drotning21 and recommended by Fink29, with shaded region corresponding to the 18 recommended uncertainty. Molten ZrO2 density is that measured by Kohara, et al. with uncertainty region shaded. Te various broken curves correspond to the regular solution model of Asmolov, et al.20 which is constrained to be consistent with their recommendation for the density of molten ZrO2 (red dashed), the 19 Breitung and Reil density, and their measurements for a 60.4 mol% UO2 corium liquid. Te various literature relations have been extrapolated beyond the measured regions, including into the supercooled states, below the melting points. Temperature error bars on the UO2 MD model points are representative of those for the other MD models. Standard deviations in the modelled densities are similar to half the symbol heights. Recent MD results employing CRG EAM potentials from Kim, et al.5 are also shown.
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