Glass - Back to the Future!

Presenting Author:
John McCloy

article posted 07 April 2016

John McCloy joined Washington State University in 2013 as an Associate Professor in the School of Mechanical & Materials Engineering, having previously been the team lead in Glass and Materials Science at the Pacific Northwest National Laboratory. Much of his research focuses on nuclear materials, particularly glass and ceramic waste forms for immobilization of radionuclides from legacy defense waste and nuclear fuel reprocessing waste. He is currently also a Visiting Professor of Nuclear Materials at Sheffield University and is a Chief Scientist Joint Appointee with Pacific Northwest National Laboratory.

In-Situ Crystallization Experiments of Nuclear Waste Glass Ceramics

John S. McCloy,*1,2,3 José Marcial,1 Ashutosh Goel,4 Brian Riley,1,2 Jarrod Crum2
1Washington State University, Pullman, WA, USA
2Pacific Northwest National Laboratory, Richland, WA, USA
3University of Sheffield, Sheffield, UK
4Rutgers, The State University of New Jersey, Piscataway, NJ, USA

A glass-ceramic waste form is being developed through the US Department of Energy – Nuclear Energy to immobilize non-fissionable waste streams of alkali (A, 137Cs), alkaline-earths (AE, 90Sr), lanthanides (Ln), and transition metals generated by the projected transuranic extraction (TRUEXplus) process. Compared to an alkali borosilicate glass wasteform, the glass-ceramic product is expected to double waste loading (to ~45%) and have improved thermal stability. These benefits are realized by the partitioning of the insoluble fission product fraction into a suite of ceramic phases through controlled crystallization. The target phase assemblage is comprised of: AEMoO4 (powellite), (Ln,A,AE)10Si6O26 (oxyapatite), and Ln5BSi2O13 (lanthanide borosilicate).
Preliminary research with crucible melt studies and a pilot scale cold-crucible induction melter demonstrated processing at ~1300°C. Upon cooling, the melt phase separated to Mo-rich and Ln-rich droplets in a borosilicate matrix, followed by crystallization. Thus far, it has only been possible to measure phase distribution on quenched microstructures, as opposed to those generated during cooling as they will evolve in the intended application. Different phase separation and crystallization will likely result from cooling versus reheating process steps.
A key barrier to maturation and exploitation of glass-ceramic technology is the gap in fundamental understanding of the molecular-scale mechanisms of phase separation and crystallization leading to the development of the desired phase assemblage and microstructure, ultimately determining long-term product performance. The challenge is in predictably achieving the targeted phase assemblages and microstructures, requiring a detailed understanding of the transformation process as a function of both cooling rate and melt chemistry.
This talk will discuss recent efforts to develop experimental techniques to monitor phase separation and crystallization in-situ at high temperature, using small angle neutron scattering and neutron diffraction. This effort is part of an ongoing US-UK collaboration to understand the crystallization behaviour of MoO3-containing glasses.

Figure: Backscattered electron micrographs of the same glass reheated and held at T shown for 4h then quenched in air, showing the effect on phase separation and crystallization.