Plasma slag stability
Assessing the safety of conditioned waste
The Belgian waste management organization, NIRAS/ONDRAF, is responsible for managing various waste streams. Some of these consist of historical low- and intermediate-level radioactive waste (LILW) encapsulated in a cement or bitumen matrix. The physicochemical properties of this historical waste are not always compatible with the surface disposal for which the waste is intended. Furthermore, it is often very difficult, even virtually impossible, to determine the chemical composition of the waste with sufficient accuracy. For some time now, NIRAS/ONDRAF has been investigating whether plasma incineration combined with a vitrification bath for such waste could offer a solution. The end product of such waste treatment is known as plasma slag, which can be regarded as glass composite materials (GCMs). GCMs are homogeneous mixtures of crystalline phases embedded in an amorphous glass matrix. In some cases, depending on the chemical composition, the slag forms an almost entirely amorphous glass matrix.
The GCM’s contain no organic material and their physico-chemical and radiological characterisation is rather straightforward. Plasma technology is therefore promising for the reconditioning of historical waste for which it is difficult or impossible to demonstrate compliance with the physio-chemical conformity criteria of the surface disposal facility where it is ultimately intended to be disposed. Additionally, the technology could also be applied to untreated, non-conditioned waste.
To assess the stability of these GCM’s in disposal conditions, a research program needs to be set up. This is where SCK CEN comes in. With our knowledge, expertise and infrastructure, our Waste&Disposal team is well equipped to investigate this type of waste.
🔗 Get a general overview on plasma technology and its intended use at the ONDRAF/NIRAS web page Plasma Research (only available in French and Dutch)
GCM’s under the microscope: zooming in on stability and compatibility with final disposal
How stable are GCM’s in the long run? And are they suitable for final disposal in a surface disposal facility? Let’s take a deep dive into the stability of these GCM’s.
The chemical and physical stability of GCMs determines how and under what conditions the waste remains safely contained in the final repository in the long term. The surface disposal facility at Dessel (Category A) involves the use of conventional construction materials, mostly cement-based, and these materials are characterised by high pH values.
So, how do these GCM’s behave in such an alkaline, cement-rich environment over decades, centuries and even millennia? How quickly will encapsulated radionuclides be released? And is there a chance that their presence will lead to processes that could negatively affect the performance of the disposal site? These research questions are being addressed by a team of experts.
Learn more about the STAB-SLAK research project
In our STAB-SLAK study, we focused on identifying the compositional range in which GCMs remain sufficiently chemically and physically stable, i.e. sufficiently resistant to dissolution and adverse side-reactions.
Discover the STAB-SLAK project: a pioneering research initiative by SCK CEN and NIRAS/ONDRAF that investigates the long-term stability of GCMs in cementitious environments.
🔍 Dive into the technical reporting on leaching tests and expansion behavior studies to understand how these materials perform under disposal conditions.
How do we evaluate the long-term stability of GCM’s?
A series of laboratory tests is being conducted to assess the long-term stability of GCMs produced by the (re)processing of radioactive waste. These tests are designed to provide information on how GCMs behave under surface disposal conditions.
Leaching tests
To determine if these waste forms retain radionuclides and other toxic elements, and how this is influenced by the composition of the GCM, leaching tests are performed. While GCMs do encapsulate these elements, the key question is the extent to which they do so sufficiently and continue to do so under relevant disposal conditions.
To this end, the slags are immersed in synthetic cement pore water (IPM water) at a pH of 12.5 to replicate the highly alkaline conditions that can be expected at the location within the surface disposal facility where these GCMs might be placed. To speed up the dissolution process without introducing unrealistic effects, tests are performed at 40 °C. Static (without water exchange), semi-dynamic (with intermittent water exchange) and dynamic (with continuous water exchange) leaching tests are conducted to evaluate the leaching behaviour of these solids in such environment.
Testing volumetric stability
A conditioned waste form is also a solid and robust material.
To evaluate the physical stability of slags in cement-based matrices, granulated slag samples are mixed with cement and water and cured in the form of prisms at 20 °C. Afterwards, the length change of the cured prisms is followed over a 120-day period, during which the prisms are stored at a temperature of 60 °C under very high relative humidity. This helps to assess whether the slags tend to expand in a disposal environment and might impose a risk to damaging surrounding systems and structures.
Microstructural analysis
To obtain a good characterisation of the slag wasteform, and to better understand the observed behaviour, microstructural analysis is key.
At SCK CEN, we use scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) and X-ray diffraction (XRD) to examine the internal structure of the slags before and after leaching. These analyses indicate how different mineral phases dissolve, transform, or precipitate over time, providing insight into the underlying processes. SEM-EDX is also applied to the cement-slag granulate mixtures following the 120-day expansion test, in order to attribute any observed expansion to an underlying mechanism.
Mechanical strength
Another way to assess whether processes occur in the cured cement-slag granulate mixtures during the expansion test is to measure the flexural and compressive strength of the prisms at the start and end of the expansion test. A strong change in strength could indicate an interaction between the slag and the cement in close contact.
Curious to see how we put this research into practice?
Explore the STAB-SLAK project and discover how we assess the stability of plasma slags to ensure the safety of conditioned waste.