Due to immense industrialization and the predicted rise in global population, energy consumption levels are anticipated to increase by 48% by 2040. However, fossil-fuel sources are fast depleting. A large part of future energy needs will thus have to be met by non-fossil-fuel sources. Nuclear fusion promises to be an efficient and reliable energy source. Using the same reaction that occurs in the sun and stars, it is a potentially safe, non-carbon producing and limitless energy source.
Making fusion happen is not easy. To fuse ions in the “fusion plasma” (ionized deuterium and tritium), the electric repulsion between them must be overcome. For this to happen, the environment must be heated to few million degrees centigrade. Considerable experimental and computational research efforts are being invested in across the world, to develop the future fusion power plants which can house these reactions, occurring at temperatures hotter than the sun.
A major focus of this research is the search for suitable plasma-facing materials. While these materials will not be in direct contact with the plasma (which is confined by a magnetic field), they will still experience very hot temperatures of around 800 – 1200 °C. Thus it is important to find a metal with a very high melting point and and good strength at these temperatures. Tungsten, with the highest melting point amongst metals, is a front-runner in the hunt for plasma facing components.
However, there lies a further challenge for these components. Neutrons and helium produced by the fusion reaction are energetic and can bombard the plasma-facing material. Essentially these particles collide with the atoms inside the plasma-facing metal and displace them from the original position. The displaced atoms may return to their original position, they may interact with the colliding particles to form clusters, or they may displace other atoms too. This process is defined as radiation damage and the defects created in the process are called the radiation defects. Over time as the defects accumulate, interact with one another and grow, the well-ordered atomic crystalline structure of the metal is damaged. As such the material properties deteriorate and material becomes weaker. Thus radiation damage can cause early failure of the plasma facing components.
To make the plasma-facing components resilient to radiation damage, it is important to first understand certain fundamental questions:
- How is radiation damage created?
- Do all defects remain and what determines how many defects remain?
- Are radiation defects created by neutrons different to those created by helium and how do they interact with one another?
- How does the retained defect population evolve with time due to the mutual interactions and as more and more helium and neutrons enter the material?
- How do these defects or their combinations impact the material properties?
- Can we predict when and how the material will fail?
Our research aims to address these important questions which are vital for a commercially feasible fusion plant. Ideally the samples for such studies would be taken from an experimental fusion plant where they have been exposed to the fusion plasma as would happen in the real case. However, such materials are radioactive and take years to cool down before they can be used. Thus, to accelerate our research, we need to find an alternative solution to mimic the radiation damage. Implanting ions into the material can create a good representation of the radiation damage.
While ion-implanted layers offer a convenient study model for studying both helium and neutron radiation damage, the implanted layer is very thin (few micro-meter only). This happens as the ions cannot penetrate deeper into the material. Thus we need special techniques to study the properties of these very thin layers. Some such techniques are:
Nano-indentation – where a diamond tip (spherical or sharp-edged) is driven into the material (penetrating up to depths of few hundred nano-meter) to test the hardness and elastic properties of few-micron-thick layers.
Electron microscopy – We can use electron microscopy to characterise the ion-implanted layers. Transmission electron microscopy, has resolution of ~ 2 nm and can directly image most defects of size greater than this dimension. This can help us image defects retained in the implanted layer. Scanning electron microscopy can help us image the tiny indents made in the ion-implanted layer and draw comparisons for example between those in pure metal with those in ion-implanted ones.
Atomic force microscopy – As seen above, SEM can show us that there are large changes between the spherical nano-indents made in pure tungsten and helium-implanted tungsten. We can examine these changes in further detail using AFM. Here using a combination of lasers and high precision cantilever tip we can measure the roughness and surface height with nano-meter resolution. Below we can see changes in surface profile induced by radiation defects, as measured by AFM with resolution of tens of nano-meter.
High-resolution EBSD & Synchrotron X-ray diffraction – From techniques like nano-indentation and AFM, we can make out if the implanted material is behaving different to the pure material i.e. if the radiation defects are making their presence felt. But to understand how and why these changes occur, we need to examine in more detail the area around and under the indents. High-resolution EBSD (electron back-scatter diffraction) and X-ray diffraction allow us to measure the strains and stresses in the thin implanted layer with 10-4 resolution. This information can be directly linked to the concentration of retained radiation defects and the pathways they adopt to change the material behaviour.
From the combination of the experimental observations we can gather two key things:
- If there are changes induced by radiation defects
- Get hints of why these changes are happening?
While these deductions are important, we must remember that they have all been gathered from the few micron thick implanted layer i.e. all these results are essentially relevant to the small-scale. To predict the performance of radiation damaged engineering components, we must translate our deductions to the macroscopic scale.
Further experiments cannot help us here, as ions cannot penetrate more than few microns. So we address this problem by using computational modelling. We use our scientific understanding gathered from the experiments on the thin ion-implanted layer, to formulate a computational material model which is representative of the ion-implanted layer. We use this material model to simulate the small-scale experiments and tune and test the model parameters to reproduce the experimental results.
We can then use this verified material formulation to build a model of an irradiated engineering component and virtually test its performance through macroscopic mechanical tests. When developing these material formulations we account for the crystallography of the concerned metal to make the predictions more accurate. These formulations are then called crystal plasticity models (CPFE). These CPFE models include a parameter to eliminate any size effects in translation from small-scale to macroscopic scale.
We hope that our combined experimental and modelling efforts can help accurately predict the performance of radiation damaged components and take us a step forward to commercially realising the fusion reactor. A final glimpse of the whole trajectory across experiments and modelling is shown below.