The process powering the sun can be harnessed as clean and safe fusion energy. Progress in fusion could be accelerated by shrinking the size and cost of reactors and the UK Government has recently announced £220 million to develop such smaller reactors. However, for them to operate continuously for several decades, certain parts of the reactor must be shielded from high energy particles. With currently available shielding materials, these parts will begin to degrade within a matter of weeks or months.
My programme of work will develop more efficient shielding materials so that these reactors can operate on a continual basis. Conventional shields use heavy atoms, which reflect the lighter particles; similar to how a ping-pong ball might bounce back off a snooker ball. My research is based on a hybrid approach, combining heavier elements with lighter ones, which instead absorb and dissipate the particle's energy; think now of similarly weighted balls colliding, like a break in snooker. The approach has been proven in theory, but I must now turn this into reality by fabricating and testing real engineering materials. In doing so I will work closely with the UK's leading fusion engineering company, Tokamak Energy, and the UK Atomic Energy Authority, both of whom seek to build energy-producing reactors within the next 10-20 years.
My first aim is to fabricate these materials. Because they are very hard and do not melt easily, I will use similar methods to the way other hard materials are made, such as those within a household drill-bit. These are made by compressing powders together at high temperature so that the powders fuse to form a solid material. I will test the properties of the materials like their strength. As part of this I will seek to understand how the geometrical arrangement of the atoms within the material - the so-called "microstructure" - affects these properties.
The second aim will be to understand how these materials degrade in the environment of the fusion reactor. They will be subjected to extreme heating, which in some areas of the reactor is similar to what is experienced in a rocket engine. I will test how the material's mechanical strength degrades at these temperatures, just like steel is softened in a blacksmith's furnace to become malleable. At the same time, the materials will also be bombarded by high energy particles in the reactor. This tends to jumble-up the arrangement of the atoms, which can make the materials more brittle; in the same way that when you bend a paperclip back-and forth, it eventually snaps. To test this, I will use specialist particle beam facilities to simulate the damage process. Because the damage only occurs on a small scale (about a tenth the thickness of a human hair) I will use very high-power microscopes to observe the jumbling-up process. I will also perform small-scale mechanical tests on the damaged areas to understand how the jumbling-up effects strength. To interpret these tests, I will work with experts in computer modelling, who can simulate individual "atomic jumps" to work out which sorts of jumps are responsible for the damage.
The final aim of the fellowship is to optimise the material's atomic arrangement to improve its damage tolerance. To achieve this, I will engineer the material's building blocks by firstly adding a cement-like layer between blocks, and secondly by flattening the blocks like pancakes. Such engineering is found in nature, where sea-snail shells are built from thinly stacked layers of relatively brittle chalk-like ceramics, with a gluey substance in between. So, when the shell is struck by predators, cracks either stop in the glue, or deflect between the layers of chalk, and the snail survives. By bringing this approach, my work will enable the materials in fusion power plants to withstand even more extreme environments and thus enable them to operate for longer, which will in turn decrease the cost of fusion energy.
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