Eurofusion November 8th 2017
Harnessing the energy generated by a fusion reactor while simultaneously generating more tritium fuel to sustain the reaction is no easy task. However, that is what is required from a blanket, the enormous structure that lines the outside of the first wall.
The energy given off in a fusion reaction comes in the form of highly energetic alpha particles and neutrons. The neutrons escape from this confinement and are eventually captured by the blanket. The kinetic energy of these particles gets imparted to the structure that captured them, generating large amounts of heat. Heat from the blanket is then transported with a coolant which goes on to drive turbines to produce electricity. This part of the reactor works in the same way you might see in a fission or coal power plant. It sounds simple enough, but there are a number of caveats that make this task difficult. For example, even the coolant can cause corrosion of the metal due to the high temperatures involved.
When neutrons collide with a solid material,they scramble the well-ordered atoms inside of it (imagine the start of a game of pool). Atoms that have been displaced from their original positions can have a profound effect on the properties of materials, causing them to crack or fail unexpectedly. On top of this, when a neutron is absorbed, a completely different atom can be created in a process known as transmutation. These new atoms further change a material’s properties and can often be radioactive. As a result, special new materials must be designed that can withstand the effects of neutron damage without becoming too radioactive.
Not only must the blanket withstand neutron bombardment, it must also be responsible for generating more tritium, required to sustain a continuous fusion reaction. There are many different otential designs for the blanket breeding system and they all have advantages and disadvantages. One design utilises a molten mixture of lead and lithium to multiply neutrons and breed tritium respectively. However, liquid metal flowing around the structure can cause problems as it interacts with the massive magnetic field of the tokamak.
PUTTING IT ALL TOGETHER
As the blanket is an enormously complex structure, it needs to be tested before operation. ITER will be built with six ports where test blanket modules can be inserted and removed. The data gathered from these test components will be vital for continuing to develop and improve what is arguably the most challenging component of the fusion reactor to design.
There are many particles that can create fusion energy, but fusing two isotopes of hydrogen – deuterium and tritium – is the easiest to achieve. This reaction produces a high-energy neutron which will escape the plasma and hit the reactor walls. Tritium is very rare, but self-sufficiency can be achievedby a “breeding blanket” lining the walls, where the escaped neutrons are augmented by a neutron multiplier and react with lithium to produce tritium. This tritium is then extracted and recycled for fuel.
In my opinion, if fusion energy is harnessed as a viable power source, it will be the greatest scientific achievement in the history of humanity. In addition to the scientific merit, it will also help reduce the world‘s reliance on carbon energy sources. It is for these reasons I have chosen to study a PhD in materials for fusion reactors.
Paul Barron (23) from Great Britain is currently based at: Manschester, UK, @paulbrrn. (Picture: private)