Investigating a material used in nuclear fusion reactors

Hi stranger on the internet! If you clicked on this webpage you’re probably interested in material science, or maybe you were just intrigued by the title, or perhaps you just misclicked…
Anyway, in the following I will briefly explain how my project ‘Computational atomic-scale modelling of materials for fusion reactors’ is structured and what are my objectives, so feel free to check it out if you’re interested.

You probably heard before in a way or another about nuclear fusion, it’s the main process that happens inside our Sun, and all the other stars, and generates the energy that makes life possible here on Earth.
Today different projects involving the world most powerful countries are trying to reproduce nuclear fusion on earth and make it a new source of energy, possibly changing forever our energy production methods.
In the context of this nuclear fusion reactors different branches of physics, chemistry and engineering come together to achieve this incredibly difficult task, but that could lead to one of the greatest achievement of mankind; one of these branches is material science. Specific materials must be used in a nuclear fusion reactor, that can sustain the incredibly harsh conditions present; to give an example ITER is currently the largest experiment of nuclear fusion under construction and at its core it will reach temperatures of around 150 million degrees, ten times higher than the temperature of the core of the Sun!

Concept image of the ITER nuclear fusion reactor

Source: ITER official web page https://www.iter.org/proj/inafewlines

One of the materials that is used in a nuclear fusion reactor is Tungsten, due to its high thermal conductivity and highest melting point of all metals. Which makes it perfect for the harsh conditions it is subject to, but it faces a critical problem: the damage caused by the particles present in the fusion reactor: the two hydrogen isotopes, deuterium and tritium, which are the input particles of the fusion reaction, helium atoms and neutrons which are the products of the reaction alongside energy.
Some of these particles can in fact hit the material causing damage at an atomic level because of their high energy, given the temperature of the core. For example neutrons can initiate chain events from an initial collision with a tungsten atom, which are called defect cascades, that will result in the displacement of several atoms from the lattice structure and the formation of vacancies.

This changes in the structure will then affect the properties of the material, and such variation need to be studied to have a clear picture of what will happen in nuclear fusion experiments like ITER.
While the work of my colleague Eoin was focused on reproducing cascades of defects inside tungsten; mine was centered around the thermal conductivity of tungsten and in particular the effect of defects on such property, like the presence of large vacancies, which can be also filled by Hydrogen and Helium atoms.To do so molecular dynamic simulations, using the LAMMPS code, have been performed. The basic idea of molecular dynamics is to create a system in which single atoms are present and each one evolves in time using the classical Newton’s law of motion and a potential that is given depending of the material of interest. Millions of atoms can be simulated using such simulations, and even if quantum effects are not considered, the great number of particles, which could not be possible in a quantum simulation, allows to study very well properties like thermal conductivity.

Example of an empty bubble inside a Tungsten atoms system in a molecular dynamics simulation. Structures of this kind will change the material properties!

(the colouring of the image is used to give a sense of depth)

If you are interested in the topic you can give a look at my next blog post, which will be focused on a procedure to calculate the thermal conductivity of a crystal using molecular dynamics and my results with Tungsten.
If the idea of the defects cascades has caught your attention my colleague Eoin has written some blog posts on the topic on his page.
In any case, thanks for your attention and I hope you enjoyed this blog post. Have a good day, dear internet stranger!