Nuclear fusion powers the Sun and it happens when hydrogen nuclei fuse together to make helium nuclei. Extremely high pressures and temperatures (around the 10-million-degree-Celsius/18-million-degree-Fahrenheit mark) inside the Sun allow fusion to occur.
On Earth the temperature inside a fusion reactor has to be much higher – around 100 million degrees Celsius (180 million degrees Fahrenheit) – to compensate for our inability to create similar pressures to those found in our Solar System’s star.
In such extreme conditions, hydrogen and helium exist as plasma – where all the electrons are stripped of atoms and can move freely. Anything else present inside a fusion reactor would therefore break down into its elemental parts as a result of this subatomic deconstruction.