UK’s latest fusion reactor, MAST Upgrade, boots up

Most recent iteration of UK's fusion reactor successfully ignited for the first time

by Ansh Bhatnagar
on 7 November 2020

in Issue 1754

What is nuclear fusion?

Nuclear fusion is a promising energy source for the future. Where nuclear fission splits heavy atoms in a chain reaction, fusion joins light atoms together. Both these processes release energy despite being opposites to one another: this is due to a quantity called the binding energy per nucleon. A graph of this quantity against atomic mass can be seen in Figure 1. This curve shows that it is energetically favourable for lighter elements to fuse to become heavier elements, and for very heavy elements to split to become lighter elements.

This is a powerful graph; not only does it show us that fusion releases a lot more energy than fission for the same mass of fuel, it also shows that iron is in a sense the most stable nucleus; it is not energetically favourable for it to split or fuse. This is why stars can only make elements up to and including iron.

Fusion requires two nuclei to overcome their electrostatic repulsion by getting close enough so that the strong force kicks in. This initial repulsion is strong; stars manage to get hydrogen nuclei close due to immense pressure and temperatures at their centres thanks to gravity, however these conditions are difficult to replicate on Earth without huge inefficiencies. Due to these inefficiencies, fusion experiments currently take in more energy than they give back. In order for fusion energy to become a practical source of clean and safe power, these inefficiencies must be reduced.

The UK’s latest £55m nuclear fusion experiment switched on last Thursday (October 29th) after a seven-year build.

Located at the Culham Centre for Fusion Energy in Oxfordshire, the Mega Ampere Spherical Tokamak (MAST) Upgrade is a successor experiment to the original MAST, which operated from 1999 to 2013.

What makes MAST Upgrade unusual is its shape; while more wellknown fusion experiments such as France’s ITER are tokamaks based on a torus (essentially, a doughnut), MAST Upgrade is a spherical tokamak, which has a shape more akin to a cored apple.

Both designs operate on the same principle: confine a super-hot hydrogen plasma in the shape of a torus using strong magnetic fields, sustaining a steady rate of fusion reactions. However, the ‘cored apple’ shape of the spherical tokamak is expected to give multiple advantages over the torus shape of the traditional tokamak, namely making it more compact and able to use magnetic fields more efficiently. The MAST Upgrade experiment aims to shed new light on the practicality of the spherical tokamak design for compact power plants.

MAST Upgrade is also the first fusion reactor to make use of the ‘Super- X divertor’, which is a system that aims to expel spent plasma by cooling it more than commonly used exhaust systems. With cooler material going through the divertor, materials used on divertor surfaces should be able to last longer before needing replacement. The Culham Centre for Fusion Energy believe that this system, if shown to work successfully, will be a key design for commercial fusion reactors.

In a Government press release, Science Minister Amanda Solloway looked forward to the future: “We want the UK to be a world leader in fusion energy and to capitalise on its amazing potential as a clean energy source that could last for hundreds of years. Backed by £55 million of government funding... [MAST Upgrade] takes us another step closer towards our goal of building the UK’s first fusion power plant by 2040.”

However, the Government’s commitment to the UK being a world leader in fusion remains in doubt. Despite calls from scientists for the UK to remain part of the European Atomic Energy Community (Euratom), the UK has not yet resolved this in the ongoing Brexit negotiations and is thus not currently party to the ITER programme, which aims to construct the world’s largest tokamak and demonstrate that fusion is indeed a viable energy source.