Alex McColgan

Let's start with those neutrons.

Being lighter than alpha particles, they carry most of the energy of the fusion reaction and so have to be captured in order for their energy to be put to use.

But controlling a subatomic particle with no charge is no mean feat.

Neutrons aren't affected by the powerful magnetic fields containing the plasma, and so stream out of the reactor in all directions at one sixth the speed of light.

To deal with this, most reactor designs utilize something called a blanket, a layer surrounding the reaction chamber that's designed to absorb high-energy neutrons and heat up.

This heat can then be used to generate steam, drive a turbine, and generate electricity.

But this approach is far from perfect.

Neutrons aren't just absorbed by the blanket.

They ping about everywhere, damaging everything they hit.

This hugely limits the lifespan of many components, particularly the reactor walls.

Another issue stems from the reactants themselves.

While deuterium is relatively common, being easily extracted from seawater, and cheap at $13 a gram, tritium is neither.

It has a half-life of just 12.3 years, and the only commercial source are Canada's 19 deuterium uranium nuclear reactors, which produce just half a kilogram of tritium a year as a waste product.

Now, ITER estimates that a commercial fusion plant would require around 125 kg of tritium a year to run.

Current global tritium reserves are around 25 kg.

And as half of Canada's reactors are due to be decommissioned this decade, this tiny reserve is quite literally going to decay away.

But of course, scientists do have a solution up their sleeves for the tritium supply problem.

As well as carrying energy out of the reactor, those high-energy neutrons produced by deuterium-tritium fusion can be used for something called tritium breeding.

The idea is that you make the blanket surrounding the reactor out of a substance that generates tritium when bombarded by neutrons.

The substance preferred by most fusion researchers is lithium.