Can we make Fusion work? A Realistic Analysis Beyond Big Promises

11 minute read

Updated on: 27 Jul 2020

In the sun’s core, fusion occurs under intense pressure and temperatures of 15,000,000°C [ref]. Can we really create such conditions on Earth? Turns out, we can!

Image of Earthly welding a sun

Earthly welding a sun

There are many types of fusion reactors and many companies and labs are working on them [ref1,ref2,ref3]. There are also proposals to use fuels other than Tritium and Deuterium [ref]. To keep this article relatively short while going into some level of detail, we will only cover the reactor type that is leading the race as of 2020: the tokamak [ref]. To explore other types, see the ‘Open Problems’ chapter!

The 3 crazy requirements for fusion

The goal is clear: to get Deuterium and Tritium to fuse, we need to make their nuclei touch while using less energy than the fusion reaction produces.

Requirement 1: Make plasma. To get the nuclei of atoms to touch, we first need to break the atoms free from their electrons. This is done by heating the mixture to over 100,000,000°C [ref1]. The resulting free-floating electrons and nuclei form a plasma [ref]. Like solid, liquid and gas, plasma is just another state of matter:

Image of Plasma - the 4th state of matter

Plasma - the 4th state of matter [ref]

Step 2: Density: Density describes how many particles are packed into a specific volume. In the case of fusion, we want to get lots of Deuterium and Tritium into a fusion reactor [ref].

Step 3: Containment?: Where are we going to keep this extremely hot, highly pressurised plasma? We need a special container that can withstand these conditions for a relatively long time [ref]. The amount of time we are able to confine this energy has increased rapidly over recent years, from 30 seconds in 2013 [ref] to 101.2 seconds in 2017 (still the record in 2020) [ref].

Image of Fusion requirements

Fusion requirements

When is a fusion reactor good enough?

There are two broad goals in fusion research and engineering:

  1. Scientific success: Reactors produce more energy than they require to run
  2. Cheap energy: Costs are low enough and efficiencies high enough for energy to be sold cheaply

So far, we haven’t even achieved the first goal [ref].

The ratio of input to output energy is often called “Q” [ref].

Image of Fusion net-energy production

Fusion net-energy production

To increase Q, we can do two things:

  1. Use less energy
  2. Produce more energy

It turns out, these two things are closely connected. As there are more and more fusion reactions (i.e. we produce more energy), the plasma heats itself more and more. This means we don’t need to continue to invest as much energy to heat it up [ref]. At some point, it could even keep going without any external heat input [ref]!

Over the last 60 years, researchers have worked on many different methods to get this right [ref1,ref2]. Sadly, almost all of these designs are stuck at Q values between 0.0001 and 0.000001 [ref]. However, there is one type (called ‘tokamak’) that has achieved Q=0.65 [ref].

Let’s see how it works!

“Tokamak” - the fusion donut

Image of Tokamak: from Deuterium to Electricity

Tokamak: from Deuterium to Electricity

Remember our three problems:

  1. Heat the plasma to around 100,000,000°C [ref].
  2. Density [ref].
  3. Confinement of the plasma for seconds to minutes[ref1,ref2].

The tokamak simultaneously satisfies the requirements of confinement and density using strong magnetic fields [ref]. These magnetic fields force the negatively charged electrons and the positively charged nuclei to move round and round on a path inside a donut-shape. Because magnets surround the plasma, they create a high pressure, which increases the plasma density [ref].

Breathe deeply. Sit back, make sure you got this so far. Because now we will touch on some of the problems fusion researchers work on today.

Image of Energy donut!

Energy donut!

Where do deuterium & tritium come from?

Deuterium is easy to find and abundant - it’s in ocean water [ref]. However, only a few kilograms of Tritium are produced by nature every year [ref] and there is no such thing as a “Tritium factory” [ref]. Today’s fusion experiments often get it from nuclear fission power plants, where tritium is produced as radioactive waste [ref1]. But where will we get it from if we stop using fission reactors?

Luckily, there is a way for fusion reactors to generate their own Tritium [ref]. In theory, we could reuse that extra neutron to make more Tritium out of Deuterium! The full reaction uses Deuterium (1p1n) and Lithium-6 (3p6n) as inputs to produce helium [ref]:

Image of Tritium breeding

Tritium breeding [ref]

Using Lithium-6 to produce Tritium actually introduces more energy to the system [ref]. Doing this in practise is very hard and one of the active areas of research and uncertainty in fusion energy [ref].

How can we get energy out of a tokamak?!

So far, so good. We have deuterium and tritium, we can heat them up, and cause fusion to happen. But how do we get the energy out?

Remember, neutrons don’t have a charge. Magnetic fields only interact with charged particles. This means that the magnetic field, as strong as it may be, can’t contain the fast neutrons coming out of the fusion reactions.

These fast neutrons are both the most valuable and most annoying aspect of fusion. Valuable because their speed is where we get the energy from [ref], and annoying because they damage the reactor’s walls [ref1,ref2]. How can we solve this?

Image of Breeding Blanket

Breeding Blanket

In the (repeated) graphic above, you see a layer called the blanket [ref] between the plasma and the magnets. The fast neutrons are slowed down inside the blanket and their kinetic energy transferred to heat it up [ref]. The hot blanket, in turn, is used to heat water, which then turns a steam turbine [ref] (just like nuclear fission and coal plants do).

This works in theory, but in practice, it’s hard to build a blanket that is efficient and resilient to damage from fast-moving neutrons [ref1].

How can we make tokamaks better?

Aside from the issues around blankets, we still haven’t achieved Q>1. There are two particularly important variables in a fusion reactor that we can control to influence how much energy is released in a fusion reactor [ref1]:

  1. R: The radius of the tokamak
  2. B: The strength of the magnetic field

So, how big do the reactors have to be to get to reasonable Q values? ITER is the biggest-ever international science experiment aiming to reach Q=10. How big is the ITER reactor?

Image of ITER Reactor

ITER Reactor [ref]

Can you see the person down at the bottom? This thing is HUGE.

Because of its size, ITER has cost tens of billions of dollars, and is taking decades to build [ref]. Remember the Q graph from before? Wondering why it stopped? Now you know! The reactors got too big, meaning they take too long to build.

Image of Fusion over time

Fusion over time

The number on the y-axis here is the so-called ‘fusion triple product’. It’s a rough indicator of how much power a fusion reactor produces and is defined as the product of the three key attributes of any fusion reactor:

Image of Fusion triple product

Fusion triple product

ITER is a science experiment, not a commercial reactor [ref]. A commercial reactor would likely need to be even bigger [ref]. Clearly, increasing the radius (R) isn’t promising. What about the magnetic field strength (B)?

Could stronger magnets make reactors smaller and cheaper?

Image of ITER vs ARC

ITER vs ARC

Inducing a magnetic field requires us to run a current through the electromagnetic field coils on the tokamak. In most materials, the current uses up energy, because some electricity is lost as heat because of resistance [ref]. However, some materials - called superconductors - have the ability to let a current pass through them without heat loss as there is no resistance to the current [ref].

Recent work on a type of superconducting magnet called REBCO (Rare Earth Barium Copper Oxide) has allowed the magnetic field strength, ‘B’, to almost double [ref1,ref2]! The limiting factor is now the durability of the steel and concrete holding everything together - at full power, the magnets would rip the reactor apart [ref].

Using REBCO magnets is likely an essential step on the path to affordable fusion reactors with Q > 1 [ref].

Importantly, ITER still uses the old, weaker superconductors [ref].

Conclusion

Fusion has always been a ‘technology of the future’, but we really are getting close to Q=1 [ref].

Private companies and university labs are now working to integrate REBCO magnets into tokamak fusion reactors [ref1]. Their progress will be a crucial indicator for fusion’s potential [ref1], but reaching Q greater than 1 is not the only problem in fusion research.

There are many open problems in blanket technology, tritium breeding, and reactor protection [ref1,ref2,ref3]. This is a massive source of uncertainty. Nevertheless, over the coming decades, we will likely find out whether fusion can be the clean and abundant source of energy we hope it will become.

Next chapter!