More Fusion Delusion

Friends, long term followers of this blog will know that I am sceptical of the relevance of nuclear fusion research to our climate emergency.

Despite my scepticism, there seems to be no end of investors willing to bet billions on projects which will inevitably fail.

This article is about a company called Helion which has a ‘new way’ of doing fusion.

There are videos describing the process which are extremely convincing and at first I just didn’t know what to make of technique: it all sounded so clever.

But eventually I came across a YouTuber (Improbable Matter) with experience in the field, and he made the major weaknesses clear.

This article is about that one major flaw in Helion’s technique which makes it inevitable that they will fail. There are many other flaws in the Helion approach, but I am concentrating on this one major and unavoidable flaw. Why? Because the processes are complicated and I don’t want to get sidetracked.

The conventional approach to fusion

To understand the novelty of the Helion technique, I will first briefly describe the conventional approach to fusion.

• The conventional approach is to fuse deuterium (D) nuclei (1 proton and 1 neutron) with tritium (T) nuclei (1 proton and 2 neutron). This reaction is chosen because it is the easiest pair of nuclei to fuse. And it’s still very hard.
• The idea is to get the mixture of these nuclei very hot – around 100 million °C – and maintain the pressure on the mixture with a strong specially-shaped magnetic field. When the pressure and temperature are sufficiently great, the fusion process starts and energy is released.
• The reaction is written D + T → 4He + n . The energetic nucleus of 4He (pronounced as helium 4) stays in the plasma and heats the plasma.
• Because the neutron (n) has no electrical charge, it is not confined by the magnetic fields, and it leaves the plasma and is captured outside the reactor and used to generate heat which is used to generate electricity in a conventional steam turbine.
• The reaction would then run continuously with the fusion reaction maintaining the plasma temperature, and a continuous stream of neutrons providing heating.
• The neutron flux from this reaction damages just about everything near the reactor and induces radioactivity in most substances.

The Helion approach

In contrast with the conventional technique,

• Helion plan to fuse deuterium (D) nuclei (1 proton and 1 neutron) with 3-Helium (3He) nuclei (2 protons and 1 neutron) using the reaction D + 3He → 4He + H (corrected on 26.2.2023)
• This reaction is called aneutronic because it doesn’t produce a neutron. This is important because it means that – in principle – the entire apparatus will not become intensely radioactive.
• The Helion process is not steady state but instead involves episodic fusion reactions every second or so.
• Their plan is to start with a mixture of D + 3He in a plasma which is then rapidly compressed using changing magnetic fields to cause the plasma to heat which triggers the fusion.
• The heat of fusion then causes the plasma ball to expand against the compressing magnetic field, and as it expands in the magnetic field, it induces electrical currents directly in coils wrapped around the fusion chamber.
• The electrical current would be ‘harvested’ directly without the need for steam generation and a turbine plant.

Helion say they have demonstrated the feasibility of this with small scale plant, and are building ever larger prototypes.

Why it won’t work

There are large number of reasons why the Helion scheme will fail. Perhaps the first and most obvious is that it uses 3He as a fuel.

The ‘conventional’ approach to fusion involves the raw materials deuterium – which is common and found in sea water – and tritium – which barely occurs naturally on Earth. Obtaining tritium is a major challenge for conventional fusionistas, but it nothing compared to the challenges of making 3He.

Helium-3 is even rarer than tritium and some You Tubers (link) are even suggesting that interplanetary mining will be the source for helium-3. Please pause at this point and reflect on just how bonkers this is.

Helion do not suggest interplanetary mining. They suggest building a completely separate and thoroughly energy consuming nuclear plant to generate helium-3.

But the reason for failure to which I would like to draw your attention today concerns the basic nuclear reactions they hope to exploit.

The graph below – taken from Wikipedia – shows how the reaction rate of several different nuclear reactions vary with temperature.

Click on image for a larger version. Graph showing the relative reactivity of different fusion reactions. Note that at 200 million °C, and assuming equal concentrations, the D-D reaction is just as likely as the Helion reaction D-3He. And the D-T reaction is around 100 times more likely.

Helion have only managed to heat their plasma to 100 million °C so far, but they state that they will shortly achieve a staggering 200 million °C. This will be tough but let’s believe them for now.

Notice that the reactivity of a D-3He plasma is roughly equal to the reactivity of a D-D plasma, and both are around 100 times less reactive than a D-T plasma. So using D-3He to start a fusion reaction rather than D-T is like using damp kindling rather than dry kindling to try to start a fire – it just makes everything harder.

But Helion insist this sacrifice is worth it because their reaction is aneutronic, and the energy of the expanding fusing plasma can be captured electromagnetically.

But let’s imagine a 50/50 mix of D/3He which starts to fuse. As you can see, the D-D reaction rate is equal to the D-3He reaction rate. So if we start out with a 50-50 mix, after a short while there will be D-3He reactions and D-D reactions.

So after a short while – nanoseconds in practice – the original 50% mixture will contain the products of both D-D fusion and the D-3He fusion. And one of the products of D-D fusions is tritium, T.

The promotional video at the start of this article discusses this (starting at 13m 58s) and says that the tritium T will be captured in the exhaust at the end of the reaction and stored. However, that won’t happen!

Because the D-T reaction is 100 times more likely than the D-3He reaction, even a small amount of T in the reaction mixture begins to ‘steal’ D, lowering the D concentration, and emitting neutrons. And leaving the 3He with nothing to react with. After spending a fortune preparing the 3He – the majority of the fuel will be left unused after the reaction cycle!

Simulation

Helion propose that the nuclear reaction ‘should’ proceed as shown in the graph below.

Click on image for a larger version. Graph showing the expected relative concentration of species as the Helion reaction proceeds. Starting with a 50-50 mixture of D and 3He, these nuclei react and the amount 4He and H increases.

In this graph I altered the initial mix from 50-50 to 49.7-50.3 to allow the lines for 3He and D to show up separately. One can see that the D-3He fuel is consumed through the reaction and there are no neutrons produced.

However, this is just not what will happen. In fact – as discussed in the video – there are several reactions that can take place. Wikipedia helpfully summarises the reactions:

Click on image for a larger version. The four most prevalent fusion reactions. The bottom reaction is the one Helion wishes to focus on. However the D-D reaction produces both T, 3He, protons (H) and neutrons. And once there is T present in the reaction mix, the top reaction (D-T) will produce 4He and neutrons.

But what is not discussed in the video is the effect of the D-T fusion reaction which is around 100 times more likely than the D-3He reaction at 200 million °C.

Given all these reactions, it can be hard to anticipate exactly how things will proceed. But I have modelled all four reactions based on their approximate likelihood i.e. on the availability of the respective reactants and their reactivity as shown in the first figure. My spreadsheet is available here: Helion Fusion Simulation.

Inevitably the model is an approximation, but it is more realistic than the single-reaction Helion vision. Some example graphs are shown below.

Click on image for a larger version. Graph showing the expected relative concentration of species as a 3He-D mixture begins to fuse. The dotted green line shows the accumulated neutron dose. Notice that compared with the previous graph, less of the 3He is consumed, the D concentration falls rapidly, and the tritium concentration remains low but non-zero. See next graph for details.

Click on image for a larger version. Detail of the graph showing the tiny but critical concentration of tritium (T).

What we see in the above graphs is that the D-D reactions produce T which does not sit inertly in the mixture. Instead it ‘steals’ the remaining D in the mixture leaving the 3He substantially un-burned. There is also a very strong neutron dose: roughly 10% of the nuclear reactions produce a high energy neutron.

Reality

So in reality, the Helion approach will not be aneutronic. Their apparatus will become just as radioactive and be subject to just as much radiation damage as in any other fusion approach.

Also their very expensive 3He will remain unburned and need to be scavenged and separated from the ‘ashes’ of the reaction.

• If just the Helion reaction occurred, then in the time window shown in the graphs above, 75% of the 3He would be consumed.
• But when one considers all the other reactions, only a maximum of 35% is consumed – even if the initial D concentration is optimised.

So does all this mean that the Helion scheme won’t produce fusion? That it won’t work?

The Helion scheme begins with a plant for manufacturing 3He. This would be a massive complex proposal which would consume vast amounts of energy. It could only be justified if the Helion fusion process were somehow a straightforward way to generate even more vast amounts of energy at very low cost (aside from the 3He).

But the Helion fusion process is definitely not straightforward. It is not ‘aneutronic’ and D-T reactions will be a real problem for them.

And then one comes back to the even more basic problem of episodic nuclear fusion which I discussed in my previous article on laser fusion. That for a modest sized plant – say with 150 MW of electrical output – one would need to build an apparatus to withstand. an explosion of 0.1 tonnes of TNT once a second. Continuously. For 30 years. Really?

What is really going on?

Discussion of fusion as a viable option for future energy generation is a distraction from the urgent task at hand – to stop burning fossil fuels as rapidly as possible.

If holding out the illusion of a future magical technology delays climate action by even a year or two then it allows big oil, big gas, and big money in general, to reap extra profits.

So I urge you to ignore the siren calls of fusionistas. Ignore the talk of cheap and clean energy. Instead, close your ears and tie yourself to the mast of your boat, and sail on to a renewable future using truly miraculous technology such as solar and wind generation, technology which actually works.