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!


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.


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.

14 Responses to “More Fusion Delusion”

  1. Bob Pugh Says:

    A very good analysis Michael. I have looked at a lot of these “fusion breakthroughs” that seem to come at fairly regular intervals. Mostly they just appear to be a way of a research team and its associated management and marketing team sucking in investors to prop up their lifestyles and secure their tenures.
    Engineers and scientists love to play with expensive toys and Net Zero is being used as cover. This situation seems to be getting worse as there is almost no critical investigation done of these claims in the media and I fear the government is not much better at spotting the lemons among the strawberries.
    I can’t see what’s wrong with using fission in moderate sized units rather than the daft EDF fiasco, pick a well proven existing design and get some built, preferably in a factory environment where quality is easier to manage using proper accounting and inspection.

    • protonsforbreakfast Says:

      Bob, Good Evening,

      Yes there does seem to an endless stream of very earnest companies funded by investors who are presumably searching for a Tax loss.

      And yes, if we had just kept building even Magnox reactors – perhaps one every 5 or 10 years – then we would have retained the knowledge, and constantly improved the technology. But I think nuclear’s time has passed now, even for fission. But I agree, we missed an opportunity to reduce our reliance on coil, oil and gas.

      Best wishes


  2. marcin Says:

    Hi Micheal,
    I appreciate your coverage of the Fusion-(con)fusion topics since there is so little or well reasoned analysis on that! Improbable Matter YouTube lectures are also great and on point with that topic.
    I personally wonder if there is something “atavistic” in the way of public enthusiasm for Fusion versus Fission. Like Fusion relates to joining, feels good, like a wedding kind of warm and fuzzy vs Fission which is about splitting which feels may have more of the “divorce” vibes… Fusion relates to warmth of the sun while Fission relates to dug out rare minerals. Realities are that Fission works and works well(easier to improve on how to do it economically vs hard physical stops for real world Fusion plants) , we know how to deal with radioactive materials safely (thousands of folks lived for months at times next to working small reactors under sea without ill effect on this planet).

    Maybe Fission should rebrand itself as using heavy metals, gifts from the earliest stars out of which all the things heavier than Helium are made. In the end Uranium and Thorium are as stellar as Fusion, and as “renowable”
    All the best wishes,Marcin

  3. Skipjack Says:

    Hello Michael!
    You are missing a few things here:

    1. D-He3 results in He4 + p not He4 + T.

    2. Helion actually wants the D-D side reactions because that is how they make their He3.
    That is why they do not have to mine the moon (which I consider silly anyway).

    D-D -> He3 +n or D-D -> T +p.
    The Tritium is stored and eventually decays into more He3, or it can be sold and traded, etc.

    That D-D reaction is still within net energy margins in their design, but it becomes more economic with D-He3 which is much more energetic.

    3. The Tritium is over 1 MeV when it is “born”. It is too hot and non- collisionional on the timescale of the pulse which is less than 1 ms.
    It will move towards the divertor very quickly.

    4. The fusion products including Tritium will be a small percentage of the total fuel in the machine. So even IF some D-T reactions were to happen, they would only consume a tiny minority of the D and there would still be enough D left to fuse with He3 (or other D).

    5. Helion plans to run at temperatures between 200 and 300 million degrees.

    6. Due to the high Ti:Te ratio – which increases with temperature – D-D and D-He3 reactivity is actually much closer to D-T reactivity.

    7. They also operate at higher densities than Tokamaks, which increases reaction rate of D-He3 and D-D even at the relatively low 20 to 30 keV temperatures.

    I highly recommend that you watch David Kirtley’s talk at Princeton from December.
    Do a search for “JPP08December2022_DKirtley PPPL” and you should be able to find it.

    It explains a lot about their theory and how and why it will (very likely) work.

    Best Regards

    • protonsforbreakfast Says:

      Dear Skipjack, thanks for those serious points. I won’t reply immediately: I’ll think about it for a bit.

      All the best


      • Skipjack Says:

        Thanks for your reply Michael!
        Let me know if there are any questions, I can help with. If there is something I do not know myself, I can ask people about it, or maybe get you in touch with them directly (the latter depends a bit on their availability, since they are very busy, but I can try).

    • protonsforbreakfast Says:

      Dear Skipjack, Good afternoon.

      Well I looked through your points and watched the hour long video presentation.

      1. Typo: sorry. Corrected.

      2, 3 and 4. You say that tritium will be a small percentage of the total fuel in the machine. My point was that since the D-T cross-section is ~ 100 x the D-3He cross-section, even 1% of tritium is significant in determining reaction outcomes.

      5, 6 In the 200 – 300 million °C temperature the relative cross-sections of the different reactions change, but in order to reach 200 million °C you need to first reach 100 million °C.

      It seems from the talk that the non-equilibrium nature of the episodic process is doing a lot of work for Helion. It seems to prevent plasma instabilities because ‘it’s all over before the instabilities can grow’ and somehow this is going to stop the tritium reacting and deliver other benefits too. And somehow, it miraculously avoids the emission of neutrons which steal energy from the reaction and activate materials of the enclosure.

      Heating to 200 million °C or more is non-trivial and no doubt Helion have a cunning plan, but I remain sceptical. It’s like someone saying they have climbed half way up Everest and they have a plan for the second half. How hard can it be?

      In the talk David Kirtley was speaking about scientific Q’s of just or 5 or so. I am super sceptical that such a low Q is sufficient to generate net energy from an engineering perspective, especially given the overheads of having to manufacture many kilograms of 3-He/month.

      My perspective perhaps differs from yours. I see a climate crisis which is already making life worse for millions and I am committed to doing what I can to minimise the damage we bequeath to the next generation. I see fusion projects as stealing billions of dollars of cash, and the capital of bright minds and spending these precious resources on something which can’t possibly help with the problem we face, even if by some miracle it technically ‘worked’.

      In any case: best wishes with your endeavours. Michael

      • Charles M. Says:

        Good afternoon, Michael.

        Like you, I’m also committed to do my part against climate change. I live in a flat so no solar panels, but I joined a solar co-op a few years back, my share is 35 kWp (from almost 2 MWp, and I’m rooting for a expansion).

        Having said that, I disagree with the idea of abandoning researching fusion.

        To start with, it is just a fact there’s no way we can force that, and, even if we could, how would we get sure that those funds get reinvested in our favored cause and not, for example, in mining, or virtual reality, or armies, or faster planes, or whatever ?

        Second, if humankind had been of that thought in the beginning, we would have never left the cave, so to speak. Think science for example. Science is a human endeavor that takes a lot of time and money, but whose fruits are rarely visible from the start. Electricity was only a curiosity for centuries before someone found any practical application. Should we had abandoned researching it because preventing famines, or curing all illnesses was still unsolved ?

        Third, even if fusion energy is not the solution to climate change, that does not mean we are not going to need it down the road. At the end of the century there will be 10 or 11 billion humans on this planet, and in my opinion we should aspire to give everyone a reasonable living standard. That means energy, a lot more energy that we are able to produce today. Renewables will be part of that, but they carry their own problems, and I don’t want to risk finding out one of them is insurmountable when it is too late. I prefer more baskets for our eggs.

        Regarding your points about the Helion reactor:

        2, 3 and 4) D-T cross section in 100 times that of D-3He only at one specific temperature. At 50 keV (the “ideal” ion temp for D-3He according the numbers showed in that Princeton presentation) that ratio is 16. And yes, I know, hotter is harder, but unless they do the research we will not know how high they can get.

        I’ve created my own spreadsheet to play with Helion’s numbers. According to them, their 50 MWe prototype reactor will work with ion densities in the order of 1E+21 to 1E+23 per m^3, while the T density would be 3 to 5 orders of magnitude less. Combine that with cycles 1 ms or less and there’s not much time to deplete the 3He (my numbers are that it shouldn’t get under 99% during a shot).

        5 and 6) They have reported having already achieved 9+ keV with a max B field of 10 T in their Trenta machine. With the one they are building at the moment the aim is 20 T. Given that temperature scales as B^2, that would mean up to 36 keV max, take out predictably growing loses and we will see how high they can get.

        And that’s enough for a day.

        Best wishes. Charlie.

      • protonsforbreakfast Says:

        Charles M:

        You speak so rationally, I would love to believe what you say is true. Except that fusion energy is going to be even more expensive that fission energy – just because of the cost of capital – let alone the cost of the technology.

        Anyway. Charles M, I don’t know where you live in the world, but if you ever pass through London UK, drop me a line: I would love to buy you a drink and have a chat.

        Best wishes: Michael

  4. Russell Robles-Thome Says:

    Michael –

    Two points are not really addressed by you above. If the Tritium is ‘born’ at 1MeV, it really does start off in a regime where it is not particularly reactive, and does not have to transition through a region where it is extremely reactive. It’s far from obvious that it remains in that condition until fusion ends, but there is at least some complex modelling to do to prove otherwise.

    Second, the point about creating large quantities of He3 isn’t really valid. If you read one of their patents, you can see they describe a regime called ‘Helion catalysed DD Fusion’. The density, temperature and fuel mix can be configured so that as much He3 is consumed in D-He3 reactions as is created in DD reactions.

    Again, far from obvious they can make that work, but at least they have a plausible means to dramatically reduce the need to manufacture He3.

    I’m probably more optimistic than you that the people handing Helion a great deal of cash have made them produce decent answers to these questions!

    Regards, R.

    • protonsforbreakfast Says:


      Good Morning. Your comments are very fair. Another commenter has also pointed out the limitations in my nuclear analysis. But I really just wanted to point out that there are detailed questions that are not addressed in the glossy promotional material. I could be wrong, but seems unlikely to me that the reaction will stay fully a-neutronic, which is critical for the technique.

      In the Princeton Seminar, CEO David Kirtley sounds very confident of his proposal, but I consider this something like a person half-way up Everest making plans for the second half. Hence I remain deeply sceptical.

      At the end they are trying to create nuclear explosions with the energy of roughly 200 kg of TNT about once a second. Mmmm. what could possibly go wrong. Aside from the whole manufacturing kilograms of 3He, the neutron activation problem, and the energy capture problem, the anticipate scientific Q = 5 is not going to be enough.

      And even if by some miracle it did work, it would be the most expensive electricity on Earth!

      Anyway: best wishes


      • Skipjack Says:

        Hey Michael!

        What detailed questions do you need to have addressed?
        Again, I can try to get you answers, if I don’t know them myself.

        To go with your comparison to Everest…
        I think that they now have the experience, the map and plan, the provisions and equipment, the right team and most of all the funding to do it.

        I do not understand the comparison to TNT.
        This is not a detonation. The plasma is very hot, but not very dense. The 100 billion Watts/m3 sound like a lot, but the pulses are only 1ms long. So you get 27 kWh/m3 by that calculation. Mind you, the 100 billion Watts/m3 is at peak and only with D-He3 reactions. In reality, the energy content of a pulse will be much lower.
        In comparison, TNT is over 2000 kWh/m3!!

        Another way to look at it: Their plants are supposed to do 50 MWe at 10 Hz pulse rate. Assuming a pessimistic conversion efficiency of ~50%, the net energy output of a single pulse would be some 2.8kWh. Assuming a Q of 3, we get <4 kWh total energy content.
        That would be equivalent energy content of about 3 kg/TNT and again the comparison is not really all that valid either because the density is much less and then the plasma is surrounded by a near perfect vacuum, which does not propagate explosive forces.

        As mentioned previously, a Q < 5 would be enough. That is because of their highly efficient, direct input energy and fusion energy recovery.
        Also note that the graphs seem to be fairly conservative and David Kirtley even mentions that they assume that none of the energy can be extracted from thermal transport and other loss mechanisms.

        Also, why do you think it would be the most expensive energy on Earth? The entire Trenta program was some 35 million (including everything). They got 500 million in funding now. That is enough for the Polaris program and building in house production capacity, paying 140 people on the team, validation, experiments, likely lots of upgrades etc, also brings them part of the way to commercialization, etc, etc.
        They plan to mass produce their machines (at 20 a day) and transport them by road.
        So I expect a CapEx of maybe 100 million/power plant. That is 2,000/kW and about the same as a combined cycle gas power plant. Their machines are much simpler. There are almost no moving parts, except pumps and some cooling fans maybe.
        The fuel is almost free.
        Now they will have some cost for maintenance, but it would not be that much.
        Most of the expensive equipment is relatively far away from the central "burn chamber" and will see a much lower neutron flux.
        The MIT assumes that their inner wall of their ARC design will have to be replaced once every 4 years and they have D-T neutrons for every reaction to deal with. Helion only has 1/3 of the neutrons and they are from D-D so much lower energy too.

        But assuming that the inner wall of the burn chamber and the compression magnets need to be replaced every year (for some reason) and they make up maybe 1/20s of the cost of the power plant (seems high), then the yearly maintenance will be 5 million. That is just a little above the 1 cent/kWh that they are proposing. And again, I am being rather pessimistic with my assumptions.
        Likely it will be far below that.

        Thanks and best regards!

      • protonsforbreakfast Says:

        To go with your comparison to Everest…I think that they now have the experience, the map and plan, the provisions and equipment, the right team and most of all the funding to do it.

        A map and a plan are great. But since no one has been to this place before, the actuality is likely to differ from the plan. Most typically in just the ways that one doesn’t want. On Everest, impassable crevasses. For Helion, fatigue stress on magnets?

        • I do not understand the comparison to TNT.

        If you want to generate 100 MWe from a Helion device you need – at 10 Hz – to generate 10 MJ per pulse at 100% efficiency in a series of nuclear explosions. TNT is 4.6 MJ/kg so at 10 Hz that’s about 2 kg of TNT per explosion. I agree it’s an explosion in a low-density gas, but the forces generated will be similar to the forces which power dynamos i.e. very large.

        •As mentioned previously, a Q < 5 would be enough. That is because of their highly efficient, direct input energy and fusion energy recovery.

        Please allow me to be sceptical. It has to cover the manufacturing of an entirely separate plant to breed 3He. As I understand it that plant would be energy negative.

        • Also, why do you think it would be the most expensive energy on Earth?
        • The fuel is almost free.
        • Most of the expensive equipment is relatively far away from the central “burn chamber” and will see a much lower neutron flux.

        The fuel is 3He – currently one of the rarest and most expensive substances on Earth. And we are talking about building a large object which will become intensely radioactive and process radioactive gases. All these things sound expensive to me. Fusion has been touted forever as producing electricity which would be too cheap to meter. I am sceptical of such claims.

        • But assuming that the inner wall of the burn chamber and the compression magnets need to be replaced every year (for some reason) and they make up maybe 1/20 of the cost of the power plant (seems high), then the yearly maintenance will be 5 million. That is just a little above the 1 cent/kWh that they are proposing. And again, I am being rather pessimistic with my assumptions.Likely it will be far below that.

        You would be talking about storing or disposing of hundreds of highly radioactive used ‘burn chambers’.
        Skipjack, I hear your optimism and your specific knowledge of this project and I wish the engineers all the best. And if it all works, I am prepared to acknowledge that my scepticism was ill-founded. But I am still sceptical.

        Let’s see how they get on the top half of the mountain.
        All the best: Michael

  5. Skipjack Says:

    Hey Michael!

    I am on good terms with people at several fusion startups, but I know the guys at Helion best. Have known them since the old days when they were still struggling for funding. It is also the fusion concept I know most about (because it is my personal favorite, followed by Zap).

    Their power plants will be up to 50 MWe, as I mentioned earlier.
    I say “up to” because they can load follow really well (better than anything else, actually), which makes them a great pairing for wind and solar.

    Cost of He3 is not really relevant. They make their own by fusing Deuterium. They will also have Tritium as a byproduct that they can sell for an even higher price than the cost of He3! In fact, selling Tritium could be more profitable for them than the energy market, at least until the Tritium market is saturated.

    To the best of my understanding, they are looking at 3 plant setups right now:

    – Run Deuterium rich and He3 lean and at somewhat lower temperatures and higher density. This is to my understanding the current design goal for the first 50 MWe machines. So (at least initially) their machines will completely self supply and there won’t be dedicated breeders at all.

    – Have dedicated breeders located at few sites and dedicated, distributed D-He3 machines (that will do a bit of breeding too just because of the D-D side reactions can’t be completely avoided). This is currently something they are investigating as an option, since it could have implications for siting requirements (among other things). “Pure” D-He3 machines would have less Tritium on site and there would be less neutrons and less activated components. So they could be located closer to customers. They might also not need quite as much equipment for the separation of fusion products, etc on site.
    Dedicated D-D breeders could be located at few select sites and likely have some design changes that make them more economic to operate (likely larger with more shielding, cooling, etc).

    – Have different modes for a single machine. E.g. individual pulses are either Deuterium- or He3- rich. This would also be a single machine, but I assume that the different modes make operation a bit more tricky. Plus neutron wall loads per pulse during pure D-D operation would be higher.
    The advantage is that they can run on just D-D (or very lean on He3) during times of low demand and then switch to pure D-He3 (or some higher concentration) when demand is high.
    There are probably also some optimizations they can do for the different operation modes that could make this attractive.

    To my understanding, all of them will be limited to 50 MWe peak output (or around that) because of wall load and other considerations. But they can co- locate several machines and share some equipment like the pulsed power components (which make a large part of the plant cost) between them (for up to 6 machines, I assume). That would mean a lower CapEx/kW for bigger plant setups.

    As for the radioactive components: This is one of the advantages of D-D-He3. The neutron loads are relatively low and the neutron energy is below the activation energy of many materials.
    That said, they will still have some activated materials, mostly 28Al (in the magnets) and some 31Si. The later will maybe make 4% of the first wall mass since 28Si and 29 Si (that make over 96%) result in stable isotopes when capturing a D-D neutron.
    28 Al has a half life of 2.3 minutes.
    31 Si has a half life of 2.5 hours.

    At their presentation to the NRC, Helion assumed a dose 4 rem/hour (40 mSievert/hr) after 1 hour from the last operational fusion pulse, 0.2 rem/hr after a day and 4 mrem/hr after a week. Those doses are assumed directly at the machine surface. So essentially, the entire thing is below background radiation after two weeks or so, max.

    Hope that answers your questions.
    Thanks and best regards again,

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