Comment by credit_guy | Hacker News Reader

Comment by credit_guy | Hacker News Reader

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> we can make tritium from lithium using the neutrons from fusion

Each fusion reaction consumes one tritium atom and produces one neutron. If that neutron hits a lithium atom, it can split that and produce a tritium atom. If everything goes perfectly and there are no losses, then you get a 100% replacement of all the tritium that you consume. If you have a 90% replacement ratio (highly optimistic), you essentially lower the cost of your tritium fuel by a factor of 10, so from $30000 per gram to $3000 per gram, or $3 MM per kilogram.

> We also get tritium from fission plants

Yes we do. Mainly from Candu reactors. There are 49 Candu and Candu-like reactors in the world, and each produces less than 1kg of tritium per year. According to [1] a 1 GW fusion power plant would consume about 55 kg of tritium per year. So you'd need to run more than 50 fission power plants to operate one fusion power plant. Most people who dream of fusion think that fission will become irrelevant, not that you'll need 50 fission power plants for each fusion power plant.

[1] https://www.sciencedirect.com/science/article/abs/pii/S09203...

That's why fusion blankets for D-T reactors use lead or beryllium as neutron multipliers. CFS for example uses FLiBe molten salt. Doing it this way a tokamak can not only sustain its own tritium supply, but periodically provide startup fuel for additional reactors.

Initial tritium load for a small, high-field reactor like CFS is much smaller than for ITER. And I'll note that the paper you linked has this conclusion:

> The preliminary results suggest that initial operation in D–D with continual feedback into the plasma of the tritium produced enables a fusion reactor designed solely for D–T operation to start-up in an acceptably short time-scale without the need for any external tritium source.

> CFS for example uses FLiBe molten salt

Ok, let's talk about that. For those who are not familiar, CFS stands for Commonwealth Fusion Systems, as startup with links to MIT. CFS aims to build a fusion reactor similar to ITER, but many times smaller, the secret sauce being that they use superconductors to achieve high magnetic fields. Back in 2022 some of the MIT guys got an ARPA-E grant to investigate the use of FLiBe to achieve atritium breeding ratio higher than 1 [1]. The results are in [2], they were published in January 2025. Here are some quotes:

  > The long-term goal of LIBRA is to demonstrate a TBR ⩾ 1 in a large volume (1000 kg ∼ 500 l) of FLiBe molten salt using D–T neutron generators. Note that a full-scale LIB in an ARC-class FPP will require ∼250 000 l of FLiBe, hence the importance of understanding tritium behavior in large salt volumes.

ARC is the fusion reactor designed by CFS. This paper states that it will need 250000 liters of FLiBe. This is an insane amount. To understand how large this amount is, consider this: this ARPA-E project that took 3 years, used a quantity of 100 ml, so 0.1 liters.

Anyway, what breeding ratio was achieved? 3.57 x 10^(-4), or 0.0357%. It's a long way to go from here to 1.

I'm not saying it's impossible, but too many things related to fusion are just "engineering details".

[1] https://arpa-e.energy.gov/programs-and-initiatives/search-al...

[2] https://iopscience.iop.org/article/10.1088/1741-4326/ada2ab/...

250000 liters of FLiBe contains 44 tons of beryllium, and the current annual production is 220 tons, so it's possible but not cheap.

Wow. The ARC reactor is supposed to deliver 270 MWe (when/if it will be built) [1]. So 20% of the world annual production of beryllium for one power plant that would deliver about one quarter of the power of an AP-1000 fission reactor.

[1] https://en.wikipedia.org/wiki/ARC_fusion_reactor

And the original ARC design used 958 tons of FLiBe.

https://arxiv.org/pdf/1409.3540

Beryllium is very efficient as a neutron multiplier, but it is also extremely rare. It would not be acceptable as a consumable for energy production, as it is much more useful for other purposes.

In the Solar System, the abundance of beryllium is similar to that of gold and of the platinum-group metals. On Earth, the scarcity of beryllium is less obvious only because it is concentrated in the continental crust, where it is relatively easily accessible, even if its amount in the entire Earth is much smaller.

Lead neutron multipliers would be preferable, because they only inter-convert isotopes of lead, so it is not destroyed, like beryllium.

However lead used for this purpose becomes radioactive, with a very long lifetime, unless expensive isotope separation would be used for it.

I mean, it has to destroy the lead eventually, since lead is being used as a source of the extra neutrons. An individual lead nucleus will be converted to lighter lead isotopes by (n,2n) reactions, but eventually it will reach Pb-203 which decays to Tl-203. Presumably the thallium (and then mercury) will also be subject to (n,2n) reactions.

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