[WorkLobster]

> you will always need at least a small fission reactor to breed some tritium

The idea for future designs is to breed Tritium in the blanket surrounding the vessel. This obviates the need for external sources.

> everything close to the reactor becomes brittle in 2-4 years

I'm no expert on solid state physics, but that seems a little short? JET is more than ten times as old, and as we speak it's in its second run of D-T experiments.

> These are all intrinsic limitations of fusion that fission reactors just don't have

To be clear these are limitations of a specific kinds of fusion reaction and/or reactor design, primarily that of Deuterium-Tritium in a solid-walled tokamak. This may be a nitpick as it's currently far and away the most promising for energy production, and the alternatives are much further from any sort of workable prototype, but I have heard of them undergoing active research: tri-alpha (aneutronic reaction, meaning no activation or embrittlement of reactor components), liquid metal for the walls and divertor (in effect, continuously replacing neutron-bombarded material), etc.

[ncmncm]

All of your alternatives make it even more expensive to build and to operate, and so even less competitive. Regardless, extracting the few grams of 3H or 3He bred for fuel in the thousand tons of molten, radioactive lithium jacket, every day, is a problem no one has solved. Good luck with that.

It is conceivable that fusion could be made to work in outer solar system spaceship propulsion, where the constraints are very different. It will never generate commercial power on Earth. The billionaires pumping cash into fusion startups are being taken for a ride. They can afford it.

By contrast, Bill Gates got US taxpayers to pony up fully half of the scratch on his pet SMR project, without giving up any ownership. So, we are the ones taken for a ride, instead.

[simiones]

> The idea for future designs is to breed Tritium in the blanket surrounding the vessel. This obviates the need for external sources.

The problem is that, at best, you can create as much Tritium in the blanket as you put in as fuel (since every emitted neutron is coming from a Tritium atom). So, to be self-sustaining, every emitted neutron would have to be caught by the blanket to form a Tritium atom, and you would have to be able to extract every single atom of Tritium from the blanket back as usable fuel - and this is assuming 100% of the tritium you put in actually fuses, which is unlikely given how hard tritium is to contain (essentially every material is porous to Tritium). So, since there are losses at each of these levels, you need to inject new tritium into the cycle.

Also note that this entire blanket design is entirely theoretical at the moment: no fusion experiment has ever attempted to do anything with the fusion products other then measure the amount of heat generated.

> I'm no expert on solid state physics, but that seems a little short? JET is more than ten times as old, and as we speak it's in its second run of D-T experiments.

I'm no expert either, but these are the estimates I have read everywhere. JET is not in any way representative, as they do a handful of fusion events per year, for a few seconds - while a DEMO plant would be running continuously, 24/7. The amount of irradiation is incomparable.

> To be clear these are limitations of a specific kinds of fusion reaction and/or reactor design, primarily that of Deuterium-Tritium in a solid-walled tokamak.

These are all limitations of the only fusion electrical power-producing technology that is anywhere close to realistic.

All other fusion reactions require much, much higher temperatures and pressures to ignite, so they are many more decades away (regardless of what some snake oil start-ups are claiming).

All other magnetic confinement D-T fusion reactions have the same problems I discussed.

And inertial-confinement fusion approaches are much less likely to ever be economical given the huge costs of the actual fuel.

[WorkLobster]

These are all excellent points—thank you for clarifying some of the parts I hadn't considered. In terms of the last and how practical it is, we're more-or-less saying the same thing; my hope was to be clear on what is a result of a tough engineering dilemma, as opposed to impossible or intrinsic, just so casual readers of this thread don't go away with the wrong idea.

[DennisP]

Fusion blankets include lead or beryllium as neutron multipliers. When a nucleus of either is hit with a neutron, it releases two more. The resulting neutrons can still breed tritium from lithium.

CFS for example uses beryllium in a FLiBe salt. General Fusion and Zap Energy use lead.