Comment by ncmncm | Hacker News Reader

Comment by ncmncm | Hacker News Reader

Parent Full thread

Their big problem is not manufacturing, not personnel, not extraction of useful energy from kinetic motion of charged particles, and not even physics of light-nucleus fusion. Those are, in principle, probably all solvable.

Their big problem is that they depend for fuel on 3He, and there isn't any. Not on Earth, and not even, usefully, in the lunar regolith. (By the way, there should be a better name for 3He as there is for 3H, tritium. "Helionium"?)

There is one source for 3He, and that is decay of 3H, tritium. Tritium can be made in high-neutron-flux processes such as in a CANDU reactor, slowing the neutrons with heavy water until they fuse with the 2H deuterium atoms in it. Then, keep it for 12+ years waiting for it to decay. We actually do manufacture a noticeable amount of tritium, and then store it in missile silos for the requisite 12 years, purifying out the 3He half periodically and topping up the tanks. Tritium is stored there ready to be funneled into the middle of a hydrogen bomb shortly before lofting to its unlucky recipients.

Operating these things is supposed to produce a little bit of 3H, which in principle could be collected, banked, and harvested. But each 3He they burn is very unlikely to make the 3H they will need to get another 3He some years on. They have to extract, anyway, the tritium they make, to avoid fusing those by accident, producing useless and harmful hot neutrons.

There is a competing technology, that has been worked on at Princeton and generically labeled "field reversed configuration", FRC, also burning D-3He, optimized for spacecraft propulsion. They hope to loft a test article by 2035.

So there are a few practical uses for this technology, if it can be made to work at all: to power spacecraft exploring the outer solar system, and maybe even for a moon base; but not for practical energy here on Earth.

For energy here, solar + wind + storage are the key to forestalling civilizational collapse. It might still be possible to prevent worldwide collapse if we radically increase the investment rate in these known-practical technologies. But not if we don't, and we haven't, enough. If civilization can be saved from collapsing, this tech could become interesting someday. Otherwise it will stall when civilization collapses out from under it. It is safe to say that would be a shame.

Actually their only primary fuel is deuterium. The fusion reactions are 2/3 of D+D and 1/3 of D+He3

D+D fusion reaction yields 50% of T+p (tritium and a proton) and 50% of He3 + neutron

So in total their fusion reaction does not consume He3. He3 is here like a catalyzer.

They still need some He3. The D-D reaction produces tritium, and the tritium decays into He3. Half life of T is 12.5 years.

Done, no issues with the helium 3.

Actually they will probably produce T in excess and could sell it.

Running deuterium-rich enough to net significant 3He out would seem to emit enough neutrons to render the reactor vessel strongly radioactive, and require the sort of cooling and shielding that makes the pure D-D reactors being pursued by others grossly impractical.

As I understand the neutrons of the D+D->He3+n reaction are slow neutrons (2.45MeV) that won't damage too much the materials.

These are not the fast neutrons of the DT fusion (14.06MeV)

They claim it won't be worse than waste neutrons of radiation therapy medical devices. To be seen...

But yes, agree this is not exactly aneutronic fusion. Around 10% of energy is lost as slow neutrons and this is definitely a problem to address.

6% as neutron radiation is the figure I've seen, from Helion and other sources.

According to the head of MIT's fusion department, even D-T neutrons only mean you'll have to bury the activated materials for a few decades. A much lower amount of lower-energy neutrons isn't going to cause any major problems.

Exactly, the neutron induced radioactivity is orders of magnitude lower than in fission by-products and is a minor issue.

The problem is more with neutron embrittlement that weakens the materials exposed to fast neutron radiations. For DT fusion in tokamaks the idea is to replace the inner core every year or so.

Helion claim embrittlement is not much an issue for them. First, because their neutrons are relatively slower and second, because they can choose neutron resistant materials since they do not fear plasma contamination thanks to the pulsed nature of their operation: in between pulses they vacuum the vessel before injecting the exact mix needed.

In tokamaks, the materials facing the plasma are designed primarily to avoid emissions that might contaminate the plasma, embrittlement comes as an afterthought. Also in DT fusion, it's not 6-10% of the energy released as slow neutrons, but 100% as fast neutrons.

Conclusion: there is a neutron problem but it seem minor and fairly solvable

I am corrected. Operated just D-rich enough to breed its own 3He (plus tritium, presumably for sale), the material damage, waste heat, and radioactivation from neutron flux produced by the D-D side reactions are much more manageable than for a plant relying on capturing those neutrons for their heat.

The waste heat from a 2 MW plant, ~200 kW, would not be negligible, but capturing and handling 200 kW in warm (guessing?) lithium-deuteride saturated heavy water is different in kind from extracting 20 GW (probably the minimum size of a the D-T plant) from thousands of tons of molten metal.

> Their big problem is that they depend for fuel on 3He, and there isn't any. Not on Earth, and not even, usefully, in the lunar regolith.

It's interesting that talking with rocket people, they point out that getting enough - enough - He3 from the Moon regolith isn't that big of a problem, and then the phrase is practically repeated - "their big problem is that 3He isn't needed by anybody on Earth, since there are no fusion plants to utilize it". Classical catch-22.

> For energy here, solar + wind + storage are the key to forestalling civilizational collapse.

You forgot the expensive but easiest solution to this, building more regular fission power plants.

Each dollar diverted from renewables to fission brings climate disaster closer. Besides displacing much less carbon emission, in operation, it doesn't start displacing any at all for a decade. During that decade, what you pay out for coal or NG could have bought the solar farm instead.

Coal producers love fission nowadays, because it means another decade of dependence on their product. They see the writing on the wall, so they only hope to put off the end. Promoting nukes is their lifeline.

Is that really how the financing of these projects works, though, to say that dollars are simply diverted from renewables to fission? I think not. As a civilization, we surely have the capacity and the imperative to do both. (That said, we should absolutely stop giving trillions in subsidies and financing to fossil fuel projects.)

Not to mention, we have a land use problem. Far too much land is given over to grossly inefficient livestock rearing, but solar installations are hardly better in land use terms, much as I recognize their necessity.

In any case, sites for solar installations are hard to find. Same deal for fission plants, for obviously different reasons, and in spite of their impeccable safety record by the numbers.

Difference being, though, that for every fission plant that overcomes NIMBYism, you get a lot more generation, albeit with considerable delay before that generation comes online, as you note.

All of which is to say that, the realities are not straightforward, it is not a binary choice. We should build as much new, non-carbon-emitting generation as possible.

Energy is the principal civilizational force multiplier. Given the damage we’ve already done that we need to undo, we need as much energy as we can get.

While we’re on the subject, let’s drop the facile pretense that any of these generation technologies are actually clean. They all have environmental externalities. I think it’s incumbent upon us to fairly represent that, and to make a whole-system analysis of this problem.

Hand-wringing about land use amounts to concern trolling.

There is absolutely no hint of shortage of land to site overabundant solar on. Solar coexists synergetically with numerous existing uses, including reservoirs and canals, pastures and farmland, and on industrial, warehouse, and even residential roofs and parking lots.

On water, solar farms operate more efficiently and cut evaporation and biofouling. In pasture and farmland, they operate more efficiently, cut water demand and heat stress, and for many crops even increase yield. They extend the life of roofs under and cars parked under them.

The area now devoted to domestic oil extraction is well over the most that would ever be needed for overbuilt solar, even without dual use.

Wind extraction, of course, has long coexisted nicely with agriculture and navigation.

So, the most effective way to get the overabundant energy we will need is to overbuild renewables.

The "environmental externalities" of renewable generation are orders of magnitude less than for others, so for purposes of planning really are negligible. Most of that is carbon emission occasioned by fabrication before the power used can be generated renewably.

Cunningham’s Law in action (unintentional on my part) [1]. Thanks for the detailed response, that gives me plenty to read up on further.

Agrivoltaics looks like a very clear win, especially for shade-loving crops [2].

[1]: https://meta.m.wikimedia.org/wiki/Cunningham%27s_Law [2]: https://youtu.be/lgZBlD-TCFE

Surely "Threlium"?

Trilium.

Android autocorrect thinks there is a word "trillium" already. So, maybe trelium or terlium. Or live with it, as we do lama and llama. Dreilium? Internet would probably go all in for threelium. Or heliumy-mc-heliumface.

Just re-use the same word. With different meaning. Big deal.

Reply on news.ycombinator.com