Hacker News Reader

Comment by pletnes | Hacker News Reader

The biggest hurdle for fusion power is, ultimately, how to deal with the neutrons. These particles are electrically neutral and their trajectories are unaffected by electromagnetic fields. The approach so far is to put a «blanket» around the fusion core to slow them down, but this fails to mention that the blanket will be slowly (or not so slowly) damaged by neutron induced transmutation. Since a lot of the energy released in fusion power comes in the form of kinetic energy of neutrons, it’s also essential to convert that energy into eg steam, for power generation.

View on HN

The blanket also has to have a high cross section for elastic collision with neutrons to efficiently capture their energy as heat. ITER is planned to have liquid lithium as this blanket because it has a high cross section and the odd neutron capture will result in helium and much desired tritium that can be extracted for use as the main fuel in the reactor. Unfortunatey, elementary lithium is a very temperamental material.

I heard a story a while ago about a facility working with a lot of molten sodium. The whole building had no running water at all, no bathrooms even, because water and sodium mixing is a problem.

Lithium doesn't seem a whole lot safer.. but I don't think a fusion power plant could be water free. As far as I'm aware there is no more efficient method of using heat to spin a generator than a steam turbine. A lithium/water heat exchanger seems pretty frightening. Do they have a better plan for that?

From somewhat outdated memory: lithium is thermodynamically less stable in the presence of water, but the reaction is slower than it is for sodium. (And so on: potassium and cesium react even more violently with water.)

But this is largely moot, as lithium in salt form (Li+) does not react with water. I would be more concerned about the beryllium salt dissolving in water and escaping. Beryllium is nasty.

If memory serves, lithium is more reactive than sodium. ITER will have some kind of heat exchanging system to cool the lithium. But I don't know what that secondary system uses.

Haven't looked at the proposal, and not commenting either way on the risk of using water in this context, but the water/reactive-metal issues have been widely investigated in other liquid metal reactor designs and generally intermediate cooling loops with some benign medium are used.

Edit to expand; having now looked it seems some of the intermediate loop designs have the SAME material (e.g. sodium). This at least means the loop at risk of mixing with water is isolated from the core (where an explosion is difficult to manage). I thought I'd seen other designs, just can't find them now.

MIT's design uses molten FLiBe salt as the blanket. It's chemically stable, and the beryllium multiplies the neutrons for breeding with the lithium.

The inner wall of the reactor is 3-D printed, and the reactor uses jointed superconducting tape that allows it to be opened up annually to replace the inner wall.

MIT's design also uses 40% of the world's annual mine production of beryllium, for a single reactor. Using the entire estimated Be resource in these reactors would supply maybe 1% of the world's primary energy demand.

That was once said about lithium and batteries. Typically once the market demand is there we suddenly find a bunch of viable reserves to tap. This is tragically happening with petroleum where we're tapping incredibly difficult reserves via deep sea drilling and fracking.

Asteroids!

I don't think asteroids would actually be a good source of Beryllium. IIRC they have approximately the same overall elemental distribution as the earth does, just more evenly distributed - which is great for finding dense materials like gold or rare-earth metals that have sunk towards the earth's core, but not so great for one of the lightest metals in existence.

Gold and the like have sunk into the core not because they are heavy, but because they chemically partitioned into a phase (liquid iron/nickel) that was heavy.

Uranium, which is also heavy, partitions into light silicate minerals, and is enriched in the Earth's continental crust by 3 orders of magnitude above the average of the planet. Without this concentration fission energy would likely never have been considered as an energy source.

It is my understanding that beryllium is concentrated in the Earth's crust by about a factor of 100 over carbonaceous chondrites. Beryllium is actually very rare on a cosmic scale, since it is one of the "X-process" elements made by cosmic ray spallation, not in stars.

Can we use neutron absorbers? Here's what wikipedia [1] says about them:

"The most important neutron absorber is B [boron] as B4C [boron carbide] in control rods, or boric acid as a coolant water additive in PWRs. Other important neutron absorbers that are used in nuclear reactors are xenon, cadmium, hafnium, gadolinium, cobalt, samarium, titanium, dysprosium, erbium, europium, molybdenum and ytterbium; all of which usually consist of mixtures of various isotopes—some of which are excellent neutron-absorbers."

By the way, boron carbide [2] has an extraordinarily high melting point (2763 C), maybe this could be handy if one wants to convert the thermal energy of neutrons into electricity.

[1] https://en.wikipedia.org/wiki/Neutron_capture#Neutron_absorb...

[2] https://en.wikipedia.org/wiki/Boron_carbide

The neutrons in a DT reactor have to be used to breed more tritium, by absorption in 6Li. One cannot afford to lose many; indeed, some neutron multiplication by (n,2n) reactions on various structural materials would be needed to make the cycle close.

I see. So you are saying it's better to use a neutron reflector [1] rather than a neutron absorbant. Either way, it doesn't appear to be an insurmountable problem.

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

Is putting a big pile of concrete around the reactor not a suitable solution? Concrete is often used as neutron radiation shielding because it's full of hydrogen (in the water chemically bound up in it) and because of its desirable structural properties.

They already do this with ITER[1] for the "bioshield" to protect humans from radiation, the facility is semi underground as well.

However by the sounds of it, a reasonable fraction of the output power is in the form of neutrons. If you had no lithium blanket and just concrete, the energy of the neutrons would go into heating the concrete, where it would either be wasted, or require the concrete to be actively cooled which would be difficult as concrete is not a great conductor.

[1] https://f4e.europa.eu/mediacorner/newsview.aspx?content=941

The main concern with the neutrons is damage to the reactor itself — you don’t want to have to replace the main chamber walls with any frequency. Fusion reactors generally do not release enough neutrons to hurt people. So shielding on the outside doesn’t do much except provide additional fail safes.

The neutron flux inside a fusion reactor is many orders of magnitude higher than the maximum allowable for human exposure. The flux is low on the outside precisely because of the many absorption lengths of shielding between the reactor and people.

> The biggest hurdle for fusion power is, ultimately, how to deal with the neutrons. These particles are electrically neutral and their trajectories are unaffected by electromagnetic fields.

Layman here, but surely they must interact or bond with something? Perhaps to produce something else that could be used.

Well, yes, they interact with most things, by colliding with their atomic nuclei, thereby turning their atoms into radioactive isotopes, and generally destroying any object or living matter in their path.

https://en.wikipedia.org/wiki/Neutron_radiation

I'm a total ignoramus when it comes to particle physics, but what you just described sounds like a particle-based process. Is there an equivalent description if you think of neutrons as waves?

Not really. At these ranges and energies a particle is the best description.

Reply on news.ycombinator.com