Originally published at: http://boingboing.net/2016/11/11/explainer-video-presents-fusio.html
…
2 Likes
Kurzgesagt videos are great.
I support them (very modest amount) on Patreon.
You can too! https://www.patreon.com/Kurzgesagt/posts
2 Likes
Fusion Reactors are not clean. They are quite polluting, just not in a way we’re used to
The problem is the Neutrons.
The basic reaction planned for ITER, for example, will be
Deuterium + Tritium reacting to form Helium4 and a Neutron
That’s a lot neutrons, which is a problem. From wikpedia
Due to the high kinetic energy of neutrons, this radiation is considered to be the most severe and dangerous radiation to the whole body when it is exposed to external radiation sources.
There are solutions to the neutron “pollution” problem, but the currently knowns solution are so expensive that even if the fusion reactor itself and the fuel was free, the final cost of the fusion energy would still be more than current fossil fuels.
I support fusion research, but even if the physics problems can be solved, there are still daunting engineering challenges.
2 Likes
That’s why we need a moon base! Collect all that sweet 3He
1 Like
Ya, but neutrons are easily shielded with water or something else that contains a lot of hydrogen, so they are not a difficult hazard to mitigate from the perspective of a radiation source. I wouldn’t call neutrons a pollution problem – they are just a hazard that can be reduced greatly with simple shielding. The primary problem with the neutrons is that they are carrying a lot of energy away that isn’t being captured. They also make inactive stuff radioactive, so now the steel/etc is going to be mildly radioactive which makes it more difficult to service the device.
The proposed methods of capturing the neutron energy bring us back to “dirty” techniques since they typically involve uranium/plutonium/fission.
4 Likes
Lithium used to capture neutrons. This leads to manufacture of more tritium.
3 Likes
I agree the shield is simple.
But based on a report I read some years ago, the amount of shielding needed for a plant producing commerical amounts of power increased the capital cost of the project so much that the power would be more expensive then coal.
Not good.
Hydrogen isn’t a great choice. First is has a small cross section. Second, when it capures the neutron, it release a gamma ray – which has it’s own issues.
When a hydrogen atom captures a thermal neutron, it turns into deuterium with the release of a 2.1-MeV gamma ray. The cross section is about 330 millibarns (not very large).
Reference Does capturing neutrons turn hydrogen into deuterium and affect the experiment?
I have no doubt progress can be made on the neutron problem – the science is easy next to plasma fusion physics – but the numbers I’ve seen suggest it’s a long way from being solved from an economic perspective.
1 Like
You don’t want to capture the neutrons with hydrogen, you just want the neutrons to transfer their kinetic energy to the hydrogen – the 1:1 mass is the most efficient for this. The gamma at the end can be problematic, but usually you have some thermal neutron absorber (e.g. boron) that emits a bit lower energy gamma or can go with heavy-water which has a much lower absorption cross-section, but is a bit less efficient at slowing them down.
3 Likes
Correct, but it also produces another free neutron, and thus does not itself solve the shielding problem.
Yes. Deuterium/helium-3 fusion does not produce a neutron, only a proton which can be easily captured. Availability, however, is the obvious and not easily overcome problem, since it is primarily one of the economics of moon harvesting. Presumably one goal of a permanent moon base would be to harvest adequate quantities for experimentation, as a larger operation would only be invested in if the reactor technology itself could be proven first. One very important thing though: don’t send Sam Rockwell or Kevin Spacey in a robot to run the harvesters. We’ve seen how that works out. It isn’t pretty.
4 Likes
Plus, unfortunately, the moon doesn’t actually have enough He3 concentration in the soil to be worth mining. Gas Giant atmospheres do, though, though that’s not quite ‘mining’.
2 Likes
It would definitely be a slow process, but presumably a solar powered vehicle could gradually separate it from the top layer of moondust where the solar wind leaves it. But yeah, scooping it out of gas giant gas would be be an essentially endless cheap supply. Maybe that will happen eventually, but a lot further to go get it.
3 Likes
Ehhh…lunar regolith has a pretty bad one-part-per-ten-million concentration of He3, it’d be sloooooow going, ten million tons of regolith for one ton of what you’re looking for.
1 Like
Sincere thanks for the article. Should be interesting reading.
My thinking is, if the robotic refineries are solar powered, and you can ship the refined He3 back to Earth to provide nearly limitless energy with a near-zero ecological footprint, then as long as you have the reactor tech and entry capital, isn’t mining lunar He3 a obviously good investment? Or are you saying Jovian He3 would be cheaper to harvest despite the distance?
1 Like
Not Jovian, but Saturnian: despite being further out than Jupiter, its much shallower gravity well requires way less fuel to push yourself out of and earthwards.
It’s also not quite a limitless energy source yet, we’ve not really tried working with it because we haven’t even cracked break-even on the the easiest (not that that’s saying much) fusion reaction, Deuterium-Tritium. The Deuterium-He3 reaction’s going to require either even bigger tokamaks or inertial confinement fusors, or fusing in thermal disequilibrium, which we’re…not quite sure can break even.
2 Likes
already solved, I saw a documentary about this
5 Likes
This topic was automatically closed after 5 days. New replies are no longer allowed.
