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So as someone who follows fusion somewhat I'm embarrassed to admit I'd never heard of this. For reference, pB here means proton-Boron [1]. To expound on what t

by cletus 8y ago

So as someone who follows fusion somewhat I'm embarrassed to admit I'd never heard of this. For reference, pB here means proton-Boron [1]. To expound on what the commenter here has said: neutrons are a big problem for hydrogen-fusion reactors for two reasons: 1. A high-energy neutron represents a loss of a lot of energy, reducing the overall efficiency of the process since that neutron is basically "lost". It obviously has no charge so can't be captured. 2. Additionally, high-energy free neutrons tend to destroy their containers. I don't know if this will make the reactor itself radioactive or not. The commenter seems to think so. Hence the idea of aneutronic fusion, which is fusion that produces no neutrons or very few. This is part of the attraction of Helium-3 fusion [2]. The neutron problem seemed so fundamental (in that neutrons are required to trigger fusion reactions, which is why you use deuterium and tritium rather than protium (Hydrogen-1) but those neutrons are the very problem that (so far) has made this method impractical. I remain skeptical if this particular problem will ever be solved. Neutrons are fine for stars. Stars are big so that neutrons don't go that far and they're big enough that gravity solves the containment issue. It's also worth noting that there's no such thing as "free" energy. There is energy where the fuel is free (solar, wind and even fusion) but that still doesn't mean the energy is free. The plant that produces it costs money and takes up space. It must be maintained and it has a shelf-life. Look at the total cost of the plant over its lifetime and divide that into the power it produces and there's your baseline energy cost. So a $50B plant that produces 5MW for 5 years for no fuel cost isn't much use to anyone. [1] https://en.wikipedia.org/wiki/Aneutronic_fusion#Proton-boron https://en.wikipedia.org/wiki/Aneutronic_fusion#Proton-boron [2] https://en.wikipedia.org/wiki/Helium-3#Fusion_reactions https://en.wikipedia.org/wiki/Helium-3#Fusion_reactions

zaarn 8y ago
I think some proposals on the design include a barrier material around the reactor chamber that would capture the neutrons and thusly convert them to heat, which can be used to drive a turbine. IIRC the aim was to find a material that would convert to a shortlived radioactive isotope instead of a long lived one, so you can simply remove the barrier and have it sit in a quiet place for a few years, then recycle it.
- pfdietz 8y ago
  Of course. Such a blanket is absolutely needed in a DT reactor, to regenerate the tritium that the reactor needs to keep running. But the materials from which that blanket is made, and the first wall separating the blanket from the vacuum vessel, are among the things that would be degraded by neutron exposure.
- ncmncm 8y ago
  The "blanket" (pumped molten salt is frequently cited, or more exotic material, many feet thick) is needed to turn fast neutrons into heat, thence to boil water to drive a turbine. But the equipment to contain the plasma, many, many tons of expensive stuff, has to be inside the blanket, and is destroyed in short order by neutron flux. The "envelope" that keeps your "blanket" from spilling out all over the floor, a monstrous apparatus of pipes, is also being irradiated, but there are materials for the pipes that can stand up to it for a while. This envelope is made in hundred-ton sections bolted together so it can be drained periodically and sections replaced. The used ones are radioactive and need to be kept somewhere safe. This is all hugely expensive, finicky work that is very dangerous to be around, so you need even bigger remote-operated machines to do it.

by cletus 8y ago

zaarn 8y ago

I think some proposals on the design include a barrier material around the reactor chamber that would capture the neutrons and thusly convert them to heat, which can be used to drive a turbine. IIRC the aim was to find a material that would convert to a shortlived radioactive isotope instead of a long lived one, so you can simply remove the barrier and have it sit in a quiet place for a few years, then recycle it.

pfdietz 8y ago
Of course. Such a blanket is absolutely needed in a DT reactor, to regenerate the tritium that the reactor needs to keep running. But the materials from which that blanket is made, and the first wall separating the blanket from the vacuum vessel, are among the things that would be degraded by neutron exposure.
ncmncm 8y ago
The "blanket" (pumped molten salt is frequently cited, or more exotic material, many feet thick) is needed to turn fast neutrons into heat, thence to boil water to drive a turbine. But the equipment to contain the plasma, many, many tons of expensive stuff, has to be inside the blanket, and is destroyed in short order by neutron flux. The "envelope" that keeps your "blanket" from spilling out all over the floor, a monstrous apparatus of pipes, is also being irradiated, but there are materials for the pipes that can stand up to it for a while. This envelope is made in hundred-ton sections bolted together so it can be drained periodically and sections replaced. The used ones are radioactive and need to be kept somewhere safe. This is all hugely expensive, finicky work that is very dangerous to be around, so you need even bigger remote-operated machines to do it.