4 ms·

If these reactors are low heat, not radioactive, and use plentiful fuel, what's the threat model? Is it just worried about availability of power?

jandrese 7y ago
"Not Radioactive" is the trick. Even if the fuel isn't hazardous you have lots of radiation being kicked off by the reaction that irradiates the surrounding equipment and the waste products.
- DennisP 7y ago
  The HB11 reaction doesn't produce neutron radiation. I've seen claims that there would be side reactions, generating neutron radiation totaling under 1% of the energy output, but the only source of that I can think of is if deuterium isn't removed from the hydrogen, and we could do that if necessary. It might not be, since neutrons from deuterium fusion are a lot less energetic than D-T neutrons. The energy output of HB11 consists of fast-moving alpha particles, which is good because that's the energy you capture, along with x-rays. None of this would activate reactor components. The waste product is non-radioactive helium.
  - Retric 7y ago
    It’s not that simple. 1% of even 1MW is a lot of radiation so it comes down to what the walls are made of more than the total neutron flux. ITER style lithium blankets would be recycled for tritium without producing much net waste. Light water nuclear reactors have several feet of water to absorb fast neutrons, which makes a huge difference. The walls of this are a more open question.
    - DennisP 7y ago
      So I looked it up, and found there actually are two side reactions from the primary fuels, no deuterium required. In a thermal plasma the resulting neutrons would carry less than 0.2% of the energy output. I don't know what the percentage would be for this reactor, since the fuel isn't a thermal plasma while fusion is happening. https://en.wikipedia.org/wiki/Aneutronic_fusion#Boron https://en.wikipedia.org/wiki/Aneutronic_fusion#Boron Neutron energy would be a bit under 3 MeV, comparable to fast fission neutrons but in much lower quantity for the power output. https://en.wikipedia.org/wiki/Neutron_temperature https://en.wikipedia.org/wiki/Neutron_temperature Interestingly, that page says the thermal neutrons in a conventional nuclear reactor are better at activating materials than fast neutrons. According to a presentation I saw by the leader of MIT's fusion program, the inner wall of a D-T tokamak would be activated by the neutrons, but would only need to be stored for a few decades. D-T releases 80% of its energy as 14 MeV neutrons. According to LPP, which is working on a different boron fusion design, a 20MW reactor would activate the walls with short-lived radiation, but it could be safely opened for maintenance after about twelve hours (iirc).
microtherion 7y ago
According to the article, the energy is generated by alpha particles, which are in principle a very dangerous form of radiation. It's not clear to me, though, whether the radiation stops as soon as the fusion does or not.
- DennisP 7y ago
  Substances which continuously emit alpha particles are dangerous. This is something different. With every laser shot there will be a burst of alpha particles. Then they'll slow down and just be plain old helium. If the direct energy conversion works, they'll get slowed down very quickly.
  - Animats 7y ago
    Substances which continuously emit alpha particles are dangerous. Only if you eat or inhale them. A sheet of paper will stop alpha particles.
- mcswell 7y ago
  Up until a decade or two ago, tritium--which emits alpha radiation--was commonly on wristwatch dials, precisely because they were not dangerous. Alpha radiation is easily stopped; it's only dangerous if you breathe it (or drink it, if that's even possible).
  - microtherion 7y ago
    Tritium emits beta radiation. It would be an interesting trick for an atom to emit radiation with a higher weight than itself. It might also be worth pointing out that "It must be safe! They even put it on wristwatch dials!" is not, historically, a sound argument.

jandrese 7y ago

"Not Radioactive" is the trick. Even if the fuel isn't hazardous you have lots of radiation being kicked off by the reaction that irradiates the surrounding equipment and the waste products.

DennisP 7y ago
The HB11 reaction doesn't produce neutron radiation. I've seen claims that there would be side reactions, generating neutron radiation totaling under 1% of the energy output, but the only source of that I can think of is if deuterium isn't removed from the hydrogen, and we could do that if necessary. It might not be, since neutrons from deuterium fusion are a lot less energetic than D-T neutrons. The energy output of HB11 consists of fast-moving alpha particles, which is good because that's the energy you capture, along with x-rays. None of this would activate reactor components. The waste product is non-radioactive helium.
- Retric 7y ago
  It’s not that simple. 1% of even 1MW is a lot of radiation so it comes down to what the walls are made of more than the total neutron flux. ITER style lithium blankets would be recycled for tritium without producing much net waste. Light water nuclear reactors have several feet of water to absorb fast neutrons, which makes a huge difference. The walls of this are a more open question.
  - DennisP 7y ago
    So I looked it up, and found there actually are two side reactions from the primary fuels, no deuterium required. In a thermal plasma the resulting neutrons would carry less than 0.2% of the energy output. I don't know what the percentage would be for this reactor, since the fuel isn't a thermal plasma while fusion is happening. https://en.wikipedia.org/wiki/Aneutronic_fusion#Boron https://en.wikipedia.org/wiki/Aneutronic_fusion#Boron Neutron energy would be a bit under 3 MeV, comparable to fast fission neutrons but in much lower quantity for the power output. https://en.wikipedia.org/wiki/Neutron_temperature https://en.wikipedia.org/wiki/Neutron_temperature Interestingly, that page says the thermal neutrons in a conventional nuclear reactor are better at activating materials than fast neutrons. According to a presentation I saw by the leader of MIT's fusion program, the inner wall of a D-T tokamak would be activated by the neutrons, but would only need to be stored for a few decades. D-T releases 80% of its energy as 14 MeV neutrons. According to LPP, which is working on a different boron fusion design, a 20MW reactor would activate the walls with short-lived radiation, but it could be safely opened for maintenance after about twelve hours (iirc).

microtherion 7y ago

According to the article, the energy is generated by alpha particles, which are in principle a very dangerous form of radiation. It's not clear to me, though, whether the radiation stops as soon as the fusion does or not.

DennisP 7y ago
Substances which continuously emit alpha particles are dangerous. This is something different. With every laser shot there will be a burst of alpha particles. Then they'll slow down and just be plain old helium. If the direct energy conversion works, they'll get slowed down very quickly.
- Animats 7y ago
  Substances which continuously emit alpha particles are dangerous. Only if you eat or inhale them. A sheet of paper will stop alpha particles.
mcswell 7y ago
Up until a decade or two ago, tritium--which emits alpha radiation--was commonly on wristwatch dials, precisely because they were not dangerous. Alpha radiation is easily stopped; it's only dangerous if you breathe it (or drink it, if that's even possible).
- microtherion 7y ago
  Tritium emits beta radiation. It would be an interesting trick for an atom to emit radiation with a higher weight than itself. It might also be worth pointing out that "It must be safe! They even put it on wristwatch dials!" is not, historically, a sound argument.