Confinement

[Mike Weber Goodenow]

Does aneutronic fusion work on inertial (electrostatic) or magnetic confinement, or could it be either one?

Are major breakthroughs in confinement capacity needed before aneutronic fusion is feasible and cost-effective?

[Tulse]

“Aneutronic” refers to the nature of the fusion reaction, namely that it produces no (or, more accurately, very few) neutrons. This characteristic is in principle orthogonal to the method used to get the nuclei to fuse (although aneutronic reactions generally require higher energies to cause fusion).

(This is all my understanding — someone else may be able to correct any errors I’ve made.)

[zapkitty]

Tulse wrote: “Aneutronic” refers to the nature of the fusion reaction, namely that it produces no (or, more accurately, very few) neutrons. This characteristic is in principle orthogonal to the method used to get the nuclei to fuse (although aneutronic reactions generally require higher energies to cause fusion).

(This is all my understanding — someone else may be able to correct any errors I’ve made.)

That’s it. Aneutronic fusion is defined as when neutrons carry no more than 1% of the total released energy. Essentially means that if a particular reaction generates any neutrons at all they would be a side-effect of the main fusion process and thus would be an annoyance instead of something required for net power generation.

And as the neutrons are <1% of the total power that means no neutron activation of reactor materials… and thus no nuclear waste.

[Mike Weber Goodenow]

Okay. Thanks.

[delt0r]

At 1% output power, activation is something that happens and that matters. It is not background. But it is manageable. Even much less than 1% (say kW or even 100W ) of neutron power over long term needs to be considered. aka Activation and corrosion modes of materials. Traditional nuclear has about 1% of the energy in neutrons for example.

[zapkitty]

delt0r wrote: At 1% output power, activation is something that happens and that matters. It is not background. But it is manageable.

Ah, I was referring to pB11, boron fusion, which is estimated to have less than 0.2% of its energy in neutrons. My apologies… I was being pB11-centric 🙂

Less than 1% is just the cutoff point above which a particular reaction is not considered to be aneutronic… the actual percentage can be much lower than that and pB11 is the only aneutronic contender being researched at all.

BTW, “less than background” only refers to conditions outside the shield, which is a meter of water, some B10 and with a layer of lead as the final backstop.

delt0r wrote: Even much less than 1% (say kW or even 100W ) of neutron power over long term needs to be considered. aka Activation and corrosion modes of materials…

… and LPP has calculated that an end-of-life core (actually, a pre-recycling core) will have the the radiation output of a classroom of kindergarteners.

(I didn’t know that school lunch programs could still afford bananas…)

That’s not radioactive waste… that’s a paperweight 🙂

[delt0r]

A 1MW plant would have the neutron power of 2kW @ .2% neutron power, even @ .02% its still 200W of neutrons. A *lot* more than “radiation output of a classroom of kindergarteners”. E. Lerner has said this even in his first Google talk. There is a cool down time of hours/days after switch off before you can get your hands inside.

Rare side reactions matter.

[zapkitty]

delt0r wrote: A 1MW plant would have the neutron power of 2kW @ .2% neutron power, even @ .02% its still 200W of neutrons. A *lot* more than “radiation output of a classroom of kindergarteners”.

Ah… as far as the kindergarten class is concerned I believe Lerner-hakase was speaking of the level of activation of the core when it is replaced after service… not the interior of the reactor during operation 🙂

So the proper comparison would be the used core sitting on the teacher’s desk … not… (beryllium, y’know) and the classroom full of kids.

delt0r wrote: E. Lerner has said this even in his first Google talk. There is a cool down time of hours/days after switch off before you can get your hands inside.

~9 hours, actually.

The culprit is C11… which is B11 that did not quite make to C12. It has a half-life of 20 minutes and decays back to B11 via positron emission. ~9 hours after shutdown you can open the reactor with no radiation hazard. Or you can open the reactor earlier if you need a free PET scan 🙂

It’s not that the C11 would be super-hazardous in and of itself… the ~9 hour wait is in order to adhere to Lerner’s apparent protocol of “less than background radiation.”

delt0r wrote: Rare side reactions matter.

Hmmm… as you seem to have misunderstood what I was trying to get at (my fault, very probably 🙂 ) perhaps we’re talking past each other?

[delt0r]

I don’t think so. The side reactions that give off neutrons are indeed rare. But once you scale up to MW its still a lot. Also the true cool down time will depend on other materials used. Neutron activation over days, weeks and years will not be insignificant. It is of course something that is reasonably easy to deal with, given proper materiel choices. Remember that we are talking about moles of neutrons here, they all get absorbed somewhere.

[zapkitty]

delt0r wrote: I don’t think so. The side reactions that give off neutrons are indeed rare. But once you scale up to MW its still a lot. Also the true cool down time will depend on other materials used. Neutron activation over days, weeks and years will not be insignificant. It is of course something that is reasonably easy to deal with, given proper materiel choices. Remember that we are talking about moles of neutrons here, they all get absorbed somewhere.

Hmmm… I thought the few neutrons produced were mostly absorbed by the water and the B10, with only a fraction of those few interacting with the core on the way out.

[delt0r]

This is unfortunately not how neutrons behave. Almost everything is nothing but space and so neutrons are quite penetrating since the don’t see the electron clouds. Then there is the prompt gammas and activation. Its not really a big deal, but it is something that will part of standard operational procedures. Water is indeed a good neutron absorber if you have a lot of it (needs lots of volume or they diffuse out). Most is absorb by hydrogen to become deuterium. The Boron will be quite dilute in gas form and will be practically transparent to neutrons.

[zapkitty]

? Neutrons behave exactly that way.

Oh, I see… I listed the occurrences in reverse order. What I meant was that only a small fraction of the neutrons interact with the core on the way out, and then they run into the shield.

And you seem to be assuming high cross-sections for some rather fast neutrons.

So, as I understand it…

1. Some neutrons are created… 0.1% of the reactions end up creating neutrons that carry less than 0.2% of the energy released… “the <0.2%" 🙂

2. Of the <0.2%, almost all would pass out of the core without hitting anything. At ~2.8 MeV and ~600 KeV these are not thermal neutrons and their absorption cross-sections are not very large.

3. Then they run into the shield. A meter of water, ~20 cm of B10 and a few cm of lead.

4. What’s left is supposed to be no greater than background radiation.

[jamesr]

The neutrons are still much lower energy than the 14MeV ones emitted by the D-T reaction.

The idea is hydrogen in the water thermalises the neutrons so that by the time they diffuse through to the B10 they are sufficiently low energy to be captured. The B10+n capture reaction then releases gammas which is what the final few cm of lead is for.

There is also the slight chance that some neutrons will be captured by the hydrogen forming deuterium.

[opensource]

We never got to the bottom of this. So, JamesR, who was right – zapkitty or delt0r? Is the amount of radiation 20 minutes after shutdown the same as background?

Furthermore, I feel the need to lay rest to the OP, whose question was not adequately answered. There are breakthroughs necessary before aneutronic fusion is feasible and cost-effective – but I don’t think one of them is the confinement of the plasma. I’m only talking about DPF technology here – fusion power that is based upon confinement (like Tokamaks) may require better confinement.

[zapkitty]

opensource wrote: We never got to the bottom of this. So, JamesR, who was right – zapkitty or delt0r? Is the amount of radiation 20 minutes after shutdown the same as background?

Er… ~20 minutes is the [em]half-life[/em] of C11. It will take several hours for the totality of any C11 produced to decay back into non-radioactive B11. Then the core chamber should be safe for maintenance.

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