Aneutronic fusion

[jgarry]

Forgive my ignorance.
A friend told me that the problem with fusion is that the neutrons chew up the reactor vessel.
In my casual perusal of polywell fusion I notice that the technology envisions using a different reation, which results in much less nuetron radiation.
Is Nuetron radiation why Dr. Bussard concluded that tokamak fusion wouldn't work?
Are there other ideas about how to handle nuetron radiation?
I remember from my days sailing on a nuclear submarine that they had a large fuel tank midships to shield the forward areas. The hydrocarbon molecules were good at stopping nuetrons.

[chrismb]

Forgive my ignorance.
A friend told me that the problem with fusion is that the neutrons chew up the reactor vessel.
In my casual perusal of polywell fusion I notice that the technology envisions using a different reation, which results in much less nuetron radiation. (1)
Is Nuetron radiation why Dr. Bussard concluded that tokamak fusion wouldn't work? (2)
Are there other ideas about how to handle nuetron radiation? (3)
I remember from my days sailing on a nuclear submarine that they had a large fuel tank midships to shield the forward areas. The hydrocarbon molecules were good at stopping nuetrons. (4)

1) neutrons sure do chew up matter, if the flux is high enough. Though alphas and protons and any fast particles do aswell, though neutrons do it much better!! (or worse, whichever perspective!)[Don't forget that aneutronic products in a thermal plasma are 'contained' by that plasma so are, figuratively speaking, harmless, whereas for IEC type devices like the polywell, the fast ions are ALSO A RADIOACTIVE/ACTIVATING FLUX, JUST LIKE NEUTRONS, which some folks appear to conveniently forget.)
2) tokamaks have fundamental magnetic confinement instabilities and a history of 50 years of not getting over it. this was his issue. (perhaps, also, that if he went around saying tokamaks would work then the funding wouldn't get to polywell??)
3) keep away from it; 1/r^2
4) hydrocarbon fuel is a very good shield for neutrons. The hydrogen atoms have ~ the same mass so classic kinetics tells you that the energy transfer of neutrons into the hydrogen is a maximum. So, anything with lots of hydrogen in is a good moderator, though what is left is thermal neutrons rattling around and they tend to then get absorbed by atoms that like neutrons, and they may become activated.

[MSimon]

Chris. Fast ions in a BFR are not radioactive. Nor are they radiation inducing.

[chrismb]

Chris. Fast ions in a BFR are not radioactive. Nor are they radiation inducing.

So, what did Ernest Rutherford win his Nobel prize for? Was it for converting nitrogen to oxygen with *neutrons*????....

And what did Ernest Lawrence use his Nobel prize winning cyclotron thingy for and why was there so much interest in it - for accelerating *neutrons* to make radioactive isotopes???....

[MSimon]

Chris. Fast ions in a BFR are not radioactive. Nor are they radiation inducing.

So, what did Ernest Rutherford win his Nobel prize for? Was it for converting nitrogen to oxygen with *neutrons*????....

And what did Ernest Lawrence use his Nobel prize winning cyclotron thingy for and why was there so much interest in it - for accelerating *neutrons* to make radioactive isotopes???....

Good point. Now the question is: with coulomb repulsion/bremms type losses is it significant? Or is it a rare event?

So let me amend that: Fast ions in a BFR are not radioactive. Nor are they very radiation inducing.

[ravingdave]

Chris. Fast ions in a BFR are not radioactive. Nor are they radiation inducing.

So, what did Ernest Rutherford win his Nobel prize for? Was it for converting nitrogen to oxygen with *neutrons*????....

And what did Ernest Lawrence use his Nobel prize winning cyclotron thingy for and why was there so much interest in it - for accelerating *neutrons* to make radioactive isotopes???....

Good point. Now the question is: with coulomb repulsion/bremms type losses is it significant? Or is it a rare event?

So let me amend that: Fast ions in a BFR are not radioactive. Nor are they very radiation inducing.

I'm confused. Neutrons are not Ions.

My recollection of how a cyclotron accelerates neutrons is that it does so with the use of a "Handle". A Proton/Neutron combo also known as "Hydrogen." The electric (and magnetic) fields of a cyclotron act on the positively charged proton of the Hydrogen ion accelerating it up to a high velocity and then smashing it into a "solid" window which is the exit from the device. The proton is stopped by the barrier, and the uncharged neutron continues onward forming a beam of neutrons.

Cyclotrons originally produced beams of electrons, then later ions, and then eventually neutrons. At least that's what I remember.


David

[chrismb]

Good point. Now the question is: with coulomb repulsion/bremms type losses is it significant? Or is it a rare event?

You've sent me scrambling for an old reference book for when cyclotrons were still the norm for isotope production, rather than your nearest friendly nuclear reactor. I've found a 1957 American Institute of Physics Handbook.

A quick scan shows, for example, an 11.5MHz 35.5 inch unit at the French Nuclear Physics Laboratory, Orsay, that can muster 10uA of 12MeV alphas/6.7MeV deuterons. Its listed purpose is 'Isotope production'. (I picked it as it has the lowest alpha output energy in the table, for closer comparison with IEC devices.)

So now we compare and contrast a 500MW Polywell with Orsay's cyclotron; a 200A output of alphas versus an isotope-making 10uA of alphas. Yet your claim is that this 200A will not be radiation-inducing?

[MSimon]

Good point. Now the question is: with coulomb repulsion/bremms type losses is it significant? Or is it a rare event?

You've sent me scrambling for an old reference book for when cyclotrons were still the norm for isotope production, rather than your nearest friendly nuclear reactor. I've found a 1957 American Institute of Physics Handbook.

A quick scan shows, for example, an 11.5MHz 35.5 inch unit at the French Nuclear Physics Laboratory, Orsay, that can muster 10uA of 12MeV alphas/6.7MeV deuterons. Its listed purpose is 'Isotope production'. (I picked it as it has the lowest alpha output energy in the table, for closer comparison with IEC devices.)

So now we compare and contrast a 500MW Polywell with Orsay's cyclotron; a 200A output of alphas versus an isotope-making 10uA of alphas. Yet your claim is that this 200A will not be radiation-inducing?

Compared to neutrons.

[chrismb]

I was just serving to demonstrate clearly that ions *are* radioactive at high energy, re: accelerating neutrons can't happen in a cyclotron, nor did Rutherford have a neutron source for his experiments.

Cyclotrons originally produced beams of electrons, then later ions, and then eventually neutrons. At least that's what I remember.

The first cyclotrons accelerated protons (and have done ever since, of course, right up to LHD). There is an energy advantage to accelerating lower mass particles, and the natural evolution is towards electrons (betatrons). Neutrons are made by nuclear collision with a window, as you say, as they cannot be accelerated electrostatically. That is to say, ions are launched at a lump of something and it kicks out neutrons. Just as will happen in a Polywell, if it ever generates any fusion products.....

[TallDave]

That is to say, ions are launched at a lump of something and it kicks out neutrons. Just as will happen in a Polywell, if it ever generates any fusion products.....

Zuh? Isn't it supposed to kick out three alphas that swirl around then exit through the cusps, then are slowed by a collection grid to the point they have given up all their energy?

Are you saying you expect the alphas to hit things (the Magrid I assume) at high energy and make lots of neutrons?

If the average path is 1000 transits then exit, how much such events would we expect, and how does this affect the neutronicity? How likely are Magrid materials to give up a neutron when struck?

I forget what Dr. Nebel's estimate was for a reactor, but I remember it was tiny and implied less than 1% neutrons iirc.

[MSimon]

I was just serving to demonstrate clearly that ions *are* radioactive at high energy,

There you go again Chris. Ions are radioactivity inducing. Unless the high energy is internal to the nucleus.

[chrismb]

Are you saying you expect the alphas to hit things (the Magrid I assume) at high energy and make lots of neutrons?

Exactly.

Isn't it supposed to kick out three alphas that swirl around then exit through the cusps, then are slowed by a collection grid to the point they have given up all their energy?

That's the idea, but the alphas apparently come out of p11B in a spectrum (bi-modal, actually) from 3MeV to 10MeV, not at discrete levels which you'd need for an electrostatic deceleration scheme. You might be able to slow the slowest ones down with an acceleration grid, but you can't slow the faster ones down. You could, perhaps, scrub off a good fraction of the energy and once alphas drop below the few MeV range, then, as mentioned, the alphas do loose a very large fraction of their radiation-inducing potential, so there is some truth in that. Not sure what would shake out of some 'real' calculations on that, though.

If the average path is 1000 transits then exit, how much such events would we expect, and how does this affect the neutronicity?

500MW is 1E21 high energy alphas per second. You only need ppm rates of neutron production per alpha collision and we're instantly talking about deathly levels of neutrons. As MSimon puts it, not as deadly *compared with* a pure 1E21/s neutron stream, but still deadly - just not *so* deadly!

I forget what Dr. Nebel's estimate was for a reactor, but I remember it was tiny and implied less than 1% neutrons iirc.

Cool. Only 1E19 neutrons/s? So, perhaps not enough radiation to kill you dead in anything less than 1ms at 10m away, whereas a pure neutron flux at that rate/energy would kill you dead in 10us.

(In a fission reactor, the energy is *only* 5% neutrons. Still, I'd not want to stand next to a 100MW fission pile just as I'd not want to stand anywhere near a 500MW polywell. Same neutron energy output?....)

It would be more convincing to me if polywell simply got on with some fusion and put in place the presumption that full neutron screening will be needed. This is the subject as it was kicked off - hydrogen-rich moderators and distance. That's how to 'fix' neutron radiation.

As has been suggested to me before in other contexts, to be in a position where there are *too many* neutrons likely in the next design evoultion is a situation that polywell should *want* to be. Claims on clean p11B appears to be a distraction and looks rather like spin for the general public.

The solution (my "being positive" bit) is just to make sure polywell power plants have the *full-business* when it comes to neutron screening. Alpha Centauri will just have to wait - we'll just have to send robots for the first polywell-powered space mission until we evolve into radiation-resistant creatures. Otherwise, this is all rather much of a nothingness, just get the shielding in place, no big deal!

[chrismb]

I was just serving to demonstrate clearly that ions *are* radioactive at high energy,

There you go again Chris. Ions are radioactivity inducing. Unless the high energy is internal to the nucleus.

I'm not really up with your definitions - why are alpha-emitters like americium-241 or polonium-210 considered 'radioactive' when they are putting out much fewer, and lower energy, alphas than a polywell reaction core would?

A polywell, if one ever functions as suggested, will put out ionising emissions of a measurable dose and of nuclear-mutating nature. That's 'radioactive' in any definition I understand!?

[MSimon]

I was just serving to demonstrate clearly that ions *are* radioactive at high energy,

There you go again Chris. Ions are radioactivity inducing. Unless the high energy is internal to the nucleus.

I'm not really up with your definitions - why are alpha-emitters like americium-241 or polonium-210 considered 'radioactive' when they are putting out much fewer, and lower energy, alphas than a polywell reaction core would?

A polywell, if one ever functions as suggested, will put out ionising emissions of a measurable dose and of nuclear-mutating nature. That's 'radioactive' in any definition I understand!?

You are confusing alpha emitters with alphas. Not exactly the same thing. You are aware that a He4 is not an Am210, no?

And of course a BFR will put out ionizing radiation (I have done some analysis of that here and there). So do smoke detectors. Is a nuclear fission plant the same as a smoke detector? Can you by a a curie of Am210 at Home Depot? Why do they sell smoke detectors there? Could it have something to do with the quantity of radiation.

Some days it seems to me like 10 different people are using your handle. One of them is quite brilliant. The other 9 seem barely knowledgeable and sloppy to boot.

[TallDave]

That's the idea, but the alphas apparently come out of p11B in a spectrum (bi-modal, actually) from 3MeV to 10MeV, not at discrete levels which you'd need for an electrostatic deceleration scheme.

I forget where, but someone had a scheme for handling a diverse set of alpha energies.

but still deadly - just not *so* deadly...So, perhaps not enough radiation to kill you dead in anything less than 1ms at 10m away, whereas a pure neutron flux at that rate/energy would kill you dead in 10us.

Shrug. It's not a tanning bed. It's just a question of how much cheaper it will be in terms of shielding and energy conversion. Anyways, here's the quote:

We've done the calculations. Neutron yield from a P-B11 Polywell machine (nonthermal) is about 1.0e12/sec. for a 100Mwe reactor. That's about 8 orders of magnitude less than a comparable D-T machine.