Proliferation

Discuss how polywell fusion works; share theoretical questions and answers.

Moderators: tonybarry, MSimon

Axil
Posts: 935
Joined: Fri Jan 02, 2009 6:34 am

Proliferation

Postby Axil » Mon Aug 23, 2010 2:08 am

Why can’t a proliferator use the polywell to build a bomb as follows:

If boron fusion is possible, then the neutron prolific 7Li(p,n)7Be reaction must also be possible.

This reaction will produce neutrons in large numbers based on the following formula:

Yn/p = 2.0 × 10−5 • E2.5
p for 2.0 < Ep < 7.0 MeV

If you can make neutrons in quantity, plutonium is a short step away.

Use lithium-7 instead of boron-11(depleted boron), then optimizes the proton energy for maximum neutron production and we have plutonium via a U238 blanket.

Why is this not possible?

See figure 3

http://www.princeton.edu/~rskemp/Kemp%2 ... rators.pdf

stevethetinker
Posts: 4
Joined: Tue Dec 02, 2008 1:16 am
Location: St. Louis

Postby stevethetinker » Mon Aug 23, 2010 3:07 am

What you say is true, of course, but I suspect that there are easier ways to get fissile material.

Making the Polywell technology widely available addresses proliferation in a different direction. A society that has sufficient energy without having to bother others is less likely to want a bomb. When electrical energy is cheap enough many things become practical, as Bussard pointed out in his Google talk, such as desalination to supply water. For a society like Iran, however, with the current leadership, whose objectives are ideological, a Polywell could well be used for bomb-making.

ladajo
Posts: 6204
Joined: Thu Sep 17, 2009 11:18 pm
Location: North East Coast

Postby ladajo » Mon Aug 23, 2010 3:11 am

And for that matter so could a bunch of fusors.

BenTC
Posts: 410
Joined: Tue Jun 09, 2009 4:54 am

Postby BenTC » Mon Aug 23, 2010 3:14 pm

A Polywell removes arguments for having operational enrichment facilities of fissile material. That is a step towards non-proliferation.
In theory there is no difference between theory and practice, but in practice there is.

D Tibbets
Posts: 2775
Joined: Thu Jun 26, 2008 6:52 am

Postby D Tibbets » Mon Aug 23, 2010 3:59 pm

Anything that makes neutrons can make plutonium. The problem with lithuim in a Polywell is that the fusion rate is generally slower than Boron11. If your goal is only to produce a lot of neutrons, a D-D Polywell reactor is much easier, and with increased fusion rates may make more total neutrons.

IF a PB11 Polywell can be designed so that conversion to other fuels is at least as difficult to do as building your own machine, there would be some impediment. Otherwise, the only sure system would be to keep tight control on the distribution and operation of the reactors. Something that has not been done with fission reactors.

The linked article describes using particle accelerators to produce plutonium, So the issue may be moot anyway.

Dan Tibbets
To error is human... and I'm very human.

chrismb
Posts: 3161
Joined: Sat Dec 13, 2008 6:00 pm

Postby chrismb » Mon Aug 23, 2010 7:38 pm

'Radiation' is the 'dirty secret' of fusion.

Fusion emits *more* radiation than fission does. It is simply that the products of fission decay much more slowly, so are a longer lived hazard. The very fact that fusion waste decays fast means that it is a much more potent radiation hazard. It's just not such a long lived hazard.

p11B is called 'aneutronic' because less than 1% of its energy comes out as neutrons. Now, that's actually a pretty funny definition because Uranium fission releases about 200MeV of which only 4.5MeV or so is in neutrons. So 'fission energy' is almost aneutronic (2.3%) by the arbitrary definition given to 'aneutronic'!!!

ANd the other thing that seems to be constantly forgotten here and in other p11B dreamlands; cutting your neutron emissions by 99% doesn't cut the required shielding by 99%. It is more like 50%.

If you can figure the analogy; just 'cos the rain is much lighter, it doesn't mean a 3" umberella will keep you dry when a 30" umberella does the job at x10 the rain fall!

93143
Posts: 1130
Joined: Fri Oct 19, 2007 7:51 pm

Postby 93143 » Mon Aug 23, 2010 8:31 pm

chrismb wrote:ANd the other thing that seems to be constantly forgotten here and in other p11B dreamlands; cutting your neutron emissions by 99% doesn't cut the required shielding by 99%. It is more like 50%.


Shielding isn't the major advantage. Bremsstrahlung requires significant shielding, and if those gamma rays are real they'll require even more.

The advantage is that all that high-level short-half-life radioactive waste is generated a lot more slowly, extending reactor lifetime - and reducing the maximum radioactivity hazard as the activation rate goes down and the equilibrium radioisotope concentration drops (though the activation rate has to come down pretty far for this last to be useful).

Also, with essentially all the reaction energy coming off in charged particles, you have a shot at direct conversion, which eliminates the steam plant and dramatically increases the practical maximum efficiency...

The major neutron-producing reactions in a p-¹¹B reactor are (according to Wikipedia) 11B + 4He -> 14N + n + 157 keV and 11B + 1H -> 11C + n - 2.8 MeV. The former is claimed to be suppressed in a Polywell due to poor ash confinement, and the latter requires a lot of energy and thus would be suppressed by the claimed non-Maxwellian ion energy distribution. Even if a high-energy tail were to form, the confinement of ions at those energies wouldn't be any better than it is for the alphas.

The result, according to Dr. Nebel, is that a Polywell running p-¹¹B instead of D-T reduces neutron radiation by about 99.999999% (a trillion neutrons per second for a 100 MWe reactor), which is nothing to sneeze at.

CaptainBeowulf
Posts: 498
Joined: Sat Nov 07, 2009 12:35 am

Postby CaptainBeowulf » Mon Aug 23, 2010 10:03 pm

A Polywell removes arguments for having operational enrichment facilities of fissile material. That is a step towards non-proliferation.


That's how I see it.

Frankly, any state in the modern world that can find a few hundred million dollars can do enrichment - even if they can't make an actual atom or nuclear bomb, they can still do a dirty bomb - huge conventional warhead with radioactive material packed around it. Detonated in the middle of a densely populated city the radioactive cloud can still kill tens of thousands of people.

Better to remove the excuse that you have to enrich fissile material because you need nuclear power.

ladajo
Posts: 6204
Joined: Thu Sep 17, 2009 11:18 pm
Location: North East Coast

Postby ladajo » Tue Aug 24, 2010 2:51 am

And a much reduced neutron population also significantly enhances reactor material lifetimes. Neutron embrittlement is a prime player when talking plant design life.

KitemanSA
Posts: 6114
Joined: Sun Sep 28, 2008 3:05 pm
Location: OlyPen WA

Postby KitemanSA » Tue Aug 24, 2010 3:57 pm

ladajo wrote:And a much reduced neutron population also significantly enhances reactor material lifetimes. Neutron embrittlement is a prime player when talking plant design life.
When you are talking high pressure containment vessels as in pressurize water fission reactors, yes. Vacuum chambers? Not so much. Many vacuum chambers are made from glass which is brittle to begin with. But they are usually under compression, not tension, so no prob, Bob!
Seems to me that the main issue with neutron bombardment would be the gas permiability of the vacuum chamber wall.

D Tibbets
Posts: 2775
Joined: Thu Jun 26, 2008 6:52 am

Postby D Tibbets » Tue Aug 24, 2010 4:20 pm

Also, neutron bombardment of the magnets- especially superconducting magnets cannot be good. The Riggatron, which was a small Tokamak proposal by Bussard in the late 70's, with smaller copper magnets inside instead of large external superconducting magnets outside was expected to last only a few months before overhaul and refurbishment.

Dan Tibbets
To error is human... and I'm very human.

93143
Posts: 1130
Joined: Fri Oct 19, 2007 7:51 pm

Postby 93143 » Tue Aug 24, 2010 5:59 pm

Yeah, there are a lot of bad things neutrons do to the reactor.

Anyway, this isn't like shielding thickness, which has a logarithmic response to the reduction factor. Cut the neutron rate in half and you double the lifetime of any part that has a lifetime governed by the neutron rate.

Decrease the neutron rate by a factor of 1000, as in a thermal p-¹¹B reactor, and a core that would have lasted six months is suddenly good for five centuries, at least as far as neutron damage is concerned (and assuming all neutrons are equal, which they aren't - p-¹¹B neutrons are lower-energy than D-T or D-D neutrons).

Decrease the neutron rate by a factor of 1e8, as predicted for a p-¹¹B Polywell, and a comparable core is now good (in a neutron-damage sense) for fifty million years...

KitemanSA wrote:
ladajo wrote:And a much reduced neutron population also significantly enhances reactor material lifetimes. Neutron embrittlement is a prime player when talking plant design life.
When you are talking high pressure containment vessels as in pressurize water fission reactors, yes. Vacuum chambers? Not so much. Many vacuum chambers are made from glass which is brittle to begin with. But they are usually under compression, not tension

Aren't you forgetting the magnets? They and their supports are under substantial loading, and considering the necessity of vacuum sealing and the fact that the reactor will not be operating in uninterrupted steady-state for its entire lifetime (startup/shutdown would likely produce substantial dynamic loading in the entire reactor, and a slight timing imperfection could induce substantial lateral loads on the coils), preventing those loads from being partially transferred to the chamber wall as shear will probably be impossible.

Never mind what happens if you're using this on a ship and it gets hit with a weapon... or in a stationary power plant and it gets hit with an earthquake... or a spacecraft and it vibrates during launch (as is pretty much inevitable) or lands a little harder than expected...

What if someone gets frustrated and kicks it?

You can't assume the forces are uniformly static and in compression. Embrittlement is no good, even if it doesn't decrease the strength of the material.

KitemanSA
Posts: 6114
Joined: Sun Sep 28, 2008 3:05 pm
Location: OlyPen WA

Postby KitemanSA » Wed Aug 25, 2010 12:23 am

93143 wrote: Aren't you forgetting the magnets? ...

You can't assume the forces are uniformly static and in compression. Embrittlement is no good, even if it doesn't decrease the strength of the material.
I haven't seen a good analysis of the magnet structure since none have been designed. Will the magnet structure be anywhere near the strength limit of the material? I don't know. If so, you have a point. But most folks thoughts wrt "neutron embrittlement" are colored by the fission world, and things are not quite the same.

Whole new issues. Deep thought needed.

MSimon suggested the magnet could be protected by a bit of Boron (10?) to absorb the neutrons. Not my field, but an interesting thought none-the-less.

ladajo
Posts: 6204
Joined: Thu Sep 17, 2009 11:18 pm
Location: North East Coast

Postby ladajo » Wed Aug 25, 2010 5:00 am

Boron has been used to enhance neutron shielding for years.

In the bigger picture, neutron flux will have to be a key design consideration for DD or DT machines. Where you have metal, and other substances you have issues. Even cooling mediums can be an issue over time. Fission plant operations provide a lot of insight to the potential issues especially given that they represent the largest base of high power high density continuous nuetron flux that we have to go on.

Even Famulus is going to have problems with his fusor over time becoming activated as well as damage to wiring insulation and other issues.

Fortunately for Polywell, there is a large historical database to draw on for engineering issues in this regard. The US Gov and Navy have spent a silly anount of money on materials science over the last 60 years.

93143
Posts: 1130
Joined: Fri Oct 19, 2007 7:51 pm

Postby 93143 » Wed Aug 25, 2010 5:59 pm

Oh yeah, cooling. Pumped turbulent flow of whatever. That introduces time-varying stress too, as well as substantial average internal pressures (-> tension) in the cooling channels... and there's a lot more cooling involved in a neutronic fusion core than in an aneutronic one.

And those magnetic forces are quite large - wasn't chrismb saying in another thread that JET (the whole thing) jumps about a centimetre in the air at the end of a run?


Return to “Theory”

Who is online

Users browsing this forum: No registered users and 2 guests