p + 15N

[chrismb]

Take total atmospheric mass as 5e18 kg, and for the 0.37% that is 15N, this is 2e16 kg of 15N, or 8e41 atoms of 15N == 6e29J.

Total annual global power consumption = ~5e20J.

So there is 1 billion years worth of global energy consumption equivalent of p+15N fusion energy from the 15N in the atmosphere, so if 10% conversion efficiency could be achieved, then that'd be 100 million years worth. (Obviously it'd be less with lower efficiency - which I think would be probable, if at all possible).

Would cosmic radiation of 100 million years be enough to re-populate the atmosphere to the current concentration of 15N?

[D Tibbets]

He states that the 12C- 4He side reaction is the most significant neutron producing reaction and it is supressed in large part because the alphas do not hang around in the Polywell.

I think the correct term is 'he speculates that...'.

Let's be very clear here that when we blow hot-air about p-11B this-or-that, in the first instance we have to at least presume the worst case for radiation. Worst case, or otherwise, the only one we've got right now is for a thermal plasma. I recommend you presume to need was someone has already recommended in a published work, and if you find polywell runs cleaner, great... if you find polywell doesn't run, well.... y'know, let's just remember that we're not talking about something 'known' here. You may well find that polywell p-11b runs more aneutronic than expected. No-one knows, so let's stick to calculations from known circumstances in the first instance, eh?

Blowing hot air indeed. Without carefull and full disclosure, extreamly missleading information can be presented as appropiate.

A more appropiate statement would be : in a hot thermalized plasma under this average temperature, this neutron output would occur. This might apply to the Polywell except, a thermalized plasma is impossible to produce net positive fusion anyway. Whether the calculation assumes the alphas quickly escapes or are retained for significant times may be a critical additional element. Taken by itself, the quote might represent the worse case scenario, but without stipulating this, it is indeed misleading.
Certainly, if such was the basis for your decisions, many, if not most advances would be still born.

Dan Tibbets

[D Tibbets]

As for the cost of purifying 15 N, I suspect the cost is based on small production of a specialty isotope. Mass production might considerably decrease cost. Also, in a worse case scenario, the fuel cost and the reactor cost would be compared to other alternatives. If a Tokamak was that alternative, the comparative cost of the electricity would be considerably greater than a coal comparison. And, of course, in applications where the cost is less important and especially if there are beneficial tradoffs (like in a Space Ship) the cost/ benefit analysis is completely different.

Dan Tibbets

[chrismb]

A more appropiate statement would be : in a hot thermalized plasma under this average temperature, this neutron output would occur. This might apply to the Polywell except, a thermalized plasma is impossible to produce net positive fusion anyway.

Can you please explain the meaning of the words 'except' and 'anyway'?

It very much sounds like an attempt to slip a non-sequitur in. You seem to be saying 'Polywell might suffer from this, but it won't because thermal plasmas already do.' Is this what you were trying to say?

[chrismb]

Taken by itself, the quote might represent the worse case scenario, but without stipulating this, it is indeed misleading.

Not at all ''it is indeed misleading". Rather, "as the performance of Polywell is yet undetermined, it may be found later that this quote is misleading." No 'indeed' about it!

To mislead is to provide information known to be incorrect. If one does not know what the information is, because it doesn't yet exist, one cannot be misleading, or even incorrect, about it.

[KitemanSA]

Try "known to be slightly misleading. How slightly remains to be determined by the final design". Does that satisfy everyone? After all, the "nothing in the way" nature of Chris' original statement IS slightly misleading, but probably not enough to change his point, which was, I believe, that this "aneutronic" reaction will kill folks quick if there is no sheilding from neutrons!

True?

[D Tibbets]

A more appropiate statement would be : in a hot thermalized plasma under this average temperature, this neutron output would occur. This might apply to the Polywell except, a thermalized plasma is impossible to produce net positive fusion anyway.

Can you please explain the meaning of the words 'except' and 'anyway'?

It very much sounds like an attempt to slip a non-sequitur in. You seem to be saying 'Polywell might suffer from this, but it won't because thermal plasmas already do.' Is this what you were trying to say?

What I am trying to say is that the assumptions are wrong. The Polywell is claimed to be a nonthermalized machine. And it is admitted, that because of Bremsstrulung radiation a thermalized machine burning P-B11 cannot reach breakeven. So, if a Polywell actually works, the thermalized assumptions about the temperature dependant side reactions is innapropiate. If, the Polywell cannot avoid thermalization, this entire argument is moot.

The prompt escape of fusion alphas is another, perhaps profound, modifier; but it is a separate argument from the above.

Dan Tibbets

[chrismb]

Try "known to be slightly misleading.

No. That's nonsense. We might found out later it is misleading, or we might find it is actually a perfectly good analysis. If it is possibly the latter then you cannot currently claim it is "known to be slightly misleading".

If it is known to be misleading, then it is a knowledge that you must have shared with EMC2, who have succeeded in running a 30kW reactor.

If EMC2 has not run a 30kW reactor, then how can you possibly know the above treatment [which begins with the assumption of a 30kW reactor] is 'slightly misleading'?!?!

(I'm done on this pedantic silliness.)

[D Tibbets]

Try "known to be slightly misleading.

No. That's nonsense. We might found out later it is misleading, or we might find it is actually a perfectly good analysis. If it is possibly the latter then you cannot currently claim it is "known to be slightly misleading".

If it is known to be misleading, then it is a knowledge that you must have shared with EMC2, who have succeeded in running a 30kW reactor.

If EMC2 has not run a 30kW reactor, then how can you possibly know the above treatment [which begins with the assumption of a 30kW reactor] is 'slightly misleading'?!?!

(I'm done on this pedantic silliness.)

Your decision to abandon this pedantic silliness is reasonable, but I cannot resist a parting shot. It is reasonable to claim that there is not a 30kW Polywell to prove it's characteristics. But, the same argument can be made in the reverse direction. Where is the 30 kW P-B11 reactor that is producing this percentage of neutrons claimed in your reference?

Without this hard evidence, you have to depend on predictions. And, these predictions are based on math, physics, and importantly for this discussion, which physics (assumption) you choose to use.

My interpretation of the reference is that their assumptions are a thermalized plasma, and an ignition machine like a Tokamak, in which the fusion products are confined long enough to significantly heat the plasma, and thus confined long enough to react with carbon, and that said excited carbon hangs around long enough (before decaying into an alpha and excited beryllium), and finally, my assumption, that this alpha- carbon reaction is the dominate source for said neutrons.

Dan Tibbets

[D Tibbets]

OK, so I lied, Thias is my last post on the subject. I think I found the reference chrisMB used
and it basically supports the arguments about removing alphas and the high energy thermalized tail to decrease the baseline thermalized neutron production rates Of course this is mostly insignificant from a biological viewpoint, because the shielding/ distance needed to protect from the high energy x-rays and gamma rays will also presumably protect from the lower neutron flux.

http://en.wikipedia.org/wiki/Aneutronic_fusion

"Residual radiation from a p–11B reactor

Detailed calculations show that at least 0.1% of the reactions in a thermal p–11B plasma would produce neutrons, and the energy of these neutrons would account for less than 0.2% of the total energy released.[28]

These neutrons come primarily from the reaction[29]

11B + α → 14N + n + 157 keV

The reaction itself produces only 157 keV, but the neutron will carry a large fraction of the alpha energy, which will be close to Efusion/3 = 2.9 MeV. Another significant source of neutrons is the reaction

11B + p → 11C + n − 2.8 MeV

These neutrons will be less energetic, with an energy comparable to the fuel temperature. In addition, 11C itself is radioactive, but will decay to negligible levels within several hours as its half life is only 20 minutes.

Since these reactions involve the reactants and products of the primary fusion reaction, it would be difficult to further lower the neutron production by a significant fraction. A clever magnetic confinement scheme could in principle suppress the first reaction by extracting the alphas as soon as they are created, but then their energy would not be available to keep the plasma hot. The second reaction could in principle be suppressed relative to the desired fusion by removing the high energy tail of the ion distribution, but this would probably be prohibited by the power required to prevent the distribution from thermalizing.

In addition to neutrons, large quantities of hard X-rays will be produced by bremsstrahlung, and 4, 12, and 16 MeV gamma rays will be produced by the fusion reaction

11B + p → 12C + γ + 16.0 MeV

with a branching probability relative to the primary fusion reaction of about 10−4.[30]

Finally, isotopically pure fuel will have to be used and the influx of impurities into the plasma will have to be controlled to prevent neutron-producing side reactions like these:

11B + d → 12C + n + 13.7 MeV
d + d → 3He + n + 3.27 MeV

Fortunately, with careful design, it should be possible to reduce the occupational dose of both neutron and gamma radiation to operators to a negligible level. The primary components of the shielding would be water to moderate the fast neutrons, boron to absorb the moderated neutrons, and metal to absorb X-rays. The total thickness needed should be about a meter, most of that being water.[31]"

Dan Tibbets

[KitemanSA]

Assume we have a very small reactor producing 30 kW of total fusion power (a full-scale power reactor might produce 100,000 times more than this) and 30 W in the form of neutrons. If there is no significant shielding, a worker in the next room, 10 m away, might intercept (0.5 m²)/(4 pi (10 m)2) = 4×10−4 of this power, i.e., 0.012 W. With 70 kg body mass and the definition 1 gray = 1 J/kg, we find a dose rate of 0.00017 Gy/s. Using a quality factor of 20 for fast neutrons, this is equivalent to 3.4 millisieverts. The maximum yearly occupational dose of 50 mSv will be reached in 15 s, the fatal (LD50) dose of 5 Sv will be reached in half an hour. If very effective precautions are not taken, the neutrons would also activate the structure so that remote maintenance and radioactive waste disposal would be necessary.}

You "assumed a small reactor", EMC2 is immaterial. You "assumed" very little shielding. It appears that you calculated using 100% of neutron reaching the 10m distance to a .5 square meter man. THIS is the "known to be misleading" part in that it is unreasonable to assume ZERO knock-down due to containment, conversion, room walls, etc. It is due to your assumption ofno significant shielding that it will be a small knockdown, thus the "slightly". No "knowledge of actual reactor" needed, merely working with YOUR assumptions a some semblence of reality.

[chrismb]

a) [again] it is not my analysis. I was requoting wiki.
b) if the wall is only plaster board and studding, I'd stick by that analysis even if it was mine. I don't think the walls should enter the calc as stopping any neutrons, though the wall may moderate them.

[KitemanSA]

[/quote]

a) [again] it is not my analysis. I was requoting wiki.

Wow! Fine, then in each place I wrote "your calculation" edit to read "the calculation you provided and have not repudiated so it is effectively your calculation"! Picky picky!

b) if the wall is only plaster board and studding, I'd stick by that analysis even if it was mine. I don't think the walls should enter the calc as stopping any neutrons, though the wall may moderate them.

Stick by it all you want, but I was responding to your rather insulting:

Try "known to be slightly misleading.

No. That's nonsense. We might found out later it is misleading, or we might find it is actually a perfectly good analysis. If it is possibly the latter then you cannot currently claim it is "known to be slightly misleading".

The statement "known tobe slightly misleading" is perfectly correct. The question is "how slightly is "sliughtly"?

[Aero]

a) [again] it is not my analysis. I was requoting wiki.

Wow! Fine, then in each place I wrote "your calculation" edit to read "the calculation you provided and have not repudiated so it is effectively your calculation"! Picky picky!

b) if the wall is only plaster board and studding, I'd stick by that analysis even if it was mine. I don't think the walls should enter the calc as stopping any neutrons, though the wall may moderate them.

Stick by it all you want, but I was responding to your rather insulting:

Try "known to be slightly misleading.

No. That's nonsense. We might found out later it is misleading, or we might find it is actually a perfectly good analysis. If it is possibly the latter then you cannot currently claim it is "known to be slightly misleading".

The statement "known tobe slightly misleading" is perfectly correct. The question is "how slightly is "sliughtly"?

Kiteman, why don't you shoot the messenger? Oh, you already did!

Known to be slightly misleading

Known by whom? Give us a peer reviewed reference or claim ownership.

[D Tibbets]

I don't understand the argument about what shielding might be provided by the vessel, and other structural materials. As pointed out in the last paragraph, the needed shielding is spelled out. What is in contention, at least for me, is the neutron output levels. As explained in a middle paragraph of the reference, the mechanisms that would reduce this neutron output is precisely explained, and match the mechanisms that I would expect in a successful Polywell

This would reduce the neutron absorption shielding need considerably. I'm not sure it would reduce the modifier thickness, unless the output was low enough that the neutron exposure was considered safe to begin with. If you increase the power of the reactor to several hundred MW, the referenced neutron numbers may apply.

The gamma ray output (10^-4 fraction) would be ~ 1/10th of the example neutron output, but these are very energetic gamma rays, and may need a similar weight of shielding to be safe as there is no mentioned mechanism to suppress them.

The view I take from the Wiki article is that the popular claims of radiation free P-11B fusion is indeed misleading, but most here already know that. Some shielding will be needed for the rare neutron, or the not quite so rare gamma ray (not to mention the energetic x-rays).
Also, need to keep in mind the energy of the neutrons. The neutron energies is less than D-D and especially D-T produced neutrons. The lower energy and vastly lower flux will make a big difference in the neutron induced damage, and secondary radiation concerns, but this cannot be ignored, as the popular claims of "radiation free" would imply.

I do wish crismb would have linked the article so that the modifiers would have been immediately obvious.

Dan Tibbets