Talk-Polywell.org

So a quick question for an oncoming sci-fi novel. What's the smallest you could make a power producing polywell, assuming that A. it works like we think it does, B. you're using P+b11.

I'm thinking of two separte size questions-- 1. how much stuff would you have to have stored to build one (think of how many container units you'd need to store the fixings) and 2. how big would it have to be to function?

Could you ever get one small enough to serve as a truck or train power sources?

I would say that it is driven by magnet technology. Ridiculous magnet power density could in turn allow for smaller units to a degree. You could also use some spin from the recent Tri-Alpha related paper on the reaction chain and simplify power capture.

and shielding.

Do I remember correctly from a while back someone stating that a 100MW p+B11 Polywell would need about a foot of concrete (or equivalent) for shielding? Or was that for the D+D version?

According to someone who knew the Polywell very well*, there is a possibility that an advanced version could be small enough to power a semi truck. A more conventional design would be the ~ 3 meter diameter magrid with a perhaps 5-8 meter vacuum vessel. AQssociated vacuum pumping and direct conversion and cooling equipment may add a similar volume.

Shielding requirements could be largely incorporated into the cooling system. And, how much radiation there would be is uncertain. The gamma ray producing side reaction may occur ~ 1/10,000th of the time, but considering the sparsity of actual P-B11 fusion reaction testing, this number may be far off.
The neutron producing side reactions (mostly D-D from non isotropically pure hydrogen?) may be only about 1 occurrence per 10s of millions of reactions. This could be several watts of neutron produced heat, and this is not trivial, but again the water cooling system could be designed to cover this. The gamma radiation at ~ 12 MeV would need heavy metal shielding and/ or significant standoff distance.

With direct conversion a P-B11 reactor may produce ~ 300 MW and ~ 30-60 MW of this would be as heat, plus the heat from Bremsstruhlung, etc. Cooling will certainly be nessisary.

An advanced P-B reactor with optimistic direct conversion at ~ 95% or more, high ion convergence (central focus), high magnetic fields and possibly POPS like effects may be compact enough to fit in a semi truck engine compartment, or at least on the trailer.


* from Robert Bussard interview on the radio program....

http://www.inference.phy.cam.ac.uk/sust ... ussard.pdf

RB: Years ago, we looked at what it would take to scale
down a pB11 clean machine. We couldn’t figure out even
with a 3rd generation system, which involved using certain
kinds of interesting physics with compression issues
in the center from instability drives; how to make anything
much smaller than the size of a 18-wheeler truck.
You could certainly build it to drive locomotives, I don’t
know about airplanes. It is basically an electrical energy
producer and I suppose you could drive airplanes.
You could certainly build rocket engines that would
make ground to orbit transit a hundred times cheaper
than anything else, but I’m not sure that constitutes an
airplane. It is a horizontal takeoK to LEO (low earth
orbit) vehicle.
Tim: For the automotive applications, you could use the fusio#
energy as the means for generating hydrogen to power them, no"
as a direct fuel source.
RB: We proposed putting a pB11 system in a 18-wheeler
to make an 1 to 2 MW electric plant. It’s more power
than they have had and its an easy way to go.

Dan Tibbets

There's an old thread around here on the topic of minimum size, in the Implications forum if I recall correctly.

As far as low power generation, I expect the Polywell could be pulsed as several Hertz if your requirements were less than the steady state capacity of the reactor.

krenshala wrote:Do I remember correctly from a while back someone stating that a 100MW p+B11 Polywell would need about a foot of concrete (or equivalent) for shielding? Or was that for the D+D version?

that question is really a lot harder than one would think. ALARP, is an acronym for an important principle in exposure to radiation and stands for "As Low As Reasonably Practicable"
Also there are different types of radiation and shielding used to achieve different effects. In some cases lower density material are more practical in slowing certain types of radiation I.G. Bremsstrahlung prevention.
So depending on what is between you and where you are from the source affects how much you need between you. I am not an expert on this I just stayed in a holiday inn express last night

krenshala wrote:Do I remember correctly from a while back someone stating that a 100MW p+B11 Polywell would need about a foot of concrete (or equivalent) for shielding?

Not concrete. Lead. At least 10", possibly as much as 14" IIRC.

That's mostly to deal with the high-energy gammas, assuming human workers spend a substantial amount of time nearby on the other side of the shielding. If the gammas don't show up, or are much less frequent than 1e-4, or if other stuff is likely to separate the reactor from the workers (and any equipment that might be rad-sensitive), the primary shielding requirement will be less. A foot of concrete would probably knock out the neutrons and most of the bremsstrahlung, but you'd have to ask a nuclear engineer to be sure.

Also, I calculated that result for a 6 GW reactor, but with the number of orders of magnitude you have to attenuate the gammas by, 100 MW would require almost the same thickness.

It would be nice if the gamma wave length didn't preclude metamaterial use in shielding, sort of like the invisibility cloak, but for gamma waves. Might drop the weight requirements. A foot of lead, gracious!

Best regards
Mike

Including all the stuff to support it, make it 40 feet by 40 feet to be believable, but small.

Tell us when the book comes out!

Water or Borated Poly works really well for neutron. There are many options for gamma, but a composite appraoch would probably best best in cosideration of space/weight desires.

Light material for neutron shielding. The neutron with a mass of one hits a nucleus, if the nucleus is hydrogen, the kinetic energy is shared between the two particles equally. If the target nucleus has a mass of 100, then only 1% of the neutrons KE is transferred (or is that 0.01%?). That is why light materials like hydrogen,or compounds containing hydrogen- like water or hydrocarbons, make good moderators(slows the neutrons down). Once slowed the neutrons can be absorbed by almost anything. Boron 10 is a particularly good absorber. Attention must be paid to the effects of the neutron, to avoid problematic secondary radiation.

Bremsstruhlung radiation at X-ray energies associated with ~ 200 KeV in a P-B11 reactor is quite penetrating (still a lot less than a 12 MeV gamma ray). A diagnostic chest x-ray may be at ~ 50-60 KeV and this penetrates water quite well. The Bremsstruhlung would be 4 times higher energy, so heavy metals like lead are needed.

The shielding thickness for a 60 MW P-B11 reactor verses a 6 GW reactor would be less. My understanding of radiation shielding is that a certain thickness of a material would be required to stop 90% (or some other selected portion) of the radiation. An additional same thickness of material would stop 90% of the remaining radiation, etc. If a foot of lead is needed to stop the gamma radiation of a 6 GW reactor, then 60MW could be two halving' s less (?). So 3 inches would be needed for a 60 MW reactor to achieve the same final radiation exposure. Also, as has been discussed in other threads, in a space ship, only shielding to protect vulnerable items is required. That plus distance could greatly reduce the total mass of shielding required. Of course the smaller power results in corresponding less available thrust. This may be OK in space but for a launch vehicle it would be inadequate.


PS: To slow (thermalize to ~ room temperature KE) D-D reaction neutrons would require ~ 4-6 inches of water. The higher energy D-T reaction neutron would require a greater thickness (~5-6 times thicker?). The absorbing material could be much thinner than the moderating material, perhaps only a small fraction of an inch thick.

Dan Tibbets

Dan,
Shielding is measured in tenth thicknesses. And yes, Hydrogenous materials make the best 0-n-1 barriers. That's what I said.

And yes again, for little zippys (as we used to call gamma), you need to take a more creative approach. As noted above, composite systems do better. And of the three factors, distance and time do better than shielding for little zippys.

The biggest issue in my opinion in any design is future activation. You can make some pretty cool setups to do the job now, but later on, after cooking for a while, you can make a right mess of it, and cause yourself some good drama. (As you also noted above).

The question of p-11B gamma intensity is far from settled. Shielding requirements may not be as severe as above.
Data. Data. Of course, they might never cough it up... ZZZZzzzzzzzzzzzz

Seem to remember it takes .4 inches of lead to half the radiation ( shows how long ago I was doing stuff i still use standard instead of metric.) I also remember drills where we stuffed everybody below the waterline as that was the best way to protect the crew. Ahh for the old cold war days now you nuke heads had other worrys as well.