Polywell transportation: how small?

If polywell fusion is developed, in what ways will the world change for better or worse? Discuss.

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DeltaV
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Postby DeltaV » Sat Jan 30, 2010 9:04 pm

Over at http://www.avrc.com/presentations/IEC_Fusion_basics.pdf they discuss radiation shielding for 100KW-class IEC fusors as being not much of a concern:

The objective would be to use a fusion powered IEC for next generation power units in the 100-kWe range to replace current [Hall Current Thrusters].

Advantages of p-B11 in Radiation emissions

- Radiation is virtually eliminated by p-B11. Exhaust with an IEC p-B11 fuel has trace radiation below NRC levels. Very few gammas are released from the fusion reaction chamber itself,
and they are stopped with modest shadow shielding.
- This is below the space radiation levels at stratosphere levels, so nothing beyond normal space radiation shielding is needed, e.g. all electronics for space is "radiation hardened" anyway.
- Exposure issues could only occur if people or electronics are very close to the IEC device while it is running - otherwise no shielding at all is needed at more than ~ 30 meters away due to 1/r2 spreading of beams and corresponding reduction of flux. Compared to space radiation, this is not even significant.

For smaller vehicles, maybe a distributed approach, using several low-power, small-diameter Polywells spread around the vehicle, could be used to reduce the oppressive shielding mass/volume requirements stated by 93143 for full coverage. Perhaps this might be good enough for atmospheric flight or road use (not for SSTO or other uses in the GW range). I'm assuming here that AVRC has their radiation types/fluxes estimated correctly and that IEC is similar to Polywell in this regard for the same power level.

Heath_h49008
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Postby Heath_h49008 » Sat Jan 30, 2010 9:26 pm

If shielding is as big a factor, wouldn't it be better to centralize the cores inside one shielding structure? It would seem to eliminate redundancy, and the associated mass.

93143
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Postby 93143 » Sat Jan 30, 2010 10:28 pm

DeltaV wrote:Over at http://www.avrc.com/presentations/IEC_Fusion_basics.pdf they discuss radiation shielding for 100KW-class IEC fusors as being not much of a concern:

The objective would be to use a fusion powered IEC for next generation power units in the 100-kWe range to replace current [Hall Current Thrusters].

Advantages of p-B11 in Radiation emissions

- Radiation is virtually eliminated by p-B11. Exhaust with an IEC p-B11 fuel has trace radiation below NRC levels. Very few gammas are released from the fusion reaction chamber itself,
and they are stopped with modest shadow shielding.
- This is below the space radiation levels at stratosphere levels, so nothing beyond normal space radiation shielding is needed, e.g. all electronics for space is "radiation hardened" anyway.
- Exposure issues could only occur if people or electronics are very close to the IEC device while it is running - otherwise no shielding at all is needed at more than ~ 30 meters away due to 1/r2 spreading of beams and corresponding reduction of flux. Compared to space radiation, this is not even significant.


They're wrong. In the first place, 100 kWf is almost certainly too small for net power, meaning 100 kWe would be a very inefficient reactor. In the second place, even neglecting neutrons and/or neutron absorption gammas, 100kWf gives you 18 W of gamma radiation, which at 5 metres from the core is more than 2 rem per minute for an average person, and 5 kW of bremsstrahlung, which at 5 metres from the core is about 600 rem per minute for an average person.

That's a fatal dose of X-rays in 3/4 of a minute. The gammas will give you the same in less than four hours, and are much harder to shield...


For smaller vehicles, maybe a distributed approach, using several low-power, small-diameter Polywells spread around the vehicle, could be used to reduce the oppressive shielding mass/volume requirements stated by 93143 for full coverage. Perhaps this might be good enough for atmospheric flight or road use (not for SSTO or other uses in the GW range). I'm assuming here that AVRC has their radiation types/fluxes estimated correctly and that IEC is similar to Polywell in this regard for the same power level.


No, Heath_h49008 is correct. Not only do you get significant mass benefits from clustering (and thus only having to shield the outside of the cluster, rather than the surface area of each core), but since the shield mass increases as R^2 (plus a very weak logarithmic dependence on fusion power) and fusion power increases roughly with R^7, power-to-weight ratio considerations strongly favour going for the largest cores you can build.

EDIT: I had forgotten to divide by body mass, which gave extraordinarily short kill times for the predicted radiation levels. Fixed.

Stoney3K
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Postby Stoney3K » Sun Jan 31, 2010 12:42 pm

93143 wrote:They're wrong. In the first place, 100 kWf is almost certainly too small for net power, meaning 100 kWe would be a very inefficient reactor. In the second place, even neglecting neutrons and/or neutron absorption gammas, 100kWf gives you 18 W of gamma radiation, which at 5 metres from the core is more than 2 rem per minute for an average person, and 5 kW of bremsstrahlung, which at 5 metres from the core is about 600 rem per minute for an average person.


You're not taking into account that the radiation will emanate in all directions (like a point source), not in a directed beam towards the unfortunate onlooker. If you're standing near the reactor, you will get nowhere near the full force of 5kW of bremsstrahlung, think of standing 2 feet away from a 5kW light bulb. As long as the beam is not focused, the numbers tell a completely different story.

1/r2 really does its magic here. It may be 5kW at the actual source (the reactor vessel), but if you get farther away in any direction, it will decrease dramatically. Also, some radiation is emanated in directions where nobody will ever get (fx. on the ground, if the reactor is sitting on the floor), so shielding requirements will be much less.
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93143
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Postby 93143 » Sun Jan 31, 2010 9:44 pm

Stoney3K wrote:
93143 wrote:They're wrong. In the first place, 100 kWf is almost certainly too small for net power, meaning 100 kWe would be a very inefficient reactor. In the second place, even neglecting neutrons and/or neutron absorption gammas, 100kWf gives you 18 W of gamma radiation, which at 5 metres from the core is more than 2 rem per minute for an average person, and 5 kW of bremsstrahlung, which at 5 metres from the core is about 600 rem per minute for an average person.


You're not taking into account that the radiation will emanate in all directions (like a point source), not in a directed beam towards the unfortunate onlooker. If you're standing near the reactor, you will get nowhere near the full force of 5kW of bremsstrahlung, think of standing 2 feet away from a 5kW light bulb. As long as the beam is not focused, the numbers tell a completely different story.


5 metres from core: 4*pi*5*5 = 314.1592654m²
Body silhouette area in sitting position is about 0.45m²:
intercepted fraction is 0.45/314.1592654 = 0.001432394
intercepted power is 0.001432394*5000W = 7.16 W
dosage is power/body mass = 7.16W/70kg = 0.102Sv/s = 10.2rem/s = 614rem/min

Satisfied?

Okay, fine, the human body will only absorb maybe 80-99% of that, depending on distribution, so it could be as low as 500rem/min...

IntLibber
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Postby IntLibber » Fri Mar 19, 2010 3:42 am

MSimon wrote:Very Very roughly 1 KW = 1 HP

1 MW ~= 1,000 HP.


Thats a bit off. As I recall its 750 W per HP.

MSimon
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Postby MSimon » Fri Mar 19, 2010 3:59 am

IntLibber wrote:
MSimon wrote:Very Very roughly 1 KW = 1 HP

1 MW ~= 1,000 HP.


Thats a bit off. As I recall its 750 W per HP.


True. But it is a good rule for rough estimation.
Engineering is the art of making what you want from what you can get at a profit.

tombo
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Postby tombo » Sun Mar 21, 2010 10:23 pm

At 75% efficiency 1kW = 750W = 1 HP
-Tom Boydston-
"If we knew what we were doing, it wouldn’t be called research, would it?" ~Albert Einstein

kunkmiester
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Postby kunkmiester » Mon Mar 22, 2010 3:39 am

Only if a 33% error is good enough. 1000 KW / 750 = 1333 HP.
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tombo
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Postby tombo » Mon Mar 22, 2010 6:05 am

If you have 1000 Watts of heat and you can get 75% of it out as work you get 750 Watts = 1 HP
-Tom Boydston-

"If we knew what we were doing, it wouldn’t be called research, would it?" ~Albert Einstein

Skipjack
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Postby Skipjack » Mon Mar 22, 2010 10:15 am

Hmm, ok, so do we have any relyable numbers on the amount of shielding that is now actually needed?
If I look at the pictures and concept studies on the EMC2 website, I never see a whole lot of material dedicated to shielding. Now that could just be because it was simply left out for understandable reasons (5 feet of lead are not that interesting, after all). I am still confused. I would assume that a PB11 polywell produces less radiation than a fission reactor. Since there are/were concepts that put fission reactors into rockets and even airplanes, I am not convinced that the shielding would have to be that heavy.

D Tibbets
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Postby D Tibbets » Mon Mar 22, 2010 4:16 pm

The shielding provided by the vacuum shell and other structures would provide some protection , but as you could not limit your exposure by adding distance or limiting time, you would have to have to add alot of heavy shielding. Some numbers for shielding against X-rays is given al this site. Gamma shielding would requir greater thicknesses. That would be counterbalanced some by the significantly larger does of x-rays in the total exposure.
http://en.wikipedia.org/wiki/Radiation_protection

Dan Tibbets
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93143
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Postby 93143 » Tue Mar 23, 2010 2:43 am

I don't recall if I've admitted this before, but I've been using a chart off Wikipedia to get X-ray and gamma attenuation lengths in lead.

The primary shielding worry is the gamma rays from the main fusion reaction (4, 12, and 16 MeV; take a look at the chart to see why that's bad). If it turns out that the one source that claims p-¹¹B can emit gamma rays is wrong about either the possibility or the intensity, or that the intensity changes with the plasma energy distribution, then the gamma shielding requirement might be reduced or eliminated.

There's still bremsstrahlung, and a pittance of 0.5 MeV neutron absorption gammas, but I can't see the lead shield needing to be even half as thick without the reaction gammas even in the worst-case scenario (high-voltage operating point with thermal bremsstrahlung, more intense than predicted)...

DeltaV
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Postby DeltaV » Tue Mar 23, 2010 6:12 am

A mass- or volume-optimized shield will involve layers of the appropriate materials with specific thicknesses and ordering, barring some "wideband" nanotech breakthrough:

http://www.talk-polywell.org/bb/viewtopic.php?t=2022

93143
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Postby 93143 » Tue Mar 23, 2010 7:00 am

No, I don't think there's much point getting fancy. Most of the radiation from a p-¹¹B Polywell is high-energy photons, and with high-energy photons lead is pretty much it. Photons are essentially stopped by mass, though lead is a bit better at it than other substances.

Tungsten shields are smaller, but they're more expensive, and it looks like they're probably heavier, even for the high-energy gammas - take a look at that chart you posted, and remember that tungsten is about 70% denser than lead.

Actually, try normalizing the attenuation coefficient on density and plotting its inverse against photon energy. Tungsten looks to be completely failing to gain any ground on lead. A tungsten shield is smaller but heavier, if this trend continues as it looks to...

Boron isn't necessarily helpful for a reactor with a neutronicity of 1e-8 and a gamma emission rate of 1e-4; if you've got a foot of lead in the way anyhow, you might as well not bother.

The only reason to complicate the shield that I can see is the cooling requirement - most of the bremsstrahlung will come out in the first few mm, meaning the vacuum vessel itself will probably take a good chunk of the load...


On the other hand, if there were some way to deflect gammas so as to pass through the shield more longitudinally, thus magnifying the effect of a given shield thickness... but that would be pretty tough even with just X-rays...


EDIT: I had forgotten that there's a neutron-producing p-¹¹B reaction that's suppressed if the high-energy tail is depleted. So maybe the gammas work similarly after all. If so, then neutrons and bremsstrahlung could be the only major components, and they require roughly comparable amounts of shielding, but of different types. The water/boron/lead shield sounds about right to me; most of the X-ray power would be dissipated in the water layer, which is easy to cool, opening the possibility of a solid layer of lead past the boron-10. IMO... and I'm not a nuclear engineer...

In other words,
Skipjack wrote:Hmm, ok, so do we have any relyable numbers on the amount of shielding that is now actually needed?

...no. Not yet. For D-D, sure, but no one cares about D-D...


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