[vansig]

belbear wrote: Direct fusion powered flight may indeed need some radical fuselage design, totally departing from the classic tube-with-wings model.
[…]
With sufficient clearance between reactor and occupants, less neutron shielding is necessary.
Shielding may only be required toward the passenger and crew, not away from them since a lot of free air is also an excellent shielding.

seems to me, that these constraints lend themselves to longer tubes, with reactor and engines in the rear, except possibly in lenticular aircraft, where the pilot might be separated from everything else by a hemispherical shell of shielding.

[vansig]

dennisp wrote: How hard would it be to make decaborane from elemental boron? What kinds of materials could we make on a small scale, given cheap power? How small and cheap could we make a fuel production plant? Etc.

scanning wikipedia, i find this sequence of reactions…

Boron trifluoride, BF3, “is manufactured by the reaction of boron oxides with hydrogen fluoride:
B2O3 + 6 HF → 2 BF3 + 3 H2O”

Diborane, B2H6, “is made industrially by the reduction of BF3, and is the starting point for preparing the higher boranes.”

Decaborane “is commonly synthesized via the pyrolysis of smaller boron hydride clusters. For example, heating B2H6 or B5H9 gives decaborane, with loss of H2.”

however, i’m not absolutely certain that the fluorine is a necessary intermediary.

With cheap power, any process that consumes lots of electricity will benefit. among them are recycling and refrigeration. Any process that needs x-rays will benefit, as well.

Electrolysis becomes more economical, leading to cheaper chemicals: aluminum, hydrogen, chlorine, etc.

A chemicals/fuel production plant will benefit from cogeneration of both heat and electricity. However the plant will not necessarily become smaller in size, since its bulk is chemical storage and reaction chambers. But cheap energy would allow such operations to scale up.

[vansig]

The short-term goal is to create a delivery service: air-drop to any co-ordinates on the planet within 90 minutes, or it’s free.
The longer-term goal is to develop a craft capable of orbital insertion with minimal expenditure of propellant.

Ramjets tend to have quite different design parameters at supersonic vs hypersonic speeds.
A cleverly designed hybrid engine could, maybe, involve shape changes, that also becomes a rocket at very high altitudes, and rescue otherwise dead-weight.

Fuel bulk is one of the problems with existing designs. With fusion, that problem goes away

http://en.wikipedia.org/wiki/X-43A

[vansig]

jamesr wrote: I still think the best way of getting rid of the waste is to just wait 50-100 years for the short live stuff to decay then drop the rest, encased in concrete, into an ocean trench.

I banish you to watch Star trek Voyager’s season 5 opener, “Night”, and then convince Greenpeace this is a good idea.

[vansig]

Brian H wrote: Don’t forget that the generator is surrounded by a shell of water and boron10. Very few neutrons will escape this shielding, […].

yes, it is only the parts inside of that shell we’ll discuss, here. I’ve edited the question to reflect that.

it does seem to suggest, though, to move any control circuitry out beyond that shell, if it has tantalum, tin, iron, or silver in it.

another question: what substance is used as the electrolyte in the capacitors?

[vansig]

zapkitty wrote:
… so what if that 5 MWt from the standard FF were to become 10 MWt once the plane was airborne? Take vansig’s presumed flow-through FF module and slide a sheath of an appropriate x-ray opaque material between the core and the onion once in the air… the air flowing through gets twice the thermal whammy but would it make up for feathering the prop? … run it hybrid?… duct design would be everything…

i like it. now, can we bring it up to hypersonic speeds?

[vansig]

Breakable wrote:
By “longer-term” I assume products that take longer to decay to background levels than a length of a single shot. I am sure there are such products, that means they are accumulating during run and I wonder what is the function describing their accumulation.

The rate of decay of potassium-40 naturally in the human body is ~4000 Bq. So, accumulation to above this level, during run, could only happen if medium-to-long-lived isotopes are generated faster than this.

We ignore short-lived isotopes, because cool-down for maintenance involves waiting for 13 hours, which is many times the half-life, so they will all be gone by the time we get near the device.

A 5MW reactor will generate ~20 kW worth of neutrons, broadly distributed in energy with upper bound ~2.9 MeV. It’s very much like the nuclear S-process in stars.. but happening at a very slow rate. So slow, in fact, that the chance of double-neutron absorption is very low, so only parent atoms deliberately part of the reactor assembly are involved.

The isotopes with > 1 day half-life are few; there is a threshold neutron energy for making them, yes. the neutron flux is low, yes; the parent atoms are not-necessarily in great abundance near the reactor, and this can be controlled by selecting reactor materials carefully. Note, also that decay modes for all these are mostly beta emitters.

So we can calculate the rate of production of radioisotopes, at a given distance from the plasmoid, from the neutron flux and the absorption cross-section of the parent.

In order by half-life, then:

Au-198 2.695 days; parent Au-197; maybe in nearby electronics
Sn-125 9.64 days; parent Sn-124 natural abundance 5.79%; in electronics
Os-191 15.4 days; parent Os-190 natural abundance; maybe in high T coatings
Fe-59 44.5 days; parent Fe-58 natural abundance 0.28%; maybe in structural material
W-185 75.1 days; parent W-184 natural abundance 30.64%; maybe in high T coatings
S-35 87.5 days; parent S-34 natural abundance 4.21%; in switch gases
Ta-182 114.3 days; parent Ta-181 natural abundance 99.9%; in nearby electronics
Sn-123 129.2 days; parent Sn-122, natural abundance 4.63%; in solder
Ag-110m 249.95 days; parent Ag-109, natural abundance 48.161%; in lead-free solder

Sn-121m 43.9 y; parent Sn-120, natural abundance 32.58%; in solder
Ni-63 100.1 y; parent Ni-62, natural abundance 3.634%; maybe in structural material
Ag-108m 418 y; parent Ag-107, natural abundance 51.839%; in lead-free solder

C-14 5,730 y; parent C-13, natural abundance 1.1%; in insulators
Cl-36 301 ky; parent Cl-35, natural abundance 75.77%; in PVC insulators
Be-10 1.5 My; parent Be-9, 100%; in the anode

Note that from an engineering perspective, the large majority of the above can be avoided entirely: by selecting materials, by avoiding placing complex electronics in the path of neutrons, and by avoiding using solder to join parts.

Edit:
Can someone please look up and post the absorption cross-section and activation threshold for the parent isotopes above, that will actually be exposed to neutrons?

[vansig]

the more i think about this, the more i believe, that in an aircraft engine, you want the highest air flow nearest the pinch, so that you can cool the anode and cause gases to expand, increasing thrust

[vansig]

zapkitty wrote:
A cartridge that sits in the vacuum side of the system and can be inserted
through an external hatch in the module without ever being brought into a
hab… that would go over well despite the extra expense involved…

i see no good reason for any of such a system to be in the hab.
it should be protected by a whipple shield, but otherwise entirely external

[vansig]

Breakable wrote: Is the neutron output energy not a probabilistic distribution? If so does that mean the high energy neutrons are just a negligible fraction?

They are .1% of reactions, .2% of total energy…

“These neutrons come primarily from the reaction
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.” — http://en.wikipedia.org/wiki/Aneutronic_fusion

[vansig]

The trouble with destroying nuclear waste is often that it is low-level. For example, the low levels of cesium-137, strontium-90, and technetium-99 in the vicinity of Chernobyl. They’re just long-lived enough, and just dispersed enough, to create a thousand-year lasting hazard.

So you need to find ways to concentrate the radioisotopes, from among the many tonnes of inert matter. It’s annoying, and very expensive to do so.

The public might not like transporting nuclear waste, but right now Chernobyl is as good a place as any to put it.

Here’s just a thought: instead of transporting your high-level nuclear waste to your fusion-fission hybrid reactor,
why not transport your reactor to the brown-field?

[vansig]

also, at 5 pinches/second, anode life would be 66 times longer than it is at 330 pinches/s. a 90-day maintenance schedule would be extended to 16 years

[vansig]

Rezwan wrote: Hi guys,
Looks like this can be split off starting at Breakable’s post on shielding requirements. Should it go under a new “shielding requirements for flights” post, or merge with “FF for jet engines” post?

No, it’s not about either of those.
but it’s still on topic, because “radioactive waste” is a non-issue.

[vansig]

it isn’t about fast versus slow neutrons, it’s about neutron flux. both short-lived and long-lived isotopes are little or no concern. it’s only medium-lived isotopes, with half-lives in years to centuries, that would be any worry, at all. but these are not produced in greater than trace amounts. and remember to compare this to that produced by cosmic rays naturally.

the anode could make:
Be-10 1.5 My

the onion could make:
Al-28 2.2 minutes
Si-31 157 minutes

steel structure could make:
Fe-59 44.5 days
Ti-51 5.76 minutes
Cr-55 3.5 minutes
V-52 3.7 minutes

high Tc superconductor could make:
Y-90 3.9 hours
Cu-66 5.1 minutes
Ba-139 83 minutes
O-19 26.4 seconds

PVC insulation could make:
Cl-36 301 ky
C-14 5.7 ky
H-3 12.3y (trace)

gases in switches:
S-35 87.5 days
F-20 11.1 seconds

electrolyte:
?

what you might expect, after years of use, is embrittlement of components, only, as nearly all these isotopes will have decayed. tritium buildup in the water shield will not be a problem, as this can be drained and replaced easily.

Fe-59 or S-35 buildup, maybe? but again, it depends on the rate of neutron production.

tritium buildup within the insulation, maybe? but this will be in trace amounts anyway, as tritrium would require double-neutron absorption.

if these are a concern, then different structural materials, and different insulation, such as teflon, could be used.

[vansig]

Brian H wrote:
The intention is to run at 330Hz, which is once every 3ms or so. So leakage control has to be a lot better than that!

note that’s 45 milliamps, above. at 3 ms per pinch, leakage wont be a problem

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