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.

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?

Rezwan - 27 June 2010 02:24 PM

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.

Brian H - 27 June 2010 11:48 AM

What longer-term byproducts?  Slow neutrons don’t make any, AFAIK.  Remember, max levels are about what you get from a classroom full of children.

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.

Neutron energy is important as many reactions have threshold energies. If you are below the threshold, the isotopes will not be produced.

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

Breakable - 28 June 2010 02:35 AM

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

Breakable - 27 June 2010 11:10 PM

Brian H - 27 June 2010 11:48 AM

What longer-term byproducts?  Slow neutrons don’t make any, AFAIK.  Remember, max levels are about what you get from a classroom full of children.

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.

Further to Eric’s comment, if low-energy neutrons caused accumulation, the classroom full of kids would do the same thing.

Back on topic, the contentious issue of “Peak Oil” sort of becomes a non-issue with FF power available, since demands on stocks drop steadily with time as electricity becomes a more and more viable substitute. Transportation is the obvious biggie here. 

As an interesting side note, I was wandering around the TeslaMotors site and came across a page with a drop-down menu to display the various states’ incentives for EV purchase and use. For Oklahoma residents, they’re huge: an income tax credit worth 50% of the purchase price of a BEV. A Tesla ‘Roadster Sport’ is around $125,000, so that’s pretty impressive.

Rezwan - 27 June 2010 02:24 PM

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?

I think less management is better than more, because it is easier to track the posts and topics.
If we had a Tree-like instead of single Thread forum (such as Reddit, or Slashdot comments) - that could help manage itself.
Reddit btw allows creating your own sub-reddit’s that have all kind of nice features for management, voting and thread display.

Breakable - 27 June 2010 11:10 PM

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;
Don’t forget that the generator is surrounded by a shell of water and boron10. Very few neutrons will escape this shielding, AFAIK. You are perhaps flogging a very small hobby horse to death!

Brian H - 28 June 2010 11:01 PM

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?

Breakable - 28 June 2010 10:47 AM

Rezwan - 27 June 2010 02:24 PM

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?

I think less management is better than more, because it is easier to track the posts and topics.
If we had a Tree-like instead of single Thread forum (such as Reddit, or Slashdot comments) - that could help manage itself.
Reddit btw allows creating your own sub-reddit’s that have all kind of nice features for management, voting and thread display.

Can you provide an example/link?

Rezwan - 04 July 2010 01:03 AM

Can you provide an example/link?

Tree like thread: 
http://www.reddit.com/r/funny/comments/clrkk/a_man_buys_a_goat/
I am not sure if you can build it on this base though… what is the name of it phpBB i know the CMS is “ExpressionEngine”, but wonder what is the forum itself?