Potential Carbon-11 Issues

[nakile]

As interest in aneutronic fusion grows, I’m sure the issues of side reactions in the reactor will need to be addressed in greater detail.

I think the big one will be carbon-11, since it’s the closest thing to radioactive waste that these reactors will create. I know that the half-life is so short that it can’t really “collect” in the reactor, but what is the maximum level that could potentially be in a unit? And what are the dangers of that amount if, though whatever means, it were to be released into the environment or ingested by a person?

[vansig]

Let’s imagine you accidentally ingested 11 grams of the stuff; that would be 6.022×10^23 atoms; all of this would be gone in 79 half-lives = 1606 minutes or about 1.11 days.

Chance of this much collecting in the reactor? none; how much of this could collect in the reactor? we can calculate this if we know the % yield of carbon-11.

[ let’s take a 5 MW reactor with system efficiency ~ 50%, so that’s 10 MJ/s of fusions. At 8.7 MeV energy release per reaction, ( = 1.39389361 x10^-12 joules) that would be
7.18 x 10^18 reactions per second; if carbon-11 yield were 100% it would take, neglecting decay, 83940 seconds or 0.97 day to build up the 11 grams, above ].

so it turns out that at above 87% yield, the carbon-11 concentration would be able to build up; but any less than that and it just cant. it turns out that yield is much lower,
(i dont have the figure, so i can just guess that it would be in the amounts of nanograms to micrograms?).

[zapkitty]

vansig wrote: Let’s imagine you accidentally ingested 11 grams of the stuff; that would be 6.022×10^23 atoms; all of this would be gone in 79 half-lives = 1606 minutes or about 1.11 days.

… er… that number is pretty meaningless without the dose rate and, while I’ve not done the calculations for an FF unit either, I [em]do[/em] know that carbon-11 is [em]hot[/em] while it lasts.

So it’s a good thing that the 100% conversion of fusion product to 11C that you speculate on is simply impossible 🙂

Very small amounts of 11C are safely used in PET scans but anything like the “11 grams” you mentioned would be near-instantly fatal inside a human.

[nakile]

Woah, thread reset… I guess I need to explain better.

If the Focus Fusion experiment, or any of the various aneutronic projects for that matter, start to show great progress, public interest will grow. And with that will come a lot of skepticism and people digging deeper trying to uncover the “catch.” I can see carbon-11 becoming that catch. Here you have a very potentially potent (short half-live) beta emitter. When dealing with short half-lives, milligrams are a big deal. Then there will be the “what happens if somebody comes along and blows these up or crashes a car/airplane into them” concerns. These are important questions that need solid answers.

I’ve read many times that in the case of the Focus Fusion, it’s nine hours from shutdown to all the 11c generated decaying to background levels. People are going to want to know what to do, if anything, in those nine hours if there ever is a loss of containment accident.

How much is in each reactor will also determine how many of these you can have in one location before safety regulations get tougher. The higher the 11c in each the reactor, the lower the amount of units people will be confortable having in one area. If it’s high enough, then distributed deployment or mobile applications (ships/aircraft) might be a tough sell or even prohibited.

zapkitty wrote: Very small amounts of 11C are safely used in PET scans…

This is a great starting point. If 11c is used in PET scans, that means there’s a lot of data on direct human exposure, which would be the worse case scenario in a loss of containment accident. Its just a matter of getting the numbers.

[vansig]

zapkitty wrote:

Let’s imagine you accidentally ingested 11 grams of the stuff; that would be 6.022×10^23 atoms; all of this would be gone in 79 half-lives = 1606 minutes or about 1.11 days.

… er… that number is pretty meaningless without the dose rate and, while I’ve not done the calculations for an FF unit either, I [em]do[/em] know that carbon-11 is [em]hot[/em] while it lasts.

So it’s a good thing that the 100% conversion of fusion product to 11C that you speculate on is simply impossible 🙂

Very small amounts of 11C are safely used in PET scans but anything like the “11 grams” you mentioned would be near-instantly fatal inside a human.

we weren’t calculating the dose, yet. i had only gotten far enough to show that the isotope could not possibly build up in the reactor

[vansig]

ok, so i still dont have the actual figure for % yield for carbon-11, but if it were 0.1%, then the amount in a reactor when you shut it down will equal the equilibrium value reached during continuous operation; as above, we have 7.18 x 10^18 reactions per second; hypothetically * 0.001 = 7.18 x 10^15 carbon-11 atoms created per second. (x 20.334 minutes = 8.76 x 10^18 atoms created during one half-life period).
at equilibrium, decay rate equals creation rate, so there would be twice this, or 1.752 x 10^19 atoms in the reactor, when you shut it down, or 320 micrograms.

make adjustments for actual yield

[jamesr]

vansig wrote: ok, so i still dont have the actual figure for % yield for carbon-11, but if it were 0.1%, then the amount in a reactor when you shut it down will equal the equilibrium value reached during continuous operation; as above, we have 7.18 x 10^18 reactions per second; hypothetically * 0.001 = 7.18 x 10^15 carbon-11 atoms created per second. (x 20.334 minutes = 8.76 x 10^18 atoms created during one half-life period).
at equilibrium, decay rate equals creation rate, so there would be twice this, or 1.752 x 10^19 atoms in the reactor, when you shut it down, or 320 micrograms.

make adjustments for actual yield

To put it in context another way: If you were standing 1m from a naked source of those 1.7×10^19 atoms of C-11 with no shielding for a few hours (ie long enough for pretty much all of it to decay) then the dose you’d get would definitely be lethal.

Working as follows:

Most C-11 decays via positron emission – with a range of energies averaging a few hundred keV. the positron will then annihilate producing two ~511keV gammas back to back.
If we discount the positrons themselves (since they won’t get very far) and just have the dose due to the gammas

gamma energy = 1.022MeV = 1.022e6*1.6022e-19 = 1.64e-13 J
1.752 x 10^19 atoms * 1.64e-13 J/decay =2.87e6 J total

fraction of surface area of sphere of a person at 1m ~ 8%

8%* 2.87e6 J = 2.30e5 J

energy distributed over ~75kg => 2.3e5/75 ~ 3000 J/kg

So you’d get very roughly 3000Gy dose!!!

Of course you’d actually get nothing like that much in reality since most of the decay energy would end up being absorbed close to the source and converted to heat, but still you get the idea that it’s a really good idea not to go in and service the device for at least 8 hours after turning it off.

After 8 hours (or ~24 half lives) the activity will have dropped to 0.5^24 =6e-8 times lower, so your dose working for a day after waiting the first 8 hours would be around 3000*6e-8 = 0.2mGy which is liveable, but still too high to meet the ALARP principle

If you wait an extra few hours, so 12 total (36 half lives) then the remaining potential dose drops to roughly 3000*1.4e-11 or around 40nGy – much more acceptable.

The question is: Is C-11 the longest lived of the by-products? The design of the device, down to the last nut & bolt will need to be considered when working out the actual dose limits. For example, some 1st & 2nd Gen fission reactors had a little cobalt in the structural steel alloys – this with its ~5yr half life now makes up a major part of the decommissioning burden.

[JimmyT]

I think a major factor in the public safety discussion has to be dispersion. If a train wreck (powered by Focus fusion) were to occur resulting in rupture of a reactor. Would those down wind need to be worried? I believe the carbon 11 in the reactor will be in the form of fine dust. Some might disperse downwind. But I’m thinking that the vast majority of it would remain with the reactor wreckage.

[Lerner]

We did do a calculation on c-11 which I will dig up and post. But it is actually quite complicated because it depends on the high-energy tail of the ion velocity distribution. I think it would be in the form of methane, so ti would disperse rapidly. Again, I think a reasonable comparison would be what happens if 3 tons of gasoline blows up.

[JimmyT]

If that’s the case wouldn’t some of it be burned up (in a nuclear sense) by the fusion pulses.

[Lerner]

Looking back at old calculations, I think that a person at 1 meter from a generator at the time of a catastrophic break in shielding, leading to a complete release of radioactive methane, would get a lethal radiation dose in only about 6 seconds. But the methane would be quite hot and the simultaneous release of the hydrogen into air would almost certainly lead to a small chemical explosion. I suspect the hot gas would rise away from the ground very rapidly so the chance that anyone would be exposed to a lethal radiation does, except for someone within a few meters of the generator, is quite low. Overall, if this, say, fell from an aircraft, the damage will be much less than for three tons of jet fuel. By the same logic, putting one in the basement of a tall building would probably not be wise. But our suggestion of putting them in substations would be perfectly safe.

[zapkitty]

Lerner wrote: Looking back at old calculations, I think that a person at 1 meter from a generator at the time of a catastrophic break in shielding, leading to a complete release of radioactive methane, would get a lethal radiation dose in only about 6 seconds.

And note that this extreme situation could only take place because of a catastrophic [em]external[/em] force applied to a generator housing. I don’t think the FF unit could supply the required energy in and of itself.

So we’re speaking of some outside force great enough to smash through the outer shell of the housing and then through a meter of lead, boron-10 and water shielding and [em]then[/em] through perhaps half a meter of the “onion” -which itself is composed of many thousands of layers of metal foil- and then through the vacuum chamber itself… and that’s the [em]shortest[/em] path to the core.

Other routes would add obstacles such as massive capacitor banks, the hybrid helium/air or helium/water cooling system and the Rogowski coils for the ion beam energy recovery system.

[Tulse]

JimmyT wrote: If a train wreck (powered by Focus fusion) were to occur resulting in rupture of a reactor. Would those down wind need to be worried?

Lerner wrote: I think a reasonable comparison would be what happens if 3 tons of gasoline blows up.

And, of course, we’ve just seen what tanker cars full of oil can do. These tanker cars wouldn’t be necessary in anywhere near the numbers they are now if FF were up and running. I’ll take the risk of release of a tiny bit of C11 to this petroleum-fueled devastation.

[JimmyT]

Touche’ Tulse.

[nakile]

So the force needed to break a reactor open would be, on its own, much more dangerous than what comes out of the reactor itself.

I suppose that for the radioactive methane (the carbon of the compound being 11c), detailed dispersion and dose calculations will probably be needed at some point. I know that in larger cities substations are located underground and next to large, tall buildings. If you have a gas release in a location like this, I could see some issues coming up, but that I all depends on how quick the gas rises and how fast it disperses into a harmless concentration. If the numbers aren’t favorable, something extra will have to be thought up and implemented to mitigate the issue.

[Author]

Posts