Mark Hibbs has written an interesting article on safeguards developments in Syria (‘Diplomatic efforts to engage Syria hindering US-led campaign at IAEA’, NF 081006). In the article he writes that, ‘According to sources close to the IAEA, a key issue is whether certain samples of carbon indicated the absence of carbon-14. That, they said, would be a signature that the carbon in the samples represents highly pure nuclear-grade graphite. That material would have been required for a DPRK-design production reactor’.Producing high-purity graphite is important for reactors, since it should be free of neutron absorbing materials. If the material absorbs neutrons, it may cause the reactor to fail.
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I am confused about this piece of information. My understanding was that carbon-14 exists in trace quantities in nature, and that most of it is produced by human nuclear activities (in particular, nuclear testing has scattered carbon-14 all over the planet, allegedly making it possible to accurately date a person’s date of birth by checking carbon-14 concentrations in their teeth).Large amounts of C-14 are produced in a nuclear reactor due to the irradiation of Nitrogen-14. The Carbon stays put within the graphite block, however.
I do not know enough about the production of high-purity graphite to make an assessment of Hibbs’s article. However, I have heard that the source is solid, and that the absence of C-14 has something to do with the calcination procedure (where the pitch coke is baked at 1300 degrees Celsius).
That said, I had a quick look at this interesting table on impurity contents in nuclear-grade graphite, and cannot see any carbon there. Then again, I read somewhere that that is because carbon is not considered an impurity.
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Have a look at this paper, which is the source for all my fancy graphics, and make up your own mind.
Apparently, an Agency report on Syria is forthcoming. We shall see...
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4 comments:
Andreas,
Carbon 14 does indeed exist in trace quantities in nature, but most of it is produced in the upper atmosphere by nitrogen 14 absorbing thermal neutrons (the neutrons are generate by cosmic ray collisions with the atmosphere). Since C 14 decays with a half life of about 5,700 years this means that there will be some equilibrium between what is produced up in the atmosphere (and which subsequently forms compunds such as carbon dioxide, to be taken up by plants or dissolve in the oceans) and what decays. Thus we have - or had - a fairly constant trace quantity of C 14 around us.
In reality the cosmic ray flux varies, as does the earth's magnetic field, which provides a small degree of protection from cosmic rays, thus the equilibrium level of C 14 actually varies slightly over the centuries. It is this variation, as well as the inherent difficulties of measuring trace quantities, that causes the uncertainty in C 14 dating. As you say, human nuclear activities, especially atmospheric weapons testing, generated big increases in atmospheric C 14, and these spikes can be used for checking ages based on body carbon.
Getting back to what you really want to know, I think your source IS somewhat confused. Nuclear grade graphite IS made by the process you describe, and since the raw material (pitch, which ultimately derives from oil) has been underground for millions of years it is obvious that it should be very low in C 14. Of course, extracting the oil may add some more recent C 14, for example the ethanol in ethanolamines used by the production process, but generally oil and its products are very low in C 14. Consequently nuclear grade graphite can be expected to be almost pure C 12.
As for the reference to the calcining process, I really don't understand that. Calcining is not much more than just heating the pitch in an inert atmosphere in order to drive off all the other hydrocarbons present - like heating coal to produce coke (the solid remainder) and "town gas" (that rather nasty brew of CO and hydrocarbons that people used for suicide attempts before we changed to natural gas, LPG, etc). There is nothing in the calcining process that is remotely energetic enough to affect the nuclear properties of C 14, or to somehow destroy the C 14.
If (almost) pure C 12 is used in a nuclear reactor then the thermal neutrons in the core very rapidly create C 14. Thus if the IAEA have found samples of graphite which are very low in C 14 then that on its own doesn't (to me, at least) prove that they have found reactor grade graphite; they have only found graphite manufactured from an ancient source of carbon. On the other hand, if they find a graphite sample that is very rich in C 14, and particularly if its crystal structure is damaged in the "right" way (neutron irradiation causes characteristic dislocations in the graphite crystals) then there's a good chance they've found graphite that was once part of an OPERATIONAL recator.
One last point, materials containing different isotopes generally have lower coefficients of thermal conductivity than isotopically pure materials. Hence the thermal characterisitics of a reactor made with pure C 12 graphite are initially slightly better than one containing C 14 (e.g. if the carbon source for the graphite were, say, sugar!). More importantly, the neutronics of pure C 12 graphite are much better than graphite containing C 14, which makes it easier to handle / model start-up criticality and loading pattern calculations. Of course, once your reactor gets going you can't avoid C 14 production, with consequent effects on moderation, but it's still nice to start with (almost) pure C 12 if you can!
Overall, then, core designers like to use graphite that is very low in C 14. But since ANY graphite produced from fossil fuels will be naturally low in C 14, the finding of "low C 14 graphite" proves only that it came from fossil materials (which is true of most electro graphites).
Hairs
P.S. Possibly your source is confusing the effect of calcining on C 14 with its effect on chemical impurities. As mentioned, calcining is intended to drive off all but the heaviest hydrocarbons, so some contaminants such as sulphur are reduced by calcining.
That's a cool picture - is that a google sketchup rendering?
Also, I'm not quite clear what you're saying in this post - is it that these sources at the IAEA think there was "reactor-grade" graphite at al kibar?
Andy. Yes it is, posted on ACW first by an unknown but very talented artist.
As for the Agency, there is less than a dozen people working on the Syria gig at the moment. Information is very rare - and they're a tight group.
But yes, it would seem like some people at the Agency believes that there was 'reactor-grade' graphite present at Al-Kibar.
Hairs. Thanks for your explanation. I've followed up with one of our advisors today, and this guy is as skeptical as you and I to the C-14 story. I have understood that it is difficult to remove all traces of C-14 from the graphite. And as you point out, the absence of significant quantities of C-14 would most likely indicate that the graphite hasn't been irradiated. No surprises there.
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