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Some nuclear reactors (like the RBMK in Chernobyl) use graphite as a neutron moderator. As far as I understand, this graphite material, either in rods or as blocks with embedded channels, surrounds the uranium fuel rods and is used to moderate the reaction.

Question: How do these graphite components become themselves radioactive?

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  • $\begingroup$ My uneducated guess is that neutrons that escape from fissioned uranium nuclei collide with graphite nuclei, fissioning them and causing instability. Thus emissions of alpha,beta, gamma etc $\endgroup$
    – Hisham
    Commented Jun 7, 2019 at 0:35
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    $\begingroup$ @Ubaid Yes, it's due to the neutrons, but carbon is far too small to fission easily. Carbon-12 & carbon-13 are stable, but carbon-14 decays by beta - emission, as do most higher isotopes of carbon. See en.wikipedia.org/wiki/Isotopes_of_carbon $\endgroup$
    – PM 2Ring
    Commented Jun 7, 2019 at 0:49
  • $\begingroup$ en.wikipedia.org/wiki/Neutron_activation $\endgroup$
    – OrangeDog
    Commented Jun 7, 2019 at 11:36
  • $\begingroup$ Wasn't it made clear in that episode that it was the tips that were graphite... $\endgroup$
    – user207455
    Commented Jun 8, 2019 at 6:49
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    $\begingroup$ @SolarMike the initial phrasing of the question was indeed wrong. Good catch. $\endgroup$
    – Emilio Pisanty
    Commented Jun 9, 2019 at 16:34

1 Answer 1

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Graphite in reactors gets radioactive mainly by forming beta decaying $\require{mhchem}\ce{ ^{14}_6C}$, mostly from naturally occuring stable $\ce{^{13}_6C}$ (1.1% abundance) :

$$\ce{^{13}_6C + ^{1}_{0}n -> ^{14}_6C}$$

with half-life $5730 \pm 40$ years. It is naturally present in traces in all carbon containing matter with recent carbon interchange with atmosphere.

The beta decay has the equation:

$$\ce{^{14}_6C -> ^{14}_7N + ^{0}_{-1}e + \overset{-}\nu_{\mathrm{e}}}$$

It is the same isotope used for radiocarbon dating, as it is continuously created in atmosphere by cosmic radiation.

Heavier carbon isotopes are very unstable and quicky undergo beta decay to nitrogen ( or even oxygen ).

Secondary source of radioactivity is contamination by fission products and radioactive products of neutron irradiation of stable isotopes.

For the former, there is wide range of formed radioactive isotopes, with 2 distribution peaks with relative masses about 2/5 ( like $\ce{^{90}Sr}$ ) and 3/5 ( like $\ce{^{131}I ^{137}Cs}$ ) of the mass of $\ce{^{235}U}$.

If you used graphite control rods, you would cause the 2nd Czernobyl.

RBMK uses graphite tipped control rods, not graphite control rods.

These tips cause problem in short reversal of control rod effects.

Graphite is not used as control rods, but as a neutron moderator, similarly as light or heavy water.

Moderators serve for slowing down neutrons to thermal speed(thermal neutrons) to improve the cross-section of the fission reaction, so the sustained fission reaction is possible even in lightly enriched or even natural uranium(heavy water nuclear reactors). Best moderators have low neutron absorption.

Moderation principle is on purely mechanical bases, as fast neutrons collides with slow particles (protons, deuterons, carbon kernels) and collisions redistribute momentum.

From mechanical point of view, protons would be best moderators, but unfortunately they considerably fuse with neutrons, so enriched uranium is needed ( in the opposite to deuterons )

Control rods are the opposite. They serve for regulation of "neutron economy" by absorption of excessive neutrons.

There are usually used 3 sets of rods,

  • Regulation set for BAU regulation

  • Adjusting sets, progressively pulled out, as neutron economy gets worse in the time

  • Emergency set, that stops the fission when triggered to be pushed in.

The rods contain usually cadmium or preferably boron, with high cross-section for neutron absorption.

Boric acid is also used in cooling water in secondary/tertiary circuits, where water serves as heat transfer medium only, not as moderator.

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    $\begingroup$ Excellent answer! It's a bit confusing that the graphite is called a moderator - it sounds like it moderates (slows down) the reaction. As you point out, it moderates (slows down) the neutrons, which makes them more likely to trigger a chain-reaction, and so it accelerates the reaction! The things that slow down the reaction are the control rods (made of boron), which absorb the neutrons... $\endgroup$
    – Oscar Bravo
    Commented Jun 7, 2019 at 9:28
  • $\begingroup$ Yes, boron is used as well. So does boric acid in water of secondary circuits, afaik. $\endgroup$
    – Poutnik
    Commented Jun 7, 2019 at 9:30
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    $\begingroup$ Without having done the calculation, I'd guess that nearly all of the C-14 production is due to single neutron capture on the naturally-occurring C-13 (which is 1% of natural carbon) rather than two-neutron capture on C-12. The capture cross sections on carbon are millibarns. By comparison the capture cross sections for U/Np/Pu are more like kilobarns, but setting up a breeder reactor that produces chemically significant amounts of transuranics by multiple-capture reactions is a technical challenge. But I can also imagine that a careful calculation might show differently. Do you have a source? $\endgroup$
    – rob ♦
    Commented Jun 8, 2019 at 18:23
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    $\begingroup$ @rob Well, all 14C production is from 13C(I am aware of its natural abundance, but did not mention it).. I do not have cross-section data for 12C to produce 13C, so I implied it is possible. It is supposed a moderator has many orders lower cross-section than fission material, but even these many orders allow considerable radioactivity of short/medium half-life isotopes. ( like tritium in light/heavy water moderator case ). $\endgroup$
    – Poutnik
    Commented Jun 8, 2019 at 18:33
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    $\begingroup$ The first 2 equations do not imply, but may look so, that all 13C in the latter comes from the former. I added the note. $\endgroup$
    – Poutnik
    Commented Jun 8, 2019 at 18:36

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