Deconstructing HBO's "Chernobyl"

Thunderf00t does a bang-up job:




Edit:  Thunderf00t is a good storyteller but weak on the nuclear stuff.  Here are my notes, addressed as an open letter to him:

First, you missed a completely obvious way to debunk the "5 megaton" garbage.  It only took 10 megatons to completely erase the island of Elugelab in the Ivy Mike test.  5 MT would have scoured Pripyat off the ground and turned the entire Chernobyl power plant to vapor.  Instead, most of the reactor building was still standing!  That wasn't a megaton or even kiloton-level explosion; it was worth, at most, a few hundred pounds of TNT.

Second, you've got a whole lot of your concepts about nuclear fission pretty badly wrong.

The reason that low-enriched uranium can't make a bomb is because you literally cannot sustain a chain reaction in pure LEU, or even LEU oxide, no matter how much of it you have.  The detail of "cross sections" comes to bite you; a fission neutron straight from a nucleus is about as likely to be absorbed by a U-238 nucleus that it goes near (and make no further neutrons) as it is to be absorbed in passing by a U-235 nucleus.  With U-238 being vastly more abundant, fission neutrons can't replace themselves and the "reaction" has no "chain"; the chain gets broken almost immediately.

So, how did the Chicago crew create a chain reaction in natural uranium (just 0.711% U-235)?  They had a MODERATOR, in the form of a big pile of relatively pure graphite bricks.  The graphite, almost pure carbon, only rarely tends to absorb neutrons but does a fairly good job of slowing them down as the neutrons bounce around.  And as the neutrons slow down, a funny thing happens:  U-235 atoms are HUGELY more successful in catching slow ("thermal") neutrons than U-238 atoms are.  When you get things slowed down JUST enough that each fissioning atom leaves neutrons that wind up splitting exactly one more atom, the chain goes unbroken:  you have a self-sustaining "chain reaction".  But for this to work, the moderator has to be between the fuel elements and slow neutrons down before they can get sucked up by U-238 or escape entirely.

What does this have to do with a reactor meltdown?  As soon as the fuel melts and runs together, it loses the moderation because the moderator is now outside the fuel mass, not between bits of it.  Ergo, the chain is broken and the reaction stops.  (In reactors using water as a moderator, losing the water also shuts down the chain reaction.  Chernobyl used graphite.)

But that doesn't stop the heat.  The OTHER thing you neglected is that the fission reaction itself is not the only source of heat in a reactor!  About 6.5% of the energy actually comes from the radioactive decay of the fission products, the daughter nuclei created by the splitting atoms.  This heat does not stop when the chain reaction stops; you have to wait for the material to "cool" as the "hottest" fission products decay away.  The stuff that decays the fastest releases heat the fastest, and goes away fastest.  Within an hour the "afterheat" is down to 1.5%, 0.4% after a day and 0.2% after a week.

Maybe you want to re-record some of your narration on your video to get those details right.  Just sayin'.

PS:  No I was not drunk when I wrote this, just fat-fingered.  All typos spotted have been corrected.



  ¶ 6/25/2019 02:25:00 PM