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Comment by duxup | Hacker News Reader

Sand is usually at the bottom of these containment vessels right? I wonder if that has something to do with it.

I think it's more likely that this is the result of radioactive decay. As elements decay, what was once a tiny piece of crystal or glass would develop defects. Among other reasons, the decay products will have different chemical properties from the original elements--so one atom may fit neatly into the structure 35 years ago, but today it's a completely different element, with a different number of valence electrons and a different size. The structure is fixed from 35 years ago when the core melted and cooled.

Imagine what would happen to your Lego buildings if some of the bricks got smaller or larger, changed the number of bumps, or split into two bricks.

Note that we're not just talking about metals here. For example, an element in a nuclear reactor might be a transition metal, an alkali metal, a halogen, or a noble gas at different points in the last 35 years--with radically different chemical properties.

I am definitely not an expert, but I did take inorganic chemistry and nuclear chemistry in college.

(Note that some of these changes in naturally occurring minerals, and the presence of compounds like noble gases trapped in minerals, are used to date minerals... you can estimate how long ago the mineral melted and cooled.)

It's called "radioactive spallation" I think. The high radioactivity plays fast and loose with all kinds of molecular bonds, even in inorganic materials.

Radioactive spallation is a part of it, but it's not the process I was talking about.

Pretty wild to think about how something we take for granted as never changing in daily life can actually change in human lifespans to things as fundamental as atoms and isotopes. Sure, there's radioactive decay but few interact with this.

It's really like a big mess of uncontrolled alchemy of matter shifting around at chernobyl. I know it's a actually a giant mess of nuclear chemistry going on with all sorts of unknown distributions and spatial proximities of various atoms but wild to think about.

Typically this stuff is well controlled and reactor configurations are well established as are the types of reactions and byproducts produced with fairly high certainty. Take those very detailed designs away and you have no idea what sort of materials are shifting states in these globs of materials. They have designs and can reason basic percentages of what may have ended up where but lots of guesswork and lots of those are not highly refined materials I'm sure, which means a bunch of impurities are incorporated in as well.

Funny you say that this stuff is "typically" well-controlled in a reactor, because the imbalance between different decay products and the lack of control was a major factor in the Chernobyl disaster (which is admittedly atypical).

For example, one important fission product in a nuclear reactor is xenon-135, which is produced from the decay of iodine-135, which is a product of nuclear fission. When you run the reactor, it produces iodine-135, which has a half-life of 6.6 hours, producing xenon-135.

Xenon-135 is a "nuclear poison." It is the the strongest known neutron absorber, and it absorbs neutrons in the reactor that would otherwise be used for fission. While the reactor is operating normally, the reactor neutrons will "burn off" the xenon-135, and the fission products will produce new xenon-135, and you can run these reactions in equilibrium.

If you turn a reactor off, the xenon-135 builds up to higher levels--remember, takes hours for the iodine-135 to decay. Because xenon-135 inhibits the reactor, you have to either wait for the xenon-135 to decay (9.14 hour half-life) or you have to pull out the control rods to compensate.

The Chernobyl operators turned the reactor off (not on purpose), xenon-135 built up, and then they pulled out the control rods to start it back up. When the reactor finally started up, the xenon-135 was burned off by the neutrons in the reactor, but this happened quickly, and the control rods were still out.

This is compounded by the fact that the boron control rods at Chernobyl were tipped with graphite (to increase volume/displacement at lower cost), so that when the reaction started to run away as the Xenon built up, the operators attempted to SCRAM, but the graphite tips on the leading edge of the control rods acted as neutron moderators before the the boron was in place, further accelerating the reaction and causing the reactor explosion.

"as the Xenon built up" should read "as the Xenon burnt off*"

>Pretty wild to think about how something we take for granted as never changing in daily life can actually change in human lifespans to things as fundamental as atoms and isotopes. Sure, there's radioactive decay but few interact with this.

Steel rusts. Masonry turns to sand. Wood rots.

They also dropped tons and tons of sand on the reactor core. I didn't read anything of sand below the reactor, so. My understanding is that they had two bio shields of concrete and some water tanks under the reactor.

I guess once Corium melts through that it becomes more glass than sand.

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