Fundamentals of Nuclear: Meltdowns

Quickly, and without looking online to find the answers, which of the following is a nuclear meltdown?

Most of them should be pretty easy but there is a good chance I might have tricked you, because the correct answer is none of them. I’ll explain what I mean in a bit, but first I’m going to show you what a nuclear meltdown actually looks like.

This picture was taken on March 28, 1979. Mere hours after the facility declared a meltdown. Notice the lack of any craters, clouds, explosions, or damage in general.
Cause and effect. Meltdown causes uncontrolled heat, heat causes water to split into hydrogen and oxygen while pressurizing the building with steam, heat then causes the hydrogen to recombine with the oxygen causing an explosion.

But wait, Fukushima was totally a meltdown I hear you say. Yes, yes it was. But that was not what this picture was. This picture is of a hydrogen explosion. And this is a very important distinction that too many people do not realize. Meltdowns are not Explosions.

I get why people have this misconception as everyone always hears about the Chernobyl explosion and Fukushima exploding, but it’s disingenuous and purposefully so. A meltdown that happens in proper containment can no more spread radiation all over the countryside than your bowl of ice cream you forgot about until after it melted could spontaneously coat itself across your entire living room. It needs some external force to contribute to that happening.

Children and explosions: almost impossible to distinguish without direct observation as both are concentrated methods of rapidly spreading large amounts of material across any and every available surface. They differ only in the speed at which they accomplish this.

Now meltdowns are still fairly dangerous on their own simply due to their temperature. To cause fuel bundles to melt takes several thousand degrees, (melting point of Zr used in bundle cladding is 1855 Celsius) but they can begin to buckle under their own weight as low as 550 Celsius. This can increase the fission rate of the fuel inside as it bends and gets brought closer to other fuel bundles that may also be sagging, which makes more heat, which pushes the reactor further along the path to a meltdown.

But remember, Meltdowns are not Explosions. No matter how hot the fuel gets, the fuel itself cannot and will never explode due to a nuclear reaction. This is due to there not being enough fissile material in a small enough area to create the conditions for a nuclear detonation. Hell even a solid chunk of 95% pure U235 metal the size of your fist can’t make a mushroom cloud without a lot of help. So there is no way that a bunch of 5% U235 oxide is going to have the horsepower to pull that off no matter how hot and bothered you let it get.

The heat of the meltdown might cause a steam explosion if the temperature rises fast enough and it vaporizes enough water fast enough to breach the pressure vessel or other part of the pressure loop like what happened at Chernobyl. Or the temperature might rise slowly but steadily and cause a chemical reaction between the cladding and the water to produce enough hydrogen to become explosive like what happened at Fukushima. I know it sounds like an academic difference at this point but it is very important for people to realize this because once again, Meltdowns are not Explosions! And if people keep conflating the two then we only end up with more needless fear and knee-jerk rejections because people are not getting the information they need to make fully informed decisions.

I keep harping on this because there is an example of a meltdown where no one was hurt, no landscape was contaminated, and all the safety precautions worked. And here’s the thing, it was Before Chernobyl happened and terrified the world. Three Mile Island. A nuclear plant right in the center of the US industrial heartland. Basically the worst possible place for something to go wrong. And what happened? Preparation and safety planning paid off and no one died or even was injured. Which is even more startling when you see what the core looked like after the event.


Determined state of Three Mile Island Core after the partial melt-down. 1. 2B inlet 2. 1A inlet 3. Cavity 4. Loose core debris 5. Crust 6. Previously molten material 7. Lower plenum debris 8. Possible region depleted in uranium 9. Ablated in core instrument guide 10. Hole in baffle plate 11. Coating of previously-molten material on bypass region interior surfaces 12. Upper grid damage Thank you to Wikipedia Commons for the image and labeling.

They call it a partial meltdown but this is still several tonnes of molten zirconium and uranium bearing ceramic that gave their best attempt at becoming a puddle. And yet there was no explosion, no damage to the building and no lasting impact on the surrounding environment or population. This is what proper nuclear safety looks like. Even in the face of the worst possible event, it is contained and mitigated with the loss of nothing but time and money.

Fun fact, lots of research is going in to making next gen nuclear reactors that have liquid fuels rather than the current solid fuels. There are lots of interesting reasons and persuasive arguments for liquid fuel reactors, but the most relevant to this is that you can’t have a meltdown if the fuel is already molten. If the reactor is already built to handle the rigors of containing a hot molten fuel either as a molten salt or a liquid metal then handling a sudden spike in power output is much easier because there is no pesky phase change from solid to liquid that ruins the ability of the core to shed heat.

So next time someone tries to scare you with the dreaded M word and how it would spread radiation and radioactive material all over the countryside remember this simple fact, say it with me now, Meltdowns are not Explosions!

6 thoughts on “Fundamentals of Nuclear: Meltdowns

Add yours

  1. Picky point here Sean. I studied containment aspects of TMI about 40 years ago when I worked for Atomic Energy of Canada. I recall there was a hydrogen burn inside the TMI “containment” building. It was contained. By the way, there was a movie, China Syndrome, just before the TMI accident starring Jane Fonda and Jack Lemmon. Lots of antinuclear activism then.

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    1. Cool, I hadn’t heard of a hydrogen burn happening at TMI. but if it was set on purpose and was contained by the containment building then I think that counts as procedures being used correctly. And if it was then I think that reinforces my point even more so as even though there was a hydrogen build up, it did not inevitably lead to an explosion due to the meltdown.

      As for the China Syndrome movie, yeah I cannot figure out how Nuclear’s luck is so bad when it comes to PR events like that.

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  2. I love Jack Lemmon and Jane Fonda, and it grieves me greatly to have seen them in that pack of lies., the China Syndrome. Nuclear energy is millions of times denser, per kilogram of reactants, than chemical energy. But a breach of the reactor’s core means that the reaction stops, except at Chernobyl where it momentarily increased, AS THE Manual Said It Would, when they did something stupid enough to turn the coolant into steam without reducing the neutron moderator’s effectiveness.
    That couldn’t happen with the reactors that use water as both the coolant working fluid, and the moderator.
    Even before Gorbachevended the Cold War between the USSR and us, The eminent pair Hans Bethe and Glenn Seaborg wrote a letter to the Washington Post, saying that nuclear weapons are a bad idea, and nuclear civilian power Is GOOD. Seaborg was the first person to synthesise plutoni
    um, and Bethe was one of the leaders on the Manhattan Project.

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  3. “Meltdowns are not Explosions”
    OK granted but one does not preclude the other. And there are other causes for explosions besides an instantaneous nuclear fission chain reaction. Like the chemical reaction hydrogen explosions at Fukushima that caused containment to be breached. Like the mechanical steam explosion at Chernobyl that breached containment and allowed the carbon moderators to burn unchecked.

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    1. I know it seems like a trite point to make, but no matter how simple it may sound, too many think that a meltdown is an explosion. Yes, a meltdown may cause situations where an explosion might occur, but they are not explosions themselves. That’s the point of the article.

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