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Comment by anonu | Hacker News Reader

Today I learned: don't bring radioactive material close together...

I know about criticality. But I always thought you needed a conventional explosive to kickstart the chain reaction.

It's only fissile material that behaves this way, not all radioactive material.

The supercritical state must be reached very quickly (with explosives) to get city-destroying power levels out of a few kilograms of plutonium. But getting enough power out of a few kg of supercritical plutonium to kill someone standing nearby is easy to do even accidentally.

You don't need the conventional explosive to kickstart the reaction - you need it to bring the material close together quickly, otherwise the early stages of the reaction blow the material apart before the chain reaction really gets going. That is, if you want to build a bomb.

Build a high yield bomb - you could probably build a relatively effective dirty bomb with the right materials and some auto springs.

You can start critical chain reaction with your hands holding a bit of Pu each, by bringing them together. What you get with implosion is keeping that chain reaction going for a ~hundred generations exponentially cracking atoms so it goes all kabloey.

It's not necessary. You just need to increase the neutron density to a point where the fission reaction can take place. You can do that by:

* Reflecting neutrons (as in the demon core incidents)

* Bringing too much fissile material together in the same place (the cause of other accidents, the "little boy" nuclear weapon)

* Compressing a subcritical mass of fissile material with explosives such that the increased density makes it critical (most nuclear weapons)

Don't forget about neutron moderators like water or graphite. Pouring water on a pile of fissile material can be enough to induce criticality. The fast neutrons interact with hydrogen atoms in water to produce thermal neutrons which are much more likely to participate in a chain reaction.

Nope, the conventional explosive is just there to keep the chain reaction from blowing the critical mass far enough apart to stop itself. Nuclear reactors do not use conventional explosives, as you probably knew.

To make it explode you are correct, you need to bring a lot of material together very fast.

This explains the difference between the two types: https://en.wikipedia.org/wiki/Prompt_criticality

You can get an explosion without. (Cf. Chernobyl and Fukushima.) Just not the mushroom cloud.

the chain reaction you get this way is more like Chernobyl than a nuclear explosion

You're absolutely right.

The idea that bringing a few bars of plutonium next to each other for a photo could result in a criticality event is completely bogus.

The real issue of course, is that you want people at a facility that handles plutonium to adhere very strictly to procedures. If you don't freak out when they take some selfies, next day they'll do something else. And one day, one of these things will be stupid and lots of people will get hurt.

I mean no, it's completely correct. Criticality happens when there's enough plutonium in a small enough space (or not enough plutonium, but in a space with a high density of neutron reflectors).

The demon core incident(s), the Cecil Kelley criticality incident, and others don't involve explosives. Just read through the wikipedia list[0], and you'll see that causes included dropping a tungsten brick, an unexpected pool of water, and a human torso all acting as neutron reflectors and causing unexpected criticality.

Like, dissolved plutonium solutions have to be one of the most wickedly, unexpectedly dangerous things out there, because pouring them from one container into another, stirring them vigorously, or standing too close to them can all cause them to go from "safe" to "deadly".

[0]: https://en.wikipedia.org/wiki/Criticality_accident#Known_inc...

Those examples, especially the Demon Core, had much higher quantities of plutonium. The Demon Core [1] was a sphere of 6.2 kg of plutonium surrounded by neutron reflecting tungsten carbide.

The photo in this article shows a few small sticks of plutonium. They are not in contact with each other. They are not surrounded by neutron reflecting materials. The chance of criticality in that configuration was insignificant.

Of course, any journalist worth his salt will make it look like we very nearly avoided an Armageddon.

I stand by my statement: the danger in that situation was not the plutonium configuration itself, but the attitude towards safety of the workers. In that immediate instance the chance of a criticality excursion was close to nil, but if not reprimanded, those workers could have done something much more dangerous next time.

[1] https://en.wikipedia.org/wiki/Demon_core

I like your thoughts here. Not sure why your parent comment is getting downvoted.

There are probably multiple facets to this story as the article alludes to: a government funded private lab raking in the big bucks and government trying to rein them in a bit with this violation in an overly punitive way. Rendering then useless so production goes elsewhere...

Somewhere between known incidents with 6.2 kg of plutonium and nothing happened incidents with some sticks there’s a point at which something could happen.

And modeling exactly where that point is is difficult, hence the rules and restrictions.

The reaction of those involved lends much more credence to the “stop being idiots and clean this up Homer” reading.

> Somewhere between known incidents with 6.2 kg of plutonium and nothing happened incidents with some sticks there’s a point at which something could happen.

Yes, but that somewhere is an epsilon below the demon core. How do I know? All nuclear bombs have a core, that's more or less the size of the Demon Core. They don't go critical until they need to.

Here [1] is a photo of scientist holding a plutonium pit with nothing but gloved hands. He is smiling. He does not look like someone thinking that the likelihood of a criticality event is all that high.

Now take another look at the few sticks of plutonium in the photo. Draw your own conclusions.

[1] https://lajicarita.wordpress.com/2012/10/16/cmrr-is-no-more-...

I was curious so I went to check, it seems that the minimum size (as a sphere) for a critical mass of Pu-239 would be around 10cm.

https://en.wikipedia.org/wiki/Critical_mass#Critical_mass_of...

Wasn’t the issue that you can make a core just below critical and shove it above critical by wrapping it in reflectors?

Ya, your probably right. But your original comment is terribly argued (actually, its not argued at all. Its just stated)

I think you're at least somewhat underestimating the quantity of plutonium in those pictures, plutonium is dense enough that there's likely more Pu in that photo than in a modern nuclear warhead.

For context, modern warheads use 3kg, much less than the demon core, and the pit that you link to the guy holding is likely a hollow shell of plutonium that is then covered in steel or beryllium. The pit itself is usually ~5cm, but would be closer to 3cm if it were solid Pu.

Like, assuming that the sharpie in the controversial image is a normal one, each of the top row cylinders are approximately 6 inches long and 1 inch diameter cylinders, meaning that the top cylinders are each 1.5kg of Pu, so that image probably has like 10kg+ of Pu, almost double what was in the demon core and 3-4x what's in a modern warhead (and I think I'm generally being conservative in those estimates, so it could well be closer to 15kg!)

And the fun part is that they’re cylinders with nothing separating them, so could be easily rolled together, by accident. And apparently they were arranged by using a glovebox, so someone’s hands being used to separate them would act as a water mediator to slow the neutrons. Fun times!

History is replete with examples of this

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