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Today I learned: don't bring radioactive material close together... I know about criticality. But I always thought you needed a conventional explosive to kicks

• philipkglass 4y ago
  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.
• kragen 4y ago
  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.
• ars 4y ago
  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 https://en.wikipedia.org/wiki/Prompt_criticality
  • ncmncm 4y ago
    You can get an explosion without. (Cf. Chernobyl and Fukushima.) Just not the mushroom cloud.
• danielodievich 4y ago
  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.
• credit_guy 4y ago
  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.
  • joshuamorton 4y ago
    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_incidents https://en.wikipedia.org/wiki/Criticality_accident#Known_inc...
    • credit_guy 4y ago
      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 https://en.wikipedia.org/wiki/Demon_core
• surement 4y ago
  the chain reaction you get this way is more like Chernobyl than a nuclear explosion
• dTal 4y ago
  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.
  • bombcar 4y ago
    Build a high yield bomb - you could probably build a relatively effective dirty bomb with the right materials and some auto springs.
• dralley 4y ago
  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)
  • 01100011 4y ago
    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.