6 ms·

> was foolishly violating the safety protocols by using a screwdriver to hold the two halves of the sphere apart. When the screwdriver slipped, the core dropped

• charles_f 2y ago
  I was also surprised. I thought you had to use an explosive to initiate the reaction. I never took the expression "critical mass" to such a literal expression, but it seems to be.
  • dudinax 2y ago
    They do experiments where they get oh-so-close to critical by dripping solution into a container.
  • psunavy03 2y ago
    Criticality is what you get in a nuclear reactor and what killed Slotkin. Supercriticality requires explosives. One is a self-sustaining chain reaction, the other is a runaway chain reaction.
    • dumah 2y ago
      No, super-criticality occurs in a nuclear reactor when ever the neutron population is increasing. You might be conflating that condition with prompt criticality.
  • Tuna-Fish 2y ago
    Criticality is simply the condition where on average a single neutron interacting with nucleii in the device will on average (through initiating fission) cause one or more additional neutrons to interact with nucleii. Geometry and mass matters here because the "default" thing a neutron does is "misses all the nucleii and exits the device", unless the device is fairly big, simply because as electrically neutral particles neutrons do no interact with electrons and only interact with nucleii when very close, so most material looks mostly like empty space to them. So in principle if you just form a large enough ball of Pu-239, it would go critical. The reason you need explosives is that in order to form that ball, you need to go from a state where there is not enough material together to go critical to a state where there is, and the criticality will immediately start releasing very large amounts of energy. This energy then heats things and drives them apart, preventing a chain reaction where the entire core goes up. In the criticality accidents listed above, that is precisely what happened. In Slotin's case, the upper half of the core kept falling on the lower half and then pushed apart.
    • pfannkuchen 2y ago
      > So in principle if you just form a large enough ball of Pu-239, it would go critical Don't neutrons lose some energy as they transit through the material? That would make this bounded in some respect anyway.
      • Cerium 2y ago
        As-in, the neutrons lose energy by hitting the material and creating more neutrons?
        • pfannkuchen 2y ago
          No, as in, as the average distance a neutron-hitting nucleus travels before the collision increases, the average energy of the neutron at collision time decreases. Or so I imagine, that's what I'm asking. The scenario was that the size of the material can increase until you guarantee a sufficiently high rate of collision, and I'm asking whether neutrons really do not lose energy as they travel prior to collision (as the scenario seems to assume).
          • simonh 2y ago
            Why would the average distance a neutron has to travel to strike a nucleus increase? I suppose it does eventually, as the number of undecayed nuclei falls, but that wouldn’t be a significant effect until the criticality reaction had very significantly progressed. In other words the reaction can’t go on forever.
            • pfannkuchen 2y ago
              > Why would the average distance a neutron has to travel to strike a nucleus increase? Because if the problem is that neutrons are escaping the object before hitting a nucleus, and we are adding more nuclei so the likelihood that they hit something increases, the new collision candidates will be further away than the old ones. In other words, adding material to the edge of the object does not affect the per distance probability of collision. It only affects the overall probability of collision. Since the per distance probability does not change while the overall probability does, the probability increase must lie outside of the average path length of a neutron through the original object.
  • NoMoreNicksLeft 2y ago
    I may be misremembering, but it seems like I've read that the explosive variation is the "supercritical mass". Critical masses aren't anything to sneeze at though, unless you like the tickle of fast neutrons massaging your internal organs.
• eig 2y ago
  You don’t need to force the halves together quickly to start a chain reaction, but you do need to put them together fast to create a bomb. If it’s not fast enough you will get a “Fizzle” [0] where some chain reaction is occurring but not over a small enough timespan to make a bomb or to stop the material from disintegrating itself. A similar slow chain reaction process is used to control energy release in nuclear power plants. [0] - https://en.m.wikipedia.org/wiki/Nuclear_chain_reaction#Predetonation https://en.m.wikipedia.org/wiki/Nuclear_chain_reaction#Prede...
  • SapporoChris 2y ago
    For further reading about the core's history. https://en.wikipedia.org/wiki/Demon_core https://en.wikipedia.org/wiki/Demon_core It's also been discussed numerous times on this board.
  • H8crilA 2y ago
    This is also what happens in nuclear reactors that go bad. These aren't full blown nuclear explosions, and if there's any major explosion at all it's usually from the hydrogen that is created when hot metals touch water.
• mlsu 2y ago
  It is fascinating for sure. I don't think there's anything in chemistry like it. It depends a lot on the geometry. A chemical reaction can be sped up or slowed down by the shape of something, but that's just because of exposed surface area. In the case of Slotin, the thing he dropped onto the core was a neutron reflector so it redirected neutrons back into the core. https://www.science.org/content/article/near-disaster-federal-nuclear-weapons-laboratory-takes-hidden-toll-america-s-arsenal https://www.science.org/content/article/near-disaster-federa... This is an interesting read, it's a story about a more recent near criticality that took place in 2011.. You can see a picture in the article of the dangerous configuration -- it's just a few rods of plutonium near each other. Any closer, if one tips over into the other, and they might go hot and release a huge amount of radiation.
  • djmips 2y ago
    Harry Daghlian dropped a neutron deflector in the first incident, Slotin allowed two halves to come together AFAIK.
    • elevaet 2y ago
      > On August 21, 1945, less than a week after Japan notified the US that it would accept the terms of the Potsdam Declaration, physicist Harry Daghlian was performing an experiment when he accidentally dropped a piece of “tamper” material, used to reflect neutrons back into the core, and triggered a critical mass. Daghlian used his bare hands to pull the mass apart to stop the chain reaction, and absorbed a fatal dose of radiation. He died three weeks later.
  • albert_e 2y ago
    > As luck had it that August day, a supervisor returned from her lunch break, noticed the dangerous configuration, and ordered a technician to move the rods apart. > But in so doing, she violated safety rules calling for a swift evacuation of all personnel in "criticality" events, because bodies — and even hands — can reflect and slow the neutrons emitted by plutonium, increasing the likelihood of a nuclear chain reaction. > A more senior lab official instead improperly decided that others in the room should keep working, according to a witness and an Energy Department report describing the incident. This part is confusingly worded. Once the dangerous configuration was noticed what was the right thing to do?
• kragen 2y ago
  you say 'such dire consequences', but given that your apparent point of comparison is atomic bombs, i would rather say that the consequences were fairly mild: no buildings were destroyed, no fallout was released, and only one person died rather than hundreds of thousands. it didn't even kill everyone in the room, and the person who it did kill survived for over a week, though possibly he wished he hadn't nuclear reactors also do not force material together at high pressure, but nevertheless achieve criticality
• colechristensen 2y ago
  Basically the faster you go from non-critical to critical, the more energy you get out of a given amount of material. If you do it slowly it just blows itself apart before it can do much real damage. If you have a small amount of material but enough to be critical and say, generate enough heat to melt itself into a puddle in a minute, it doesn't explode or anything, but before it melts and likely starts itself on fire, everybody nearby is going to get a lethal dose every few seconds. In other words, there's a lot of room between "self-sustaining nuclear reaction" and "bomb". Even storage of materials in warehouses has to be done carefully because too much too close can cause dangerous amounts of reactions.
• 1wd 2y ago
  Feynman has an interesting story about critical mass: > Los Alamos was going to make the bomb, but at Oak Ridge they were trying to separate the isotopes of uranium ... he saw them wheeling a tank carboy of water, green water - which is uranium nitrate solution. He says, “Uh, you're going to handle it like that when it's purified too? Is that what you're going to do?" They said, “Sure -- why not?" "Won't it explode?" he says. Huh! Explode?" ... he noticed certain boxes in big lots in a room, but he didn't notice a lot of boxes in another room on the other side of the same wall ... what you would have to do to fix this. It's rather easy. You put cadmium in solutions to absorb the neutrons in the water, and you separate the boxes so they are not too dense ... https://calteches.library.caltech.edu/34/3/FeynmanLosAlamos.htm https://calteches.library.caltech.edu/34/3/FeynmanLosAlamos....
• dredmorbius 2y ago
  Rehashing some of what's been said and adding to it: A nuclear chain reaction occurs where more neutrons enter into a fissible mass than leave it, where those neutrons trigger additional fission events. "Criticality" is the point at which that neutron emission is just balanced: the same number are added as are consumed. This is often fairly stable, and can be further controlled with moderating systems (e.g., control rods, circulating water, or neutron reflectors which increase neutron flow). There's also the matter of "prompt" vs. "delayed" neutrons. The first, prompt neutrons, are emitted immediately following a fission event, the latter occur after some delay, from milliseconds to minutes or longer. The ratio of prompt to delayed neutrons also matters in controlling a nuclear reaction. A nuclear reaction at criticality is not a bomb, at least not necessarily. What it is however is sustained, which is to say that the nuclear reaction will continue unless circumstances change. A nuclear bomb, and specifically a fission bomb, requires not only a critical mass but a supercritical one, with a large amount of the material going critical at once. The challenge for the engineer is that nuclear reactions release so much energy that the explosive material itself can be blown apart before enough of it has time to react. So the trick is to transition between subcritical and supercritical masses quickly. For Uranium-235, the reaction is slow enough that a "bullet-style" design is sufficient. A supercritical mass is arranged in two pieces, which are separated until detonation is desired, at which point one (usually smaller) mass is shot into the other, like a bullet down a gun-barrel. Plutonium-239 is so fissile that this would result in premature criticality and only a small fraction of the material would fission before being blown apart. Instead, an implosion design is used, in which a subcritical mass of plutonium is surrounded by explosive charges which, when detonated, compress the core sufficiently that it does achieve criticality, and the much larger nuclear explosion follows. The Uranium bullet-style device was considered sufficiently reliable that it was not tested. The Hiroshima bombing was the first detonation of this style of weapon. The Trinity test was to confirm the theory of a plutonium implosion-style design, and Nagasaki saw the second explosion of such a weapon. In the case of the Hiroshima (uranium) bomb, about 1 g of matter was converted to energy, and about 660 g of a total fissile mass of ~51 kg actually reacted, or about 1.3% of the total mass. Essentially the bomb was already coming apart before any more material could engage in fission. See: <https://old.reddit.com/r/askscience/comments/1546rcv/why_did_only_1g_of_the_hiroshima_bomb_go_through/jsqq7v9/ https://old.reddit.com/r/askscience/comments/1546rcv/why_did...> I believe values are about the same for the Nagasaki weapon. More on fission weapon designs: <https://nuclearweaponarchive.org/Nwfaq/Nfaq4-2.html https://nuclearweaponarchive.org/Nwfaq/Nfaq4-2.html>
• dghughes 2y ago
  Dropping/gravity had nothing to do with it or an impact. When it dropped it fell onto the pile and once there the total mass was now enough for it to go supercritical.