4 ms·

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,

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.

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.