I’ve been binge watching YouTube about nuclear criticality events, and I get the basic principle that you have two radioactive items and they come in contact long enough to create an event.
But I am not quite getting it. So for example, I watched a video about the demon core, but I am not sure how the shims were preventing criticality.
Thank you.
In: Physics
First its important to understand what criticality isn’t and is. Something being critical does not mean that it will instantly explode like a nuke, but it does mean the temperature will start to ramp up and will ramp faster the further into the critical region the mass is
A mass of fissile material(atoms that can be split like U-235 or Pu-239) is critical when every time an atom randomly splits it causes one other to split. If it causes more than 1 other atom to split then its super critical, if it causes less on average then its sub critical. You can change the chances of a chain reaction by reflecting the neutrons back in so they get another shot, or adding more material so its likely to hit something on the way out, or by squeezing the material so its denser so its harder to miss.
If you have a 9.9 kg sphere of Plutonium-239 it is sub-critical so it’ll sit there being kind of warm and fairly radioactive but it’ll stay the same temperature because its subcritical.
If you have a 10 kg sphere of Pu-239 then you have a critical mass of it and the sphere will get really hot and be quite radioactive. If you do anything to squeeze it or put a beryllium shell around it to reflect the neutrons back in then it’ll become supercritical and the temperature and radioactivity will both start to rise. This is what happened with the Demon Core, they dropped the Beryllium shell closed around it so their relatively small radioactive core was now supercritical and the reaction rate started to climb causing a lot of radioactivity
Now you’ll notice that it didn’t become a bomb. That’s because there are levels of super critical. If you get it supercritical so each event causes 1.001 events then the rate of radioactivity will climb slowly, but if each event causes 3 then it will climb very quickly.
A critical mass of U-235 has 15 splits per second. If you make it 10% above the critical level then after one step you have 16 splits per second, and in 10 steps you’ll measure 39 per second. If you squeeze it down in a nuke and reflect all the neutrons back in so that every neutron causes a split then the first one to split causes 3 which cause 9 which cause 27 and 10 steps later you’re at 59,000 and 50 steps later you’re at 7×10^23.
While something being “super critical” means its going to get warmer just being super critical isn’t enough for a nuke. The core needs to be *very* super critical
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