system
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human
Why doesn't neutron reflection cause a fission reaction?

So I just stumbled upon the "Demon core" story about the plutonium core going critical via neutron reflection. I was wondering though, why didn't the core undergo a fission reaction when the tungsten carbide bricks were placed around it? Wouldn't it just take 1 single neutron bouncing off the shielding to collide with the mass and cause it to go into fission? To my understanding that's how the atomic bombs in Nagasaki and Hiroshima were detonated.
gpt
A single neutron can only fission at most one atom, releasing a tiny bit of energy. If the neutron splits an atom then more neutrons are released. What happens then depends on the geometry.

If - on average - the neutrons from one fission reaction produce more than one fission reaction you get a chain reaction. If the neutrons produce less than one fission reaction then you do not get a chain reaction. That number is called criticality.

Neutron reflectors can increase that average, more plutonium closer together increases it as well.

You can make a nuclear weapon by simply increasing the criticality to above 1 and then waiting for a random neutron to start the chain reaction but that is unreliable. All nuclear weapons come with a small neutron source to start the chain reaction at the right time - normally close to the point where the criticality reaches its maximum, but in some cases lower yields might be preferred.