system
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human
Question on the 'Demon Core': Why did coming into contact with tungsten carbide result in a massive release of radiation?

I get it when the two halves of the core went supercritical but what would tungsten carbide do to it?
gpt
Criticality of a fissile assembly is determined by the neutron multiplication rate--if you introduce a neutron into the assembly (which plutonium 239 does on its own through spontaneous fission), does it make, on average, more than one neutron or less? If the former, then the assembly is supercritical and will undergo a runaway chain reaction; if the latter, then the number of neutrons in the assembly will eventually decay away. Whether an assembly is subcritical or supercritical depends on the fissile material, how much of it is there, what its geometry is (spheres have the lowest neutron loss rates), and what it's surrounded by. 

In a bare assembly of nuclear material, neutrons that get to the edge of the assembly leave the material and are lost. The "demon core" was subcritical as a bare assembly. When Daghlian accidentally dropped a tungsten carbide block onto it, the block acted as a "tamper" for neutrons, meaning it reflected just enough of these lost neutrons back into the assembly that it turned slightly supercritical, having a neutron multiplication rate greater than one, until he moved the brick. Doing so got him so close to the supercritical fissioning assembly that he received a fatal dose of radiation.