Nuclear Physics Theory: What would happen?

[exdeath]

but why does simply meeting or exceeding the critical mass cause an explosion? is it that the spontaneous fission of some of the plutonium atoms (and therefore the release of some free neutrons) is enough to start the fission chain reaction on its own?

Density, prompt neutrons, and probability. A certain mass and density of fuel in a certain geometrical configuration has neutron gain < 1, = 1, or > 1. When an assembly is prompt critical you have gain > 1; the number of neutrons released by a fision cause one or more additional fision events immediately causing near instantaneous exponential chain reaction and consumption of the whole fuel source at once.

Think of it like you have a bunch of red hot coals. Seperate, they are no threat, but bring a pile of them together, and you have a potent concentrated heat source.

You wouldn't get a nuclear explosion, it would just heat up and expand rapidly and explode from the sudden heating and break apart the core and stop the reaction. This is nothing special. If you had the capacity to heat up any dense metal from room temperature to 1000s or more degrees instantly from an external source, the same thermal explosion would occur.

If handling plutonium, if you're lucky, you will be killed instantly by that explosion.