Recent comments in /f/askscience

Liberty-Justice-4all t1_jbbdq3c wrote

Reply to Why do ice cubes crack in certain temperatures? by opbananas

I know why they crack at all, but I'm actually surprised that almost boiling water doesn't cracked them best.

The cracking is due to the fact that almost everything expands as it gets warmer, and if one part expands (or shrinks) more than other parts the stress forces can cause the connecting areas to snap and disconnect.

You get the same effect and can shatter glasses and mugs by very rapidly cooling or heating them (think hot tea in a cold glass, or ice cold drinks in a glass from a hot dishwasher).

My best guess on it NOT cracking in super hot water would be that it melts the surface quicker than the layer of warmed (but still frozen) ice can get thick enough to have enough stress built up to shatter free.

It's also possible the latent heat absorbed by melting off that surface is acting to cool the transition a bit, so the faster it melts the (effectively) better the cube is insulated, as sort of an ablative thermal stress guard, but I would expect that effect to be swallowed up by the vastly greater thermal difference without much notice.

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drhunny t1_jbbc5d2 wrote

Reply to comment by marcusregulus in How do plutonium based atom bombs work? What chemical reactions happen that make them explode? by L0RD_E

That's mostly correct. You can't breed Pu-239 without also breeding a little Pu-240, and you basically can't get it out of the Pu-239. Pu-240 has a very high spontaneous fission rate (atoms occasionally just fall apart, often releasing a neutron or two). At the levels present in a few kilograms of Pu, there's a random neutron every microsecond or so.

For a gun-type design, it spends a fraction of a millisecond in a configuration where a chain reaction is possible but generates a dud. And the Pu-240 spontaneous fission makes it very likely that such a dud chain reaction will happen.

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marcusregulus t1_jbay1yq wrote

Reply to comment by drhunny in How do plutonium based atom bombs work? What chemical reactions happen that make them explode? by L0RD_E

My understanding of the issue of a critical mass and nuclear fission with Plutonium is that you need Pu239. The nuclear reaction to create Pu239 also creates Pu240. Pu240, being only one mass number difference is very hard to separate from Pu239. A gun design is not fast enough to initiate a fission reaction with Pu, thus an implosion design is needed.

Basically, to generate the heat, pressure, and neutron flux to ignite a fusion reaction, takes a fission reaction first.

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