Recent comments in /f/askscience

TheJasonKientz t1_j4to0cc wrote

Reply to What happens to the energy of sound in space? by full_hammer

A sound doesn’t have any energy of its own, it’s all kinetic energy of interacting matter. So when a sound wave from earth reaches the edge of the atmosphere, the molecules will just not bump into another molecule and the sound wave ceases to exist. But the molecules all still have their kinetic energy. And eventually that will dissipate as heat.

The person who talked about the tuning fork is right. I’ll add, though, that that tuning fork in space technically never made any sound because there was no medium for the vibration of the tuning fork to propagate through.

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SharkAttackOmNom t1_j4tfd7x wrote

Reply to comment by zemsten in If nuclear fission in U-235 causes the atom to be split into 2 smaller atoms (such as Kr-92 and Ba-141) then how is it that U-236 is produced as waste since the U-235 was just split into smaller peices? by Ian98766

Yes when referencing the negative temperature coefficient, that’s the water (and in general the heat of the whole core) which lowers reactivity due to inefficient neutron-slowing. The effect is even more pronounced of the water is allowed to boil to steam. The steam bubbles making “voids” which won’t slow neutrons, basically at all.

Neat trick can be played here. Thermal output can be controlled in a BWR reactor by increasing or decreasing the coolant flow. Faster flow will drive the threshold of boiling water higher, allowing more of the fuel rod to fission. If they want to slow the reactor, slow the coolant flow rate. The water will boil lower and reaction rate slows at the top of bundles. BWR control rods insert from the bottom so it can control reaction rates from bottom up and top down.

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zemsten t1_j4t7qs2 wrote

Reply to comment by SharkAttackOmNom in If nuclear fission in U-235 causes the atom to be split into 2 smaller atoms (such as Kr-92 and Ba-141) then how is it that U-236 is produced as waste since the U-235 was just split into smaller peices? by Ian98766

This is due to the temperature of the moderator (water) though, isn't it? At least mostly? This is important because a fast neutron has a higher probability of slowing down through water as a moderator when that water is more dense. More slowing down -> more thermal neutrons -> higher likelihood of a thermal fission. Fast fissions are negligible except for during source range reactor startups.

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Pedromac t1_j4t4005 wrote

Reply to comment by bikedork5000 in If nuclear fission in U-235 causes the atom to be split into 2 smaller atoms (such as Kr-92 and Ba-141) then how is it that U-236 is produced as waste since the U-235 was just split into smaller peices? by Ian98766

I feel like what you just said was so painfully obvious but it just never occurred to me the temperature would fluctuate so much at the atomic level but reading what you said makes perfect sense. Thank you for that

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