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War_Hymn t1_j96qqcd wrote

You're comparing natural decay with induced fission.

In ambient conditions, radioactive elements with unstable atomic structure are basically falling apart slowly. In turn, they only released a small amount of energy as they do so.

With fission, you speed up the process by shooting a bunch neutrons at the radioactive atoms so they fall apart much, much faster. So much faster that neutrons in the radioactive atoms explode out, hit other atoms and cause them to break apart too. If the effect is strong enough, you get a chain reaction that produces a lot of energy (but also causes all your radioactive fuel to "fall apart" faster).

Natural decay is a rickety building falling apart slowly over years or decades into rubble. Fission is when you topple that rickety building so that it hits the rickety building beside it, which then also tips over and hits another building, and another, etc. domino effect.

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superbcheese t1_j96wlzq wrote

WHO IS BUILDING THESE RICKETY BUILDINGS?!

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War_Hymn t1_j96zg0g wrote

White Dwarf & Red Giant Construction Ltd: "Meh, just slap a few more neutrons into the structure. It should hold."

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breckenridgeback t1_j96rc16 wrote

More simply: a fission reactor is designed to artificially increase (EDIT: decrease - rate go up, half life go down) the half-life of materials by a factor of a million or so, so as to release millions of years of potential decay in just a few years. It does this by using one decay to trigger another, which triggers another, and so on.

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Taxoro t1_j9a3axl wrote

It's very important to understand that fission and nuclear decay is not the same thing. You cannot compare the two.

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bradles0 t1_j99scfu wrote

is it possible to create an "induced fission" environment for the nuclear waste to help it decay faster?

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Taxoro t1_j9a46rf wrote

Not really.

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But it is possible to have different forms of fission reactors that produce wastely less waste. It is even possible to recycle some of the waste we currently have and then reduce the time to decay

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Kriss3d t1_j96uqq8 wrote

Wouldn't moving the rods further apart make less neutrons hit others? I'd think that would kill the chain reaction.

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RhynoD t1_j97ot6k wrote

Maintaining a stable chain reaction is surprisingly difficult. The neutrons that come from out of the fissioning heavy atoms are going way too fast. The neutrons aren't blasting into the other atoms so much as merging with them and causing them to become unstable. If the neutrons are going too fast, they just bounce off and nothing happens.

Yes, putting fuel pellets close to each other increases the odds that the neutrons will bounce around, slow down, and then cause another atom to split. However, unless you just put a lot together, that won't happen - at least, not at a rate that maintains the chain reaction.

Nuclear bombs get around this by using conventional high explosives to compress the nuclear fuel while also setting off another nuclear fuel with a much lower half-life that spits out a huge number of neutrons. Enough neutrons are released and the nuclear fuel has been compressed enough that even with a pretty low chance of the neutrons causing more atoms to split, enough of them split anyway and boom.

Nuclear power plants use a moderator like water, which slows the neutrons down without (hopefully) absorbing too many of them. The pellets are separated by the moderator so that as a few atoms naturally fission the neutrons are slowed down by the moderator. When they get to the other fuel pellets, the neutrons are going slow enough to have a high chance of interacting and causing more fission - releasing more neutrons, etc. The process is controlled by controlling how much of the fuel is exposed to the moderator and other fuel pellets, and by controlling "shields" made out of a material (like boron) that absorbs the neutrons without doing anything, blocking the chain reaction.

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