Fission Conspiracy

[Matchka]

Now, I have BA in English so please be gentle with me if this is a dumb question. What goes on in a nuclear power plant is nuclear FISSION, right? That means they're breakin' stuff in the reactor and capturing the resulting heat. As I recall, isn't the fuel for the reaction Uranium 235? And isn't some of the funky goo that results from the reaction some isotope of Plutonium?

How does anyone (even an extremely clever nuclear physicist) break something (Uranium, atomic #92) and get something bigger (Plutonium, atomic #94)? I can see breaking Uranium into Gold (#79) and Aluminum (#13). And I can see some extremely greedy types hoarding the resulting byproducts and creating humongous airlines and soda can factories--and using Plutonium (nasty goo indeed) as the nuclear fall guy to wit, "Don't bust no U235 at home, kids, cuz you'll have to get rid of the Plutonium." It would also explain why we go nutz when someone else tries to develop the technology--gee maybe N Korea really wants to corner the soda can/airliner skin market and make a little gold on the side to buy some economy with.

I know I sound paranoid, but I can't ditch the argument. What is it about the incredibly cmplex workings of nuclear fission that I fail to understand?

[Desmostylus]

It's not the ₉₂U²³⁵ that turns into Pu.

A breeder reactor works by converting the otherwise fairly useless ₉₂U²³⁸ into Pu.

A ₉₂U²³⁸ nucleus absorbs a neutron to become ₉₂U²³⁹. After a time (half-life 24 minutes), it emits an electron to become ₉₃Np²³⁹. After a bit more time (half life 2.3 days) it emits another electron to become ₉₄Pu²³⁹.

[kanicbird]

I don't remember the actuall reaction or even much of the specifics but to give you an idea till someone else comes along, by adding a high speed neutron into the nucleus of U238 (or something like that), it will break down to a proton and an electron and become Np239 which is unstable and release 2 high speed neutrons becomming Np237. Np might become Plutonium from a natrual breakdown of Np or might require a further neutron but either way a neutron is converted to a electron and a proton raising the atomic number once again.

The conversion of the neutron to an electron and a proton will increase the atomic number but (by itself) not change the atomic mass.

[Desmostylus]

Thanks, kanicbird, that was a big help.

[Matchka]

OK, I don't understand this business about emitting an electron. It sounds like neutrons like to break apart into proton, electron (and presumably toss out a dash of energy in the process). That right?

So when my hapless U(235 or 238 or whatever) gets hit by a flying particle, one of its neutrons gets broken in the aforementioned fashion turning it into Np, which subsequently 'rots' into the still bigger Pu. Am I on the right track or am I trying to address a question that is best answered by me quitting my job and going back to school for the degree?

As an aside, are there other elements associated in any significant quantity with this reaction?

[Desmostylus]

The emission of an electron is also called beta decay. (And electrons are sometimes called beta particles). In basic terms, yes, a neutron turns into a proton by emitting an electron (and an antineutrino).

You are almost there. This bit isn't quite right:

So when my hapless U(235 or 238 or whatever) gets hit by a flying particle, one of its neutrons gets broken in the aforementioned fashion turning it into Np, which subsequently 'rots' into the still bigger Pu.

If ₉₂U²³⁵ gets hit by a neutron it'll fission. If ₉₂U²³⁸ gets hit by the neutron, it'll absorb it and turn into ₉₂U²³⁹, which then "rots" into ₉₃Np²³⁹, which then "rots" into ₉₄Pu²³⁹.

Note also that there are upper and lower bounds on the energies of the neutrons involved in the collisions. Neutrons that are too fast or too slow won't have the desired effect.

[Q.E.D.]

Not quite. The electron emission isn't stimulated by a "flying particle", it's a naturally-occuring decay process called beta decay (as you correctly assumed, it's the decay of a neutron into a proton + an electron. Well ok, it's somewhat more complex than that, but that's close enough). Notice that in those two beta decay steps, the atomic mass (239) remains the same, and the atomic number (the number of protons) increases by one. The energy released is in the form of a beta particle, an electron.

[Q.E.D.]

That "Not quite" was for Matchka, not Desmostylus.

[MC Master of Ceremonies]

As QED says, nuclear decay is an inevitable but statiscal process that happens without any 'outside' interference.

[Nametag]

We should perhaps add that this property of U-238 (grabbing neutrons and not splitting) makes it impossible to sustain a chain reaction. This is why "enriched" uranium fuel (containing extra U-235) is used. The left-over uranium from the enrichment process (containing extra U-238) is called "depleted uranium," and is used to make armor-piercing shells and stuff.

[Polycarp]

Originally posted by Q.E.D.
Not quite. The electron emission isn't stimulated by a "flying particle", it's a naturally-occuring decay process called beta decay (as you correctly assumed, it's the decay of a neutron into a proton + an electron. Well ok, it's somewhat more complex than that, but that's close enough). Notice that in those two beta decay steps, the atomic mass (239) remains the same, and the atomic number (the number of protons) increases by one. The energy released is in the form of a beta particle, an electron.

U-238 does not naturally decay except over a billion-year time span -- but in the presence of fissioning U-235, it absorbs a "slow neutron" (i.e., one traveling at a speed relatively slow by relativistic standards, a small fraction of c) to become U-239, which rapidly decays by beta emission as described above by other posters.

And one extremely minor nitpick: most of the energy from the beta breakdown is carried not by the electron but by an antineutrino which is released at the same time; different beta emissions release electrons with different energy (and hence speed) thanks to this, providing one way to detect what it was that broke down.

[Matchka]

Neat. Being an atheist is difficult sometimes. Thanks all!

[Q.E.D.]

Errrrrr....what's being an atheist got to do with understanding nuclear physics?

[kunoichi]

<sarcasm>Geez, Q.E.D., didn't you know scientists are not allowed to believe in a divine being?</sarcasm>