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PlutoniumChemist t1_j4q3tia wrote

This is a great question, and yes. 239U has a half life of less than 30 minutes and will radioactively decay into 239Np. This happens because there are too many neutrons in the 239U nucleus, so one of the excess neutrons will "decay" into a proton & an electron (a form of radioactive decay called beta minus decay). When that happens, the nucleus goes from 92 protons to 93 protons, which makes it Np instead of U. The 239Np will then decay into 239Pu using the exact same method with a half life of a couple minutes.

That's how we create 239Pu for nuclear weapons.

The 239Pu has a half life of 24,000 years, so only a small amount of it will radioactively decay during irradiation. Some of it will fission in the reactor, but the rest of it has a chance to "capture" more neutrons to form heavier isotopes of Pu, like 240Pu, 241Pu, & 242Pu. If this is allowed to go on long enough, then the Pu is no longer suitable for a nuclear weapon. This is why commercial power reactors don't produce weapons grade Pu - the fuel sits in the reactor too long and produces these heavy isotopes of Pu

241Pu can decay into 241Am, which can capture neutrons and decay into heavier elements, which can then capture neutrons and decay into heavier elements again, etc etc. This process can repeat all the way up to the element Fm inside a nuclear reactor.


TerjiD t1_j4q996f wrote

This is a really interesting explanation. Made me smarter.

I gotta ask though - is "decay" the correct term, when an element goes up in protons?

Only curious, just trying to get smarter.


PlutoniumChemist t1_j4qdvq8 wrote

There are multiple types of radioactive decay, some make the atomic number go up, some make the atomic number go down, but it's correct to refer to them all as radioactive decay.


cited t1_j4qfpjm wrote

Remember it also goes down a neutron when it does it. Everything wants to get to the happy line on the chart of the nuclides.


biggyofmt t1_j4qn9ek wrote

This type of decay is known as Beta minus decay. A beta minus particle (aka an electron) and a photon are emitted from the nucleus. This has the effect of raising the atomic number by one, as a neutron was converted to a proton. It is still decay because particles were emitted from the nucleus


Wiz_Kalita t1_j4s134f wrote

It's decay because the atom is going from a higher energy state to a lower energy state. It's very much like rolling down a hill except instead of picking up speed it converts potential energy into radiation.


kira01070 t1_j4ran62 wrote

Graat explanation!! Just curious, why stop at Fm?


PlutoniumChemist t1_j4rcme7 wrote

Beta minus decay is the only mode of radioactive decay that increases atomic number.

These heavy radionuclides become more and more likely to decay by alpha decay or by spontaneous fission as their atomic numbers increase and less likely to decay by beta minus.

The half lives of these radionuclides also decrease as they get heavier, meaning they don't "survive" long enough to reliably capture neutrons before they decay into something else

By the time you get to Fm, there are no accessible beta minus decaying isotopes of Fm that can be created through neutron capture before the nuclide decays by some other mode.

This logic applies to nuclear reactors. Nuclear weapons have... a much higher neutron flux. This means the Fm isotopes could potentially capture a very large number of neutrons in a very short period of time in order to create an exotic beta minus decaying isotope of Fm before it decays by some other decay mode. Not sure if this was ever observed during the various nuclear weapons testing phases across the world