Because you can generate plutonium-239 from uranium-238 in a breeder reactor, and then separate the plutonium-239 from the Uranium simply by using chemistry methods because you've created an element with different valence electrons.
Isn't it somewhat funny that it is easier to purify a synthetic element than a naturally occurring one?
Thorium-232 by Ciro Santilli 37 Created 2024-08-14 Updated 2025-07-16
GPHS-RTG by Ciro Santilli 37 Created 2024-08-14 Updated 2025-07-16
Figure 1.
GPHS-RTG diagram
. Source.
Figure 2.
Cassini probe's RTG before installation
. Source.
Polonium-209 by Ciro Santilli 37 Created 2024-08-14 Updated 2025-07-16
Polonium-208 by Ciro Santilli 37 Created 2024-08-14 Updated 2025-07-16
Plutonium-240 by Ciro Santilli 37 Created 2024-08-14 Updated 2025-07-16
This isotope shows up as an inevitable contaminant in Plutonium-239 for nuclear weapons, because it emits neutrons too fast and makes it harder to assemble the critical mass without fizzle.
Wikipedia explains that Pu-240 is formed by Pu-239 Neutron capture:
About 62% to 73% of the time when 239Pu captures a neutron, it undergoes fission; the remainder of the time, it forms 240Pu.
so its presence is inevitable.
Plutonium-239 by Ciro Santilli 37 Created 2024-08-14 Updated 2025-07-16
This is the isotope that is produced for nuclear weapons by irradiating Uranium-238 with a neutron.
Plutonium-240 is a contaminant.
Plutonium-238 by Ciro Santilli 37 Created 2024-08-14 Updated 2025-07-16
Strong alpha emitter. Can be used as an atomic battery.
Figure 1.
Plutonium-238-oxide pellet glowing under its own heat
. Source. Unlike for nuclear applications, we don't need the pure metal, so the oxide 238PuO2 is used instead as it is more chemically stable.
K-25 by Ciro Santilli 37 Created 2024-08-14 Updated 2025-07-16
Thermal neutron by Ciro Santilli 37 Created 2024-08-14 Updated 2025-07-16
These are neutrons that have reached the thermal equilibrium according to the Maxwell-Boltzmann distribution after having bounced around many times without undergoing neutron capture.
Good fissile material is material that is able to absorb thermal neutrons and continue the reaction, because that's the type of neutron you end up getting the most of.
Neutron cross section by Ciro Santilli 37 Created 2024-08-14 Updated 2025-07-16
Figure 1.
Uranium-235 neutron cross section as a function of neutron energy
. Source.
Figure 2.
Neutron cross section for various uranium isotopes
. Source.
Figure 3.
Neutron cross section of two isotopes of Boron as a function of neutron speed
. Source.
Gas centrifuge by Ciro Santilli 37 Created 2024-08-14 Updated 2025-07-16
Gaseous diffusion by Ciro Santilli 37 Created 2024-08-14 Updated 2025-07-16
This isotope separation method was the first big successful method, having been used in the Manhattan Project, notably in the K-25 reactor.
This method was superseded by the more efficient gas centrifuges.
Figure 1.
Gaseous diffusion diagram
. Source.
Comptes rendus de l'Académie des Sciences by Ciro Santilli 37 Created 2024-08-14 Updated 2025-07-16
Apparently there were biweekly reports, that were grouped and published biannually on January and July, each one with a sequential tome number.
For example, both Marie Curie's Polonium paper and Marie Curie's Radium paper were published in the second half of 1898 and fell in tome 127.
TODO how to download a PDF from? I can't even turn the pages...
Cobalt-60 by Ciro Santilli 37 Created 2024-08-14 Updated 2025-07-16
Wikimedia project by Ciro Santilli 37 Created 2024-08-14 Updated 2025-07-16
Uranium ore by Ciro Santilli 37 Created 2024-08-14 Updated 2025-07-16

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