Plutonium-240 is a contaminant.
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 for various uranium isotopes
. Source. 
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This method was superseded by the more efficient gas centrifuges.
Gaseous diffusion diagram
. Source. 
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.
Public domain publication list: archive.org/search?query=comptes+rendus+academie+des+sciences&sort=-date&and%5B%5D=collection%3A%22pub_comptes-rendus-hebdomadaires-academie-des-sciences%22 but some years are randomly missing like 1898?
OK from here you can find all of them more clearly: www.academie-sciences.fr/en/Transmettre-les-connaissances/comptes-rendus-de-l-academie-des-sciences-numerisees-sur-le-site-de-la-bibliotheque-nationale-de-france.html
The RaLa Experiment by Our Own Devices
. Source. Has some good mentions of Uranium vs Plutonium in nuclear weapon design.Bibliography:
- Open Q&As:
- Closed Q&As:
There are unlisted articles, also show them or only show them.