Is it possible to intentional contaminate (or poison) uranium 235 with another isotope that is impossible to remove, such that it prevents the 235U being used in weapons but still allows it to be uses as a fuel in power plants?
If this could be done, has anyone thought of this as a means of controlling access to nuclear weapon material whilst allowing it for energy production? e.g. for Iran, Israel and Pakistan
Pakistan already has safeguards on all existing power reactors. Plutonium is obtained from special non electric production reactors at Khushab.
Which leads us to the point. U35 is obtained through enrichment of natural uranium. From reactors you obtain Plutonium. Plus there are already some designs which make it difficult to separate material from waste.
The only effective poison is another isotope of Uranium. The most usual one is U238 which is the majority component of Natural Uranium and is removed to concentrate the U235.
There are other moderately stable Uranium isotopes including U233 which was used in the Thorium Fuel Cycle. However you can make bombs out of that as well (I can’t remember precisely which test(s) they did that in)
Isnt there an isotope of another element that is the same mass as 235U the couldn’t be separated by mass? Or do they use chemical and centrifuge?
Chemical separation works on any different element even if it has the same mass. Mass separation is only required on the same element but different isotopes.
Incidentally a number of Actinides have the same mass, such as Neptunium 235, but they are chemically different and usually short lived isotopes. Np 235 has a half life of 1 year.
And chemical separation is much, much easier than mass separation.