The two primary isotopes of uranium, 235U and 238U, are separated from each other by chemical techniques.
Uranium 235 is a medium-heavy element, one of the heaviest and most radioactive of all naturally occurring elements. Uranium has a half life of 710 million years, meaning that half of it will have decayed into other elements after that amount of time has passed.
Uranium 235 is used in nuclear reactors as well as making nuclear weapons. It's also the most efficient fuel for nuclear reactors because it produces more energy than any other form of uranium when it fissions (breaks down).
Uranium 235 is a medium-heavy element and is used in the manufacture of armor-piercing ammunition.
Uranium is the heaviest natural element. It is heavier than lead and tungsten but lighter than gold.
The most common way that uranium 238 is used is in the manufacture of armor-piercing ammunition. It's also used to make so-called "depleted" uranium, which is used to coat the tips of some bombs and other weapons. Depleted uranium has been stripped down to leave behind just 0.7% of the original mass—so while it's still radioactive, it doesn't have any significant nuclear power.
Uranium 238 isn't used in nuclear reactors because it's too heavy: when you put a lot together, they tend to fall apart into smaller pieces rather than build up into something bigger. That means it takes a long time for them to accumulate enough energy for us humans (or aliens) to do anything useful with them!
So what about those really big reactors we sometimes hear about? Those use enriched uranium (uranium 235), which has been processed so that there are more neutrons around per atom than normal. This makes the atomic reaction happen faster—so much faster that we can use those reactions as a source of electricity or heat without waiting billions or even trillions of years before anything interesting happens!
The two isotopes are separated by their different chemical and physical properties which are not affected by radioactive decay. The separation of uranium-235 from uranium-238 is achieved through a series of chemical reactions that use gaseous diffusion and centrifugation technologies.
The two isotopes can be separated by chemical and physical differences. They are chemically identical, but have different atomic weights due to their different numbers of neutrons. Both are heavy elements, but uranium 235 has only one more proton than uranium 238 while having three less neutrons in its nucleus. This makes it slightly heavier than the other isotope which allows it to be separated from the rest using a process called fractional distillation