Uranium-236 (236U or U-236) is an isotope of uranium that is neither fissile with thermal neutrons, nor very good fertile material, but is generally considered a nuisance and long-lived radioactive waste.
The most significant contribution to uranium-236 abundance in the environment is the 238U(n,3n)236U reaction by fast neutrons in thermonuclear weapons.
However, the neutron capture cross section of 236U is low, and this process does not happen quickly in a thermal reactor.
With a much greater cross-section, 237Np may eventually absorb another neutron and become 238Np, which quickly beta decays to plutonium-238 (another non-fissile isotope).
A small number of fast reactors have been in research use for decades, but widespread use for power production is still in the future.
(Plutonium-244, which has a half-life of 80 million years, is not produced in significant quantity by the nuclear fuel cycle, and the longer-lived uranium-235, uranium-238, and thorium-232 occur in nature.)
The decay chain of uranium-238 to uranium-234 and eventually lead-206 involves emission of eight alpha particles in a time (hundreds of thousands of years) short compared to the half-life of 238U, so that a sample of 238U in equilibrium with its decay products (as in natural uranium ore) will have eight times the alpha activity of 238U alone.
Even purified natural uranium where the post-uranium decay products have been removed will contain an equilibrium quantity of 234U and therefore about twice the alpha activity of pure 238U.