What makes Uranium-235 ideal for nuclear fission? Why not use another element all together?
In: Physics
U235 is great as a fission source in nuclear reactors for many reasons: abundance, reaction cross-sections and products.
Abundance: uranium is relatively abundant. In the early 50s and 60s, uranium was thought to be more rare, so alternative fuel sources such as thorium were considered. This fell out of favor after uranium was discovered to be roughly as abundant as Thorium overall, with less processing needed to produce fuel.
However Thorium 232 much like U238 are both breeder material. That is they require capturing typically a faster neutron to eventually become u233, P239 which then fissions when hit by a neutron. U235 in particular is convenient because it has a particularly high “thermal” neutron crossection or reacts with fission more readily with slower (having the equivalent energy of a room temperature) neutrons. Remember it’s easier to slow a neutron down that it is to speed it up. The other fissionable nuclides likewise have higher lower energy cross sections. Fission neutrons are typically fast, and you can create reactors that breed their own fuel to burn as fission, however those higher energy neutrons will more readily damage your vessels and mechanics, reducing the lifetime to around 20-30 years, where some light water thermal u235 reactors will have been operating for 60+ years.
Now breeder reactors do have the benefit that the higher neutrons will essentially cause all kinds of reactions reducing the overall mass and volume of waste, however it has to be processed,refined and recycled to retrieve that other 95% of potential energy, and those processes are far more expensive.
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