Nuclear Fission of Uranium-235

[NewPhysi]

TL;DR Summary

Nuclear Fission of Uranium-235

I'm learning about nuclear fission for fun, well based on my research, 82% of the time that uranium-235 absorbs a neutron it will fission, the uranium-235 will briefly turn into an excited uranium-236 and after that, it will split, the other 18% of the time it will just emit radiation and produce uranium-236, that one being radioactive waste, but isn't the uranium-236 unstable to the point where it will split in the same way? What's the difference between the two situations?

I may have said something wrong, correct me please, thanks.

 

[anuttarasammyak]

Atom nuclei for larger atomic numbers become unstable due to short range of attractive strong force and larger repulsive Coulomb force. Uranium is on the verge. A neutron difference in nuclei dominates the future of the nuclei. In general even mass numbers make it stable and the odd one make it unstable.

 

[mfb]

The U-236 is produced in an excited state which is likely to fission. If it emits a photon then it goes to its ground state, which is unlikely to fission. So unlikely that it's essentially not happening at all. U-236 is still radioactive but it has a long half life and the dominant decay mode is an alpha decay.

 

[malawi_glenn]

Nuclei with a certain number of protons (Z) and neutrons (N = A-Z) can exists in different states. The ground state is the state with lowest energy and correspond to the configuration of those Z protons and N neutrons which has the highest amount of binding energy (which is equivalent to lowest total mass). But there are many ways one can re-arrange those protons and neutrons - those states are called excited states and have lower binding energy than the ground state (which is equivalent to higher total mass).

Ground states does not have to be stable and can thus decay to other nuclei and particles, but they are waaaay more stable than excited states. This is the case for U-236, its ground state is not stable, which was mentonied in the post before mine here.

Gamma decay is when the protons and neutrons rearrange in an excited state and emitts high energy photons to get rid of excess energy. Quite similar to de-excitation of eletrons in an atom which you might be familiar with.