B U235 Criticality: Neutron Capture and Alpha Emission

[Congrute]

TL;DR Summary

U235 is an Alpha Emitter, so how can it reach Criticality.

How does U235 reach criticality via neutron capture when its natural decay process is via alpha emission. I know that U235 will reach criticality in a suitably shaped container after a specific concentration has been reached. In addition, I've read that the original U235 bomb (little boy) was furnished with a neutron initiator but only so as to increase the fission efficiency. According to the text I've read, the bomb would have gone off without the neutron initiator, but with a lesser yield. So, how are the required neutrons generated in either case? What am I missing? Thanks in advance.

 

[Vanadium 50]

U-235 has a small probability of decaying via spontaneous fission:
https://en.wikipedia.org/wiki/Spontaneous_fission

 

[mfb]

The alpha decay doesn't matter.

U-235 can decay via spontaneous fission, providing an initial neutron to start the chain reaction. You can't control the time of that, however. If you get a spontaneous fission event too early or too late your yield is much lower, and if you don't get spontaneous fission while the core is critical then you don't get a nuclear explosion at all. A separate neutron source can inject neutrons at the time of maximal criticality, and you are just left with the risk that spontaneous fission starts the chain reaction too early. The implosion design compresses the material faster which reduces this risk. A gun-type plutonium bomb wouldn't be practical because plutonium has too many spontaneous fission decays.