I've been watching an MIT course which explains the way that a nuclear bomb works. I am not a physicist or physics student, however; I am a computer scientist. As such, I am not looking for a level of detail as required from someone who studies physics. I also had a look at this question From what I understand so far can be summarized as follows:
Uranium-235 is an isotope which, when the individual atoms receive a neutron, the atoms will split, release "energy," and give off amongst other things, more neutrons flying through the air.
If there is some density and mass of Uranium-235 atoms all in close proximity, it follows that the above action will cause a chain reaction because once one atom splits and sends out neutrons, those neutrons will in turn hit other atoms which will split and do the same.
When atoms split, neutrons will often fly out and not actually cause fission in other atoms... Thus, many neutrons are "lost." To combat this, there is a shield put around a U-235 mass in one of the production bombs to help reflect neutrons back in toward other atoms to induce more fission.
My question is two-fold. If the complexity of answering these 2 questions with medium to low detail is too high for one thread, please let me know and I will remove one and separate it to another thread.
Question #1: Does subcritical mass mean that say, I could have a tennis-ball sized chunk of Uranium-235 in my hand, and NO atom-splitting will be occurring at all because it is not at critical mass? My confusion here lies in that I am picturing many many U-235 atoms in this ball, and the probability that one of those atoms will receive a neutron from the surrounding environment or another of the atoms, is relatively high. Is this an incorrect assumption here?
Question #2: How exactly would taking 2 such balls (for example), both at subcritical mass, and slamming them into each other, suddenly bring them at critical mass and fire off neutrons in a way that simply having 1 ball wouldn't already achieve if a neutron happened to hit it? My misunderstanding here probably has to do with critical mass/density. Say for example, 5,000 atoms inside of one of my U-235 balls split and then the process stops because it's not at critical mass... Would this energy emission be noticeable? It's hard for me to conceptualize just how much energy each individual atom gives off as it splits, and the relate that back to something that a human could perceive. Obviously, when one of these balls explodes in a bomb, it's safe to say that it is VERY perceptible to many people. But, where is the threshold for that? How many atoms must split for heat or some emission to be "felt?"