Uranium in TPT is not fissile. Plutonium however is, and when mixed with neutrons, it detonates. Its difficult to properly explain all of how the radioactive elements section works, so perhaps checking out the wiki section may clarify your questions. The wiki can be found as a tab on the top tabs bar here on the website. Once you are there, click radioactive section. There you may find most of your answers. If not, please come back to this thread and fire away.
@fanjet(View Post) Yes. That is how the majority of plutonium bombs are created. A more spontaneous way of making a bomb is through using compressed deuterium [Deut]. To compress the deuterium, put a lot of it in a container and use the cooling tool to slowly compress the deuterium. The colder the deut, the more explosive it becomes after coming in contact with neutrons. I have a save with highly compressed deut somewhere. You can copy and past it into your own bombs.