It is not something that can be easily conceptualized. However, if we consider that God created the world out of nothing, that He caused the whole being, the form and matter for instance of water, of created things to be from nothing, it is certainly not beyond his infinite power that he can change the whole substance of one thing into the whole substance of some other thing. We may be able to take as an analogy the instance where Jesus changed the water into wine at the wedding in Cana. This can be conceptualized as Jesus changing the being of water into the being of wine.
The appearance of the uranium in a nuclear reactor as you describe it is not going to come from nothing. The matter in the plutonium is the same matter that was in the uranium whether the matter of the uranium turned into energy as science might call it and then back again into matter or mass. In modern scientific theory you have mass = energy = mass as well as the conservation of mass and the conservation of energy in which they say that neither mass or energy is created or destroyed. In transubstantiation, there is no common subject that was in the substance of the bread or wine that is also in the substance of the body and blood of Christ after the conversion of the elements, neither of the form or matter.
Blood remains blood whether it is carrying carbon dioxide or oxygen and the red blood cells which are a part of the blood and which do the work of carrying carbon dioxide and oxygen remain red blood cells whether they are carrying carbon dioxide or oxygen. There is no substantial change going on here. Nor are the carbon dioxide and oxygen elements converted into one another.