> But if you get it a little too close together, it goes boom. It won't go boom. One of the difficulties (of many) of the Manhattan project was that it was rea

> But if you get it a little too close together, it goes boom. It won't go boom. One of the difficulties (of many) of the Manhattan project was that it was really really difficult to get fissile material together fast enough to cause a significant explosion. Those conditions are impossible in reactors. What does happen is that gaseous hydrogen is generated and that subsequently explodes.