I still don't get why we can't use the waste, it gives off heat and radiation, so why can't we turn that into electricity? Is it not potable enough? What is the problem?
But if you get it a little too close together, it goes boom. So for all the reasons that nuclear reactors are expensive, a waste heat generator would be larger and at least as complex.
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.
A company called TerraPower is planning to build a test reactor soon.