What you describe is basically a radioisotope thermal generator, like those used by space probes. In such a device, you use natural decay heat of unstable radio

What you describe is basically a radioisotope thermal generator, like those used by space probes. In such a device, you use natural decay heat of unstable radioisotopes. In a nucler reactor, things are a bit different. You start with uranium that is only slightly rsdioactive and does not produce any usable quantities of heat. You place in ib the corrrct geometry and start a controlled nuclear chain reaction. You jave neurons split uranium, which produces heat and more neutrons. Controlling this reaction so that it actually runs runs, but not so much as to melt your reactor is what, as far as I understand it, makes nuclear reactor design hard.