im no expert on nuke reactors but uranium generates heat under pressure? thats all i can say rly, like i said i dont know much on this...

What you could do is use Plutonium and Neutrons to to start the reactor. When it undergoes fission it releases Uranium, Lava, and pressure. Uranium becomes incredibly hot under pressure and if you can keep the pressure from escaping the reactor will be able to run solely on this initial reaction. This will serve as the core.

The next part is what generates the electricity. What you need to do is use something like DMND to connect the core to an area where water will evaporate, pass through a detector, and then be cooled by a heat exchanger and fall back down and flow into the the boiler again. You can look at other reactors for this part, which is the generator.

The next thing you will need is something along the lines of a Heat Exchanger or a Heatsink. This will be connected to the Generator to cool down the steam and turn it into water again. You could use a simple method such as a heat sink with cloned Liquid Nitrogen cooling or go for a slightly more advanced(and generally more effective for size) method such as NaCl(yes, this is salt) cooling.

The final thing you will need it quite simple, an output. You can do this with a simple wire, or use complex electronics and/or transformers to make electrical output more stable.

Once you've done this you have a functioning Nuclear Reactor.

if you want a simple (maybe not as effeciant) reactor, try making a core, then through a tube, make a wall that only alows air, above that put over the wall that generates electricity when stuff is inside, let the water steam out into the atmosphere like most real life reactors do. do fission at the bottom, then put water (non-conductable prefferibly) and it will steam out trough the lectricity producing wall. only problem is harnessing the electricity safly.