Well, you could start off by creating a chamber where uranium is exposed to high pressure. The heat from the uranium would then be routed to (probably by having the uranium chamber inside it) a chamber of water where the water can evaporate and (since you can't really make turbines) have DTEC on the exaughst that sends out electricity when the WTRV passes by it. That would be the basic start of the reactor itself. It would be interesting to see what kind of stuff you could add to it (change how much heat is being transferred to increase/decrease electricity being produced? Using different elements?)

A realistic one or a just nuclear one? For a realistic one do what dom2dom said or use PLUT explosions, although you will need to make a casing to contain it. For a just nuclear one you can use VIBR (SPRKs near detonation, EXOT, fusion and use solar pannels to collect the electricity, etc...