At its core, a nuclear fission power plant requires:

• A nuclear core stabilizer vessel to hold fuel rods so they don't melt through the floor or burn through the ceiling too quickly.
• Nuclear fuel rods.
• A containment vessel (preferably thick walls of lead or some other strongly resistant material.)
• A method of venting the heated gas within the chamber to provide power (generally a thermoelectric generator).
• A method of cooling the chamber with cold gas/liquid.
• Radiation collectors (optional) for generating more power from less stable reactions.
• Holding tank (optional) for auxiliary coolant source in the event of leaks or pressure loss.
• Drainage tank (optional) for draining fluid from the chamber in the event of excessive radioactivity not being absorbed.

The most efficient way to build one, and the riskiest, is to do it yourself. Otherwise, you can use a Fission Reactor Flatpack with the CE's Blueprints to create an area near the edge of the ship. This flatpack includes its own cooling bank as well as an emergency vent to space should things get out of hand, and all you need to do is supply the fuel rods, begin filling the containment chamber with water, and move to the control console to activate it. It is notably not the most optimal design, though, merely the safest functional design.

https://nsv.beestation13.com/wiki/Guide_to_the_Nuclear_Reactor

https://nsv.beestation13.com/wiki/Guide_to_the_Stormdrive_Engine