Construct an arbitrary reactor out of fuel housings, fluid compartments, and moderators to create and manipulate neutrons to create heat for steam.
To start, you'll want to produce some fuel housings, which store fuel cells and produce neutrons, and some fluid compartments, which will hold a coolant and output a heated coolant (for example, water into steam).
To further improve the reactor's output, you can create more fuel housings, which will start to cause a chain reaction, allowing the reactor to produce more heat and thus more heated coolant.
The next step up is to produce some moderator blocks out of carbon (blocks of coal) and water. This reduces the energy in the neutrons, making them less useful for heating water, but making them more likely to continue a chain reaction.
All input and output can be automated using any form of fluid and item pipe, from any side. Neutrons are emitted on the XZ plane, so it is perfectly acceptable to leave the area above and below just air.
Future updates will implement radiation as well as core meltdowns, so you should keep your reactor cooler than 773 K!
Note: This mod should be combined with another mod that has a form of steam, as well as a mod that provides fluid transport!