From HBM's Nuclear Tech Wiki

Fluids, generally being liquids, gases, or even plasmas, are an important part of the Nuclear Tech Mod, as fluids take a major step in your progression and help you create new things that you will use in your playthrough and allow you to progress further in the mod. Generally, these fluids are transported via the different fluid ducts the mod offers, or in the container items, also being able to be stored for long-term bulk in tanks & barrels. Some of these many fluids are either radioactive, corrosive, flammable or combustible, allowing them to be used in their own respective ways such as being used as heat or energy for your machines and base. Another major paper of fluids in the Nuclear Tech Mod is in the Petroleum Processing. From it, you will be able to get the many different fuels which will help you get even more fluids that will help you in the mod progression. Fluids are also a major part of the mod Chemistry, which is generally made in the Chemical Plant, from it, you will be able to make many other fluids and also items.