Mekanism fission reactor calculator

Set the size of your reactor, click the grid to place fuel columns, and read off how fast it can burn before it overheats, how much steam and water it moves, and which industrial turbine turns that steam into power. The link in your address bar keeps the layout.

How the reactor heats up

Every millibucket of fissile fuel burned releases 1,000,000 J of heat, and each Fission Fuel Assembly lets the reactor burn 1 mB more per tick. That heat raises the core's temperature. How far depends on the shell: every casing block, glass block and port adds 1,000 J/K of heat capacity, so a larger reactor warms more slowly and settles lower.

Coolant pulls the heat back out, but only in proportion to how much fuel it can reach. An assembly is fully cooled when it has four or more faces open to coolant; packed side by side, its neighbours hide those faces. That is why the checkerboard is the usual layout: every column keeps all four sides clear and the reactor cools at 100% efficiency. Filling every column doubles the maximum burn rate but can cut the efficiency to a fifth or less, and the reactor overheats long before it reaches that rate.

Water carries off half as much heat per degree as sodium, so the same reactor on sodium runs twice as fast before it overheats. Either way, one millibucket of fuel per tick ends up as 20,000 mB of steam per tick. On water that is 20,000 mB of water a tick, or 400 buckets a second, and a reactor that runs out of water keeps heating with nothing to stop it.

Above 1,200 K the reactor takes damage every tick. Once damage passes 100% it can melt down, which destroys the reactor and leaves a radiation zone. Below 1,200 K it slowly repairs itself.

How the turbine turns steam into power

A turbine's steam flow is capped twice. Its Pressure Dispersers limit it to 1,280 mB per disperser for every block of volume below the complex. Its Turbine Vents limit it to 32,000 mB per vent, and in a big turbine the vents are always the tighter limit. Each mB of steam yields up to 10 J when the turbine has 28 blades and 7 Electromagnetic Coils, and proportionally less with fewer. Adding rotors adds blades but takes away wall space for vents, so the best build of a given size is a trade-off. The calculator tries every rotor count and keeps the one with the highest output.

Saturating Condensers return the steam as water, 64,000 mB/t each, which you can pipe straight back into the reactor.