Modern Industrialization nuclear reactor planner

Lay out the hatches of your reactor, then let the planner run it for ten minutes of game time. You get the steam and EU/t it settles at, how much of the fuel's energy ends up as steam, how hot every hatch runs, and which parts would melt. Pick a component on the right and click hatches to place it. The link in your address bar always holds the current layout, so you can share it.

How a reactor behaves

Every fuel rod gives off a trickle of neutrons on its own, and each neutron it absorbs makes about six more. Most of those leave fast. Fuel catches slow neutrons far better than fast ones (roughly 72% of thermal hits are absorbed against 34% of fast ones), so a rod surrounded by water, which slows almost every fast neutron that scatters in it, burns hotter than one packed against other rods.

Heat is what you sell. A fluid hatch above 100°C boils its water in proportion to how hot it is, up to 8,192 EU/t of steam at the reactor's 3,250°C ceiling, and that boiling is what cools it. Heat spreads between neighbouring hatches, faster through heat exchangers and high-pressure water, and leaks out at the edge of the grid.

Running hot has a price. Uranium rods lose efficiency above about 975°C and stop reacting near 2,975°C; MOX rods start dropping earlier, at 850 to 950°C. Parts also melt: a Cadmium Control Rod at 1,900°C, a Large Heat Exchanger at 1,800°C, a Carbon Large Plate at 2,500°C. Cadmium soaks up nearly every neutron that touches it, which is how you calm a reactor that runs away; Invar plates barely conduct heat, so they insulate.

The planner assumes every fluid hatch is kept topped up and its steam is taken away. Results come from a random walk of neutrons, so two runs can differ by a percent or so.