Thorium Cells are mainly produced as a by-product of processing Uranium Dust in an Industrial Centrifuge. One of the products of processing 16 pieces of uranium dust with some Empty Cells produce 4 thorium cells.
To craft Thorium Cells you must first gather Thorium Dust and then compress it, using an automatic compressor or industrial compressor which will make Thorium Ingots. Thorium Ingots and empty cells are then put into an Automatic Canning Machine, which will make Thorium Cells for Nuclear Reactors.
Thorium cells are used in Nuclear Reactors to produce energy. Without proper cooling it will cause the reactor to go into a meltdown stage where it explodes, leaving a huge crater for you to fill in assuming you survive.
Thorium cells produce 5 times less energy and heat per tick than Uranium Cells but last five times as long.
Thorium Cells produce 2 EU/heat per tick (4 EU/heat if next to 1 other Cell, 6 EU/heat if next to 2 other Cells, etc.) (This behaviour can be customized inside the config files) and between 2-10 million EU per cycle (it takes several hours for thorium cell to run out). However, placing reactor cooling elements directly adjacent to Thorium Cells will affect their heat generation, with every directly adjacent cooling element beyond the first reducing a Thorium Cell's heat generation by 20% for up to a 60% reduction.
As thorium cells generate much less heat per tick, it's easier to manage the reactors, allowing much more efficient power generation per cell than other cells.