Make reactor fuel rods. The powder is compressed and baked to produce ceramic pellets. The pellets go inside aluminum tubes. They are welded shut at both ends to make reactor fuel rods. North Korea has one fuel rod factory.
Fuel rods slide into holes in a graphite block to make the reactor core.
The tiny percentage of U235 causes the reaction, because its atoms are unstable. Enough fuel rods are massed in the core so escaping neutrons are likely to hit other U235 atoms, splitting off their neutrons, and so on a chain reaction. It is critical to mass enough rods to keep the reaction going. Too little mass means no reaction, too much an atomic explosion. Each split nucleus releases heat.