In a generic reactor, fuel rods are carefully spaced inside the moderator — water, say — so that each neutron generated inside a fuel rod and slowed down in the modera­tor produces just one neutron when it causes fission in another fuel rod. The fuel is uranium oxide, UO2, a black powder created from UF6, pressed into pellets, sin­tered, and ground to size. The pellets are slid into thin tubes about the diameter of a pencil and 5 m long. The pellets cannot be large because the heat generated inside has to escape to the coolant. Also, since most of the uranium is U238, the neutrons have to get out of the pellet into the moderator before they are absorbed by the U238. The coolant is usually the same kind of water as the moderator, but it gets hot and carries the output energy. Hundreds of fuel rods make up a fuel assembly, and hundreds of assemblies make up the fuel load, which can weigh 100 tons. The fuel lasts about four years, and one-fourth of it is renewed each year. There have to be enough fuel to make up a critical mass, ensuring that at least one neutron from each reaction will find another U235 nucleus to split. The fuel assemblies have to be spaced just right inside the moderator for this to happen. When fuel assemblies are renewed, they are shuffled so that the new ones and the half-used ones are dis­tributed evenly. The heat produced is carried away by the coolant and is used to generate electricity at 30% efficiency in steam turbines. One ton of fuel can gener­ate 30 MW of power and 40 GW-days of energy.