Several natural features help ensure the nuclear plant's safe operation. The uranium fuel is formed into ceramic pellets, which resist the effects of high temperature and corrosion during operation. Also the concentration of fissionable U-235 in the fuel pellets is very low. They are made of zirconium alloy, which resists heat, radiation and corrosion. The tubes are precisely arranged as assemblies within the reactor, with spaces between them for the control rods.
Water flowing up through the assemblies removes the heat of the chain reaction.
If the neutrons are moving too quickly, fission is unlikely to occur. Any loss of water slows the chain reaction. Certain changes take place in the fuel pellets during their time in the nuclear power plant. Some of the plutonium itself undergoes fission, adding to the production of heat in the reactor. During the life of the fuel, these radioactive fragments collect within the pellets.