Fuel rod

Boiling water reactor fuel

In boiling water reactors (BWR), the fuel is similar to PWR fuel except that the bundles are "canned". Each fuel bundle is surrounded by a thin zircaloy channel box which directs the flow of water inside each bundle. This is primarily done to prevent local density variations from affecting neutronics and thermal hydraulics of the reactor core. In modern BWR fuel bundles, there are either 91, 92, or 96 fuel rods per assembly depending on the manufacturer. A range between 368 assemblies for the smallest and 800 assemblies for the largest BWR in the U.S. form the reactor core. Each BWR fuel rod is backfilled with helium to a pressure of about 3 standard atmospheres (300 kPa).