Brennstäbe

Brennstäbe are elongated tubes that enclose nuclear fuel and form the basic units of fuel assemblies in nuclear reactors. Typically, the fuel consists of uranium dioxide pellets stacked inside a corrosion-resistant metal tube, called cladding. The cladding is usually a zirconium alloy chosen for its low neutron absorption and good resistance to radiation and corrosion.

The inner space of a Brennstab includes a small plenum at the top to allow for fuel swelling and the buildup of fission gas. End plugs seal the pellets, and the rods are assembled into bundles held together by spacer grids to create a fuel assembly that can be inserted into the reactor core.

In operation, neutrons cause fission in the fuel, releasing heat that is carried away by the reactor coolant—commonly water. The coolant circulates through the gaps between rods and around assemblies, transferring heat to a secondary circuit that drives a turbine to produce electricity.

In many reactors, the uranium fuel is enriched to a few percent of U-235. The cladding serves to contain fission products and minimize neutron absorption, while withstanding high temperature, pressure, and irradiation during operation.

Fuel management involves replacing rods after they reach planned burnup limits. Spent Brennstäbe are removed from the core and stored in cooling pools or in dry cask storage; reprocessing may be used in some facilities and jurisdictions.

The term Brennstäbe is German for fuel rods; in English-language nuclear literature they are referred to as fuel rods or fuel elements.