Nuclear Engineer

(1) Fuel: Any fissionable material. This can be either fissile material such as 233U, 235U, 239Pu, or 24lPu or fissionable material such as 232Th, 238U, or 240Pu. Most modern power reactors utilize this fuel in a ceramic form either as an oxide such as UO2, carbide such as UC, or a nitride, UN.

(2) Fuel element: The smallest sealed unit of fuel. In an LWR or LMFBR the fuel element is a metal tube containing ceramic pellets of fuel (such as UO2). In an HTGR the fuel element can be regarded as either a tiny (300 μm diameter) particle of uranium carbide coated with pyrolytic graphite layers, or as a cylindrical fuel pin composed of these fuel particles bound together with a graphite binder.

(3) Fuel assembly or bundle: The smallest unit combining fuel elements into an assembly. For example, in a LWR the fuel assembly is composed of several hundred fuel elements fastened together at top and bottom with coolant nozzle plates and with several spring clip assemblies along the length of the fuel. In an HTGR the fuel assembly is a hexagonal block of graphite with holes into which the cylindrical fuel pins are inserted. Fuel is usually loaded into a reactor core or replaced one fuel assembly at a time. A typical power reactor core will contain hundreds of such fuel assemblies.

(4) Moderator: Material of low mass number which is inserted into the reactor to slow down or moderate neutrons via scattering collisions. Typical moderators include light water, heavy water, graphite, and beryllium.

(5) Coolant: A fluid, which circulates through the reactor removing fission heat. The coolant can be either liquid, such as water or sodium, or gaseous, such as helium or carbon dioxide. It may also serve a dual role as both coolant and moderator, such as in the LWR.

(6) Coolant channel: One of the many channels through which coolant flows in the fuel lattice. This may be an actual cylindrical channel in the fuel assembly, as in the HTGR, or an equivalent channel associated with a single fuel rod, as in a LWR.