In today's applied light water reactor (LWR) technology, the fissile material is embedded in a ceramic matrix, pressed, and sintered to pellets, which are then filled into the cladding tube of fuel pins that are assembled to a fuel bundle. This is the most widespread and well-known concept, which is also mostly adapted for the present fast breeder reactor (FBR) technology. Many alternative fuel forms have, however, been researched, seeking simplified fabrication routes and other advantages. When fissile isotopes are coming from spent fuel that is chemically separated (reprocessed), particle fuel with its direct filling of fuel particles into the fuel pin offers several advantages. Two major types of particle fuel are discussed here: Sphere-pac and Vipac fuel.