HFIR is a beryllium-reflected, light-water-cooled and -moderated, flux-trap type reactor that uses highly enriched uranium-235 as the fuel. The image on the right is a cutaway of the reactor which shows the pressure vessel, its location in the reactor pool, and some of the experiment facilities.
The preliminary conceptual design of the reactor was based on the "flux trap" principle, in which the reactor core consists of four annular regions of fuel surrounding an unfueled moderating region or "island" (see cross section view). Such a configuration permits fast neutrons leaking from the fuel to be moderated in the island and thus produces a region of very high thermal-neutron flux at the center of the island. This reservoir of thermalized neutrons is "trapped" within the reactor, making it available for isotope production. The large flux of neutrons in the reflector outside the fuel of such a reactor may be tapped by extending empty "beam" tubes into the reflector, thus allowing neutrons to be beamed into experiments outside the reactor shielding. Finally, a variety of holes in the reflector may be provided in which to irradiate materials for later retrieval.
The original mission of HFIR was the production of transplutonium isotopes. However, the original designers included many other experiment facilities, and several others have been added subsequently.