David Cook

HFIR currently operates at 85 MW—following a derating from 100 MW in the early 1990s because of embrittlement of the reactor pres­sure vessel—using a U3Og-Al dispersion fuel that is 93 percent enriched in uranium-235. HFIR’s original primary mission was the production of transuranic isotopes. With the addition of the cold neutron source in 2007, the facility began hosting world-class cold and thermal neutron scattering research. The facility also meets critical needs for materials irradiation and the performance of neutron activation studies.

The reactor core is cooled and moderated by pressurized light water and is very small (50.8 cm active fuel height and 43.5 cm diameter). The HFIR core contains one inner and one outer cylindrical fuel element (see Figure 3-5). At the center of the inner fuel element is a 13 cm-diameter

hole (the “flux trap”) that contains vertical experimental targets for isotope production (californium). Outside the fuel elements is a beryllium reflector that contains additional experimental positions.

The inner fuel element contains 171 fuel plates, and the outer fuel ele­ment contains 369 fuel plates. The fuel plates, are involute-shaped, and the fuel meat is radially contoured along the involute—the fuel distribution is peaked in the center and thinner on the edges to suppress power peaking (see Figure 3-6). The inner element plates also contain a burnable poison (boron-10). These fuel plates are complex to manufacture because of the plate form and the welds at the sideplates.

HFIR has a number of unique design features that complicate conver­sion; consequently, it will be the most complex—and the last—of the U. S. research reactors to convert from HEU to LEU. Conversion will not occur until it is clear that the reactor’s primary operating missions and safety will not be significantly impacted. This includes maintaining the very high fluxes that HFIR is capable of generating, particularly in the flux trap region.

HFIR cannot be converted until an appropriate LEU fuel has completed development and qualification. Like MITR, HFIR’s unique fuel assemblies and highly compact core complicate conversion. Also like MITR, the use of high-density UMo monolithic LEU fuel along with additional changes in the reactor design is likely to allow for conversion. The new fuel will be 19.75 percent enriched in uranium-235 and have a density of 15.5 gU/cm3.

Radial

Contouring

Profiles

FIGURE 3-6 Radial contouring of fuel plates for reference LEU fuel design. The reference LEU fuel elements are shown in cross — section. The inner fuel element, shown on the left, will be more dramatically asymmetric than the outer fuel element, shown on the right, although both fuel elements are noticeably asymmetric. SOURCE: Cook (2011). Image courtesy of Oak Ridge National Laboratory.