The gas-cooled fast reactor (GFR) has received increased attention in the past decade under the impetus provided by the Generation-IV International Forum. The GFR given principal attention is a version using helium as a coolant. However, the work presented here is for a core using supercritical carbon dioxide (S-CO2) as a coolant, in a direct Brayton cycle, which has comparable cycle efficiency (~45%) at much lower temperatures (e.g., 650°C versus 850°C) than helium-based cycles.

A reactor core for use in this direct cycle S-CO2 GFR has been designed that satisfies established neutronic and thermal-hydraulic steady-state design criteria, while concurrently supporting the Gen-IV criteria of sustainability, safety, proliferation, and economics. Use of innovative tube-in-duct fuel has been central to accomplishing this objective, as it provides a higher fuel volume fraction and lower fuel temperatures and pressure drop when compared to traditional pin-type fuel. Further, this large fuel volume fraction allows for a large enough heavy metal loading for a sustainable core lifetime without the need for external blankets, enhancing the proliferation resistance of such an approach. It was not possible to achieve a sustainable core (i.e., conversion ratio = 1.0) using conventional pin-type oxide fuel.

Use of beryllium oxide (BeO) as a diluent is explored as a means for both power shaping and coolant void reactivity (CVR) reduction, similar to the studies carried out earlier for the sodium-cooled European Fast Reactor. Results show that relatively flat power profiles can be maintained throughout a batch-loaded "battery" core life of more than 15 yr using a combination of fissile concentration and diluent zoning, due to the moderating effect of the BeO. Combining BeO diluent with the innovative strategy of using a thick volume of S-CO2 coolant in the radial reflector yields negative CVR values throughout core life, a rare, if not unique accomplishment for fast reactors. The ability to maintain negative CVR comes from a combination of the effects of spectral softening due to the BeO diluent and the enhanced leakage upon voiding of the S-CO2 radial reflector.