Optimized core design of a supercritical carbon dioxide-cooled fast reactor

Abstract

Spurred by the renewed interest in nuclear power, Gas-cooled Fast Reactors (GFRs) have received increasing attention in the past decade. Motivated by the goals of the Generation-IV International Forum (GIF), a GFR cooled by supercritical carbon dioxide (S-CO2), fueled with Light Water Reactor spent fuel transuranics, and directly coupled with a Brayton cycle is under investigation as part of a larger research effort at MIT. While the original GFR chosen by the GIF is a 600MWth version using Helium as a coolant, the work presented here is for a 2400 MWt, core using S-CO2 as a coolant, which has comparable thermal efficiency (-45%) at much lower temperatures (650TC v. 8500C) A reactor core for use in this direct cycle S-CO2 GFR has been designed which 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 (TID) 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. Use of Beryllium Oxide (BeO) as a diluent is explored as a means for both power shaping and coolant void reactivity (CVR) reduction in fast reactors. Results show that relatively flat power profiles can be maintained throughout a batch-loaded "battery" core life using a combination of enrichment and diluent zoning, due to the slight moderating effect of the BeO.

(cont.) Combining BeO diluent with the innovative strategy of using a thick volume of S-CO2 coolant as 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. In support of assessing the neutronic self-controllability of this core, a simple first-order steady state design metric is developed, modified from other established methodology to suit the uniqueness of this core concept. The results of this analysis show that the core will passively shut itself down without violation of established core thermal limits in the event of several limiting Anticipated Transients Without SCRAM (ATWS) scenarios, except for a Loss of Coolant Without SCRAM at End of Core life. Since most of the requisites for passive core shutdown have been demonstrated within the parameter uncertainties of current estimates, the candidate core design is deemed sufficiently safe. Further, design solutions for fixing this deficiency are proposed.

(cont.) Alternative cores using traditional pin-type fuel and innovative Internally-Cooled Annular Fuel (ICAF) have also been evaluated. While the performance of the TID core is superior, the results of the pin-type core show promise, pending design modification and relaxation of the imposed core pressure drop constraint, which would come at the expense of cycle efficiency and increased decay heat removal power requirements. Nevertheless, no improvement would be able to achieve a sustainable core (i.e. conversion ratio=l) using oxide fuel without the use of external blankets for pin fuel, even without the use of diluent in the fuel. A comprehensive comparison of the thermal hydraulic and neutronic performance of TID fuel with that of the traditional pin-type fuel, as well as with the ICAF is also made, showing the fundamental reasons for their difference in performance.