Core design concept of the supercritical water-cooled fast reactor for in-vessel retention

The fast reactor core design concept (Super FR) of the Supercritical Water-cooled Reactor (SCWR) has been developed to manage fuel debris criticality during severe accidents. For the anticipated total core melt-down and In-Vessel Retention (IVR) of the fuel debris, the relationships between the core design parameters, the core characteristics, and the fuel debris criticality have been clarified through the equilibrium core design, neutronics and thermal-hydraulics coupled core analyses, and in-vessel debris criticality evaluation. The small (650 MWth) and the mid-size(2073 MWth) cores achieve design targets (average core outlet temperature ≥ about 500 °C; average linear heat generation rate: 15.0 kW/m; and operation cycle length > 360 days (average discharge burnup >44 GWd/t)) under design criteria (maximum cladding surface temperature ≤ 650 °C; maximum linear heat generation rate < 39 kW/m; negative void reactivity coefficient). Through design by analyses, the followings have been revealed. Firstly, reducing the fuel rod pitch to harden the neutron spectrum is an effective way to reduce debris criticality without sacrificing the core performance (core outlet temperature and average discharge burnup) but with tradeoff to the core power peaking, which may ultimately limit the core power. Secondly, enlarging the core size had limited influence on the in-vessel debris criticality as the impact of the reduced Pu enrichment was compensated by that of the increased fuel inventory. Lastly, the new concept of In-Vessel Retention with Extended Control Rod (IVR-ECR) has been proposed and preliminary evaluations have shown its high capability to achieve subcriticality of the in-vessel debris. • Supercritical Water-cooled Fast Reactor core for In-Vessel Retention is designed. • Small (650 MWth) and mid-size (2073 MWth) cores have been developed. • The control rods are extended to reach the lower plenum with HfO 2 as the absorber. • Meltdown of the core is managed subcritical in the lower plenum of the vessel.