Comparative analysis of the efficiency of a CO2-cooled and a He-cooled pebble bed breeding blanket for the EU DEMO fusion reactor.

Highlights • The balance of plant of DEMO have been described in this paper. • A new kind of coolant for solid breeder blanket of CCPB have been proposed. • Comparative analysis of the efficiency of HCPB and CCPB for DEMO have been given. • Parametric analyses of BB outlet temperature and CCPB pressure drop have been shown. Abstract The EU DEMO is considered to be the last step before building a commercial fusion power plant. Up to now the Helium Cooled Pebble Bed (HCPB) Breeding Blanket (BB) is one of the four BB concepts being investigated in the EU for their possible implementation in the EU DEMO. While He is the reference gas cooling for the BBs in DEMO, CO 2 has been the coolant choice in the nuclear fission industry for gas-cooled reactors during the past 60 years. In the recent years, it is also gaining interest as a secondary coolant for the Balance of Plant (BoP) to replace the traditional vapor (Rankine) cycle. Despite the lower heat capacity and thermal conductivity of CO 2 with respect to He, the larger density of the former can lead to an advantageous performance on heat dissipation rate per unit of pumping power. This feature, coupled with its relative transparency to neutrons, the molecular stability at the operating temperatures of the BB and the industrial scale ready turbomachinery makes CO 2 an attractive alternative to He. The aim of this paper is to make a parametric study of the gross and net efficiencies and the design of a simplified BoP of a DEMO power plant based on a CO 2 -cooled compared to the reference EU DEMO HCPB BB, as a function of the BB outlet temperature and turbine inlet pressure. From this study it is concluded that the vacuum vessel and the divertor heat sources are benefitial in order to improve the net plant efficiency for both coolants. Also, it has been observed that the plant net efficiency is better for the case of a PHTS with CO 2 as heat transfer fluid under the assumption that both BBs share the same geometry. In the case that the BBs do not share the same geometry, the study concludes that a PHTS with CO 2 leaves a pressure drop margin of about 1.5 before reaching the same net efficiency as a HCPB BoP DEMO plant. [ABSTRACT FROM AUTHOR]