Analysis and optimization of the secondary circuit for the option WCLL BB direct coupling with small ESS of the EU DEMO power

The Primary Heat Transfer System (PHTS) of the European Demonstration fusion power plant (EU-DEMO) transfers heat from the reactor sources (Breeding Blanket (BB), Divertor and Limiters) to the secondary Power Conversion System (PCS). Two reference concepts of the EU-DEMO BB and the respective PHTS are currently under consideration: the Helium Cooled Pebble Bed (HCPB) BB with large Energy Storage System (ESS) and indirect coupling between PHTS and PCS, and the Water Cooled Lithium-Lead (WCLL) BB directly coupled to the PCS and with small ESS. According to the new EU-DEMO Energy Map there are significant uncertainties regarding the splitting of fusion power among different reactor sources. Because of these uncertainties 12 possible operational scenarios were formulated for each of the HCPB and WCLL options. In 2022 a design of the PCS circuit for the WCLL variant was developed, in which all the components were sized to be able to absorb the maximum possible thermal power of all reactor sources ("maximum of maximum" scenario). However, we observed that this PCS circuit could not reach the most problematic operational point A1-2, in which thermal power of BB and Divertor, was the lowest and the highest, respectively. In the present work we proposed a modified "maximum of maximum" PCS cycle for the WCLL BB option and we verified that it should be able to operate according to the A1-2 scenario. Operation of the considered PCS cycle during the plasma pulse and during the dwell phase was simulated using the GateCycle software.