First thermal-hydraulic analysis of a CO2 cooled pebble bed blanket for the EU DEMO

Abstract

Over the years, the Helium Cooled Pebble Bed (HCPB) Breeding Blanket (BB) has been intensively studied for the EU DEMO. However, several feasibility issues remain open for a HCPB-type DEMO reactor. Some of these issues are linked with the use of He as coolant and are related to the large size of the Primary Heat Transfer System pipework, the resulting large coolant inventory and therefore large expansion volumes required in an ex-vessel loss of coolant accident, the limited operational experience with relevant size He-turbomachinery, the large circulating power and possible considerable pipe leakage, among others. Due to the larger density of CO2, the use of this gas as primary coolant for DEMO can lead to key advantageous features, mitigating most of the issues present for He-cooling and resulting in a higher net efficiency than that of HCPB, as reported in a previous study. Therefore, a CO2-Cooled Pebble Bed (CCPB) has been proposed as an alternative coolant to He for the EU DEMO. After identifying that CO2 have a negligible influence on the neutronic performance, making the CCPB’s TBR almost equal to the HCPB’s one (TBR ≈ 1.15), a first set of thermal-hydraulic analyses with the commercial code of ANSYS CFX are reported here. The analyses are based on the newly proposed design of breeding zone (BZ) in the enhanced HCPB fuel-breeder pin concept for the EU DEMO. Despite the lower heat transfer capability of CO2 compared to He, the fuel-breeder pin design BZ improves the thermo-hydraulic performance to a point that this coolant is able to meet the materials’ temperature requirements. The results show that the CCPB can satisfy the basic thermal hydraulic requirements and that CO2 is also a realistic option as primary coolant for gas-cooled fusion reactors, with the key advantage of the existing >60 years operational experience in fission industry with this coolant.