EU-DEMO Concept design of chemical and volume control system: Design main drivers and challenges

The EU-DEMO nuclear fusion power plant features four independent Chemical and Volume Control Systems (CVCS): one each for the Breeding Blanket (BB), Divertor Cassettes (DIV CAS), Divertor Plasma Facing Components (DIV PFC), and the Vacuum Vessel (VV). Due to the large wetted surfaces (both inside and outside the VV) and the high Corrosion Rates (CRs) of in-vessel materials, a significant amount of metal is released into the coolants of the Breeding Blanket Primary Heat Transport System (BB PHTS). As a result, the CVCS associated with the BB PHTS – an auxiliary system responsible, among other functions, for the purification and chemical control of the primary coolant – faces far more demanding requirements than those of similar systems used in the Reactor Coolant Systems (RCS) of Light Water Fission Nuclear Power Plants. The CVCS is a critical system, as it ensures the integrity of pressure boundary materials and helps manage radiation dose levels in the environment. This study focuses on comparing the design-driving parameters of the EU-DEMO BB PHTS CVCS with those of fission nuclear power plants such as the AP1000. It examines how these differences impact system size and integration within the Tritium Building. A key point highlighted is the relationship between the high CRs in DEMO and the system's required performance – especially the need for high purification flowrates to keep the concentration of activated corrosion products in the primary coolant within acceptable limits. This, in turn, necessitates the use of very large components, including valves, piping, heat exchangers, resin bed demineralizers, degasifiers, tanks, and pumps. In contrast, the CVCS of fission reactors like the AP1000 operates under less demanding conditions: lower CRs (mainly due to the material used and the corrosion law of the fission nuclear plants), smaller wetted surfaces, and reduced coolant volumes all contribute to smaller system dimensions and more relaxed spatial requirements. Furthermore, the complexity and scale of the EU-DEMO CVCS – exacerbated by stringent safety design constraints – demand an innovative engineering approach to ensure the system can be integrated without compromising the overall plant layout. In conclusion, while CVCS systems in fission power plants benefit from relatively mild operational conditions, EU-DEMO's CVCS must overcome unique challenges inherent to nuclear fusion. These challenges, especially regarding material corrosion and system scale, require dedicated design strategies and tailored engineering solutions to achieve a feasible and efficient system.