Analysis of supercritical carbon dioxide Brayton cycles for a DEMO-like fusion power plant

Abstract

Fusion energy is a clean, abundant and almost-unlimited power source for a future decarbonised energy market. The progressive disappearance of carbon-based power produced has sparked the necessity of finding reliable energy sources that can complement renewable energies. In this context, the international ITER project is aiming on demonstrating the physical and engineering feasibility of producing net thermal power from a nuclear fusion reactor. In parallel, the European Union is designing the next step after ITER, DEMO, a power plant whose goal is to be the first nuclear fusion power plant. Currently, two designs are under research for DEMO: DEMO1, a conservative design based on a pulsed reactor and DEMO2, an optimistic approach where the reactor is working on steady-state. DEMO1 is the most established option as it is the most feasible one. DEMO1 is used as the current official design in the DEMO Baseline 2018. PROCESS is a system code developed by the Culham Centre for Fusion Energy (CCFE) that models fusion power plants from a physics and engineering point of view. This code has been used to obtain the DEMO Baseline 2018, that contains all the plasma physics and engineering parameters and constraints that define DEMO1. A series of supercritical carbon dioxide (S-CO2) Brayton cycles have been proposed for a DEMO-like fusion power plant. S-CO2 Brayton cycles present good characteristics for fusion reactors as they present good efficiency for medium range temperatures (DEMO Baseline 2018 maximum temperature is 500ºC) and good tritium recovery (tritium is expected to leak into the carbon dioxide from the reactor). A total of eleven layouts are presented, nine for the pulsed DEMO1 and two for the steady-state DEMO2 scenario. As DEMO1 is a pulsed reactor, a thermal energy storage (TES) system is proposed to cover the gaps during pulses. For each layout, an optimization process has been carried out with the electric cycle efficiency as the figure of merit. For the pulsed state cases, a maximum efficiency of 34.47% is achieved and 56.1% for the steady-state. The PROCESS code has been used to do an economic assessment of the most efficient pulsed layout and the two steady-state cases. This code estimates the plant cost and the cost of electricity (COE) of each layout. DEMO1 COE is set around 400$/MWh and 200$/MWh for DEMO2. This means that only the DEMO2 scenario is competitive with the current market prices. But subsidies are expected during the first generations of fusion power plants as happened with renewable energies.