Investigating dynamic scenarios of a spherical tokamak fusion power plant

Spherical Tokamak for Energy Production (STEP) is a programme led by the United Kingdom Atomic Energy Authority (UKAEA) to design and build a fusion energy power plant that can demonstrate net electricity generation. One of the unique challenges of the STEP Prototype Plant (SPP) is the dynamic transient operations that are associated with operating as a prototype. Specific dynamic constraints linked to plasma operations translate into rapid and frequent start-up and shut down regimes relative to conventional powerplants as well as potential power fluctuations in steady state. Operational flexibility will be required for all phases of the SPP's prototypic operations. Initiating a fusion reaction requires a significant amount of electrical energy due to its high inherent electrical parasitic losses and the amount of time required to reach steady state. Similarly, the power generation cycle, used to convert thermal energy into useful electrical output, also requires a large energy input to start-up. Energy storage and auxiliary heat solutions are typically used to meet these energy demand requirements. This paper explores how the operational sequencing of dynamic scenarios such as start-up and shutdown can be adjusted to optimise energy storage requirements and reduce the reliance on the electricity grid as a power source. Another factor is the amount of heat that must be rejected to the environment during the period of transience adding crucial cooling capacity constraints. Different parameters for various energy storage technologies are explored. This includes response time, the associated footprint and the impact on commercial viability. Most importantly, each energy storage solution is evaluated against the key requirement for flexibility to enable the transient operations. This analysis has significant implications for wider power plant technology selections, the final operational profile of SPP and the possible energy storage mix of a future commercial variant.