Supercritical CO2 Brayton Cycle Integrated System Test (IST) TRACE Model and Control System Design

by Michael Hexemer, Kevin Rahner & Brett Siebert
Knolls Atomic Power Laboratory

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A TRACE model of a 100kW S-CO2 Brayton loop has undergone major updates to include a detailed representation of the IST facility heat source and heat sink. The model has been the primary tool for thermal-hydraulic and control system design for all three sub-systems: oil heating system, chilled water system, and Brayton loop. This model forms the basis for defining test operations and pre-test predictions. Important lessons learned from building the model include how to correctly model highly throttled valves in TRACE, the need to extrapolate turbomachinery performance maps, and the effect of oil (PG-1®) fluid properties on code calculational stability. The resulting IST model is able to effectively simulate nearly any operation where the CO2 remains above the supercritical pressure (1070 psia). The current model has gone through many iterations as component design and control features have been revised to meet overall functional requirements. For example, system valves, heat exchangers and control strategies have been modified during a preliminary design and analysis period. Important lessons learned through this process includes the impact of over-sizing Brayton loop and cooling water system components on transient performance, as well as the relationship between control strategy and compressor surge margin.

The lessons learned during the updating of sub-systems and subsequent transient analysis will be discussed in detail in this paper. The importance of performing detailed transient analysis early in the S-CO2 plant design and why a simple full power heat balance should not be used to select system components will be highlighted.