Publications

ntentAdvanced Nuclear Power Program

Dynamic Response and Safety Implications for Supercritical CO2 Brayton Cycles Coupled to Gen-IV Reactors

N.A. Carstens, P. Hejzlar, and M.J. Driscoll

MIT-ANP-TR-114 (May 2007)

Abstract

The supercritical carbon dioxide (S-CO₂) recompression cycle is a promising advanced power conversion cycle which couples well to numerous advanced nuclear reactor designs. This report investigates the dynamic simulation of, control strategies for, and selected transient results for an indirect S-CO₂ recompression cycle as part of a DOE/Sandia Gen-IV research project “Energy Conversion – SCO₂ Systems and Dynamic Response Analysis.”

The cycle analyzed is a 600 MWth, highly recuperated, single shaft recompression power conversion cycle with a turbine inlet temperature of 650°C. The cycle features relatively high net efficiency (~47%) at relatively low heat addition temperatures, primarily due to efficient compression. The bottom of this cycle approaches (but in the steady state does not cross) carbon dioxide’s critical point, where high fluid densities (~600 kg/m^3) allow efficient compression.

Dynamic simulation of this cycle is complicated by its key features: single-shaft constant-speed turbomachinery, main and recompression compressor in parallel, operation of the main compressor inlet very close to the critical point, and rapid fluid property changes surrounding the critical point.A dynamic simulation and control code for gas-cooled Brayton Cycle reactor power conversion systems (PCS) has been significantly modified and enhanced to use supercritical carbon dioxide as the working fluid.These modifications include the incorporation of accurate yet fast fluid properties, more detailed modeling of turbomachinery performance, and rapid yet accurate calculation of heat exchange in printed circuit heat exchangers, even with rapid fluid property changes. Of particular significance are the methods devised to overcome convergence problems caused by compression near the critical point of CO₂, and the attendant large variations in properties in the main compressor, precooler and low temperature recuperator. Coding innovations have made faster than real time simulation possible (on today’s off the shelf hardware), which makes plant simulator and control applications feasible.

This code, designated GAS-PASS/CO₂, was used to devise and investigate some of the major control strategies required to operate the cycle:high and low temperature control, and three variations of turbine bypass, and inventory control. Using these strategies various transients were investigated including part-load operation, loss-of-load, loss of heat sink, over-power, and startup/shutdown.

The results of these simulations show that the S-CO₂ recompression cycle can be controlled for the transients analyzed, including steady state operation, a variety of part-load operation methods, and the loss-of-load transient. The plant also shows the ability to move between zero and 100% power and can operate at part-load efficiently by combining several control methods. Of particular note, are the advantages of a method devised for simultaneously using properly coupled low temperature and inventory control during part-load operation to avoid rapid fluid property changes near the critical point.