Hybrid systems analyses are conducted in two stages: simplified steady-state analysis to determine technical feasibility of a candidate system architecture, followed by detailed dynamic analysis of promising integrated system configurations for system optimization, detailed performance analysis, and control system design.

Top-level system analyses are performed using steady-state modeling and analysis tools. These analyses help establish system and subsystem boundary conditions, such as operational temperatures, pressures, and flow rates necessary to maintain overall energy balance, providing the essential information to determine if a proposed configuration is technically feasible. Introduction of preliminary economic analysis tools then allows a system designer to determine the potential economic value (return on investment) associated with the proposed integrated system configuration.

Dynamic system modeling integrates more detailed component and subsystem models, using validated component models and modeling tools where possible. For advanced subsystems, such as advanced SMR concepts employing non-water coolants or novel heat exchanger designs, validated component models may not be available. In these instances the model developer should use reasonable assumptions for subsystem design parameters and performance behavior using validated modeling tools. Separate effects tests and subsystem testing may be used to later validate those assumptions.

Dynamic simulation of hybrid system performance is where some of the key questions regarding the reliability, resiliency, and efficiency of hybrid system operation can begin to be answered. From the perspective of the reactor subsystem, these questions might include the following:

• To what extent can the power of a single reactor module be varied to accommodate load variations?

• What benefits, if any, can be realized by incorporating multiple small reactor modules in the integrated system?

• What factors limit the rate and magnitude of increasing/decreasing reactor power, including the frequency or magnitude of the proposed fluctuations in system load, frequency, and voltage?

• If rapid response time is required by the balance-of-plant, what mechanisms can be used to buffer the reactor(s) from rapid transients, and how can these be implemented with high reliability?

• What steps are necessary to develop and demonstrate system-level control that will minimize individual reactor cycling?

• How will the overall system safety analysis be performed to reflect the relationship between reactor safety characteristics and the overall system safety?

• How can electricity demand changes be managed? A decrease in electricity demand would trigger a switch from electricity generation to thermal power storage or direct use as thermal energy in the integrated industrial process. What type of interface (valving, controls, operational strategies) might be required to divert reactor thermal power to the appropriate subsystem, and what is the characteristic response associated with those components?

A well-defined dynamic simulation will provide an excellent platform for control system development. The hybrid system control must first establish control hierarchy, including priorities for electricity production (i. e. first meeting the electricity demand before considering other output streams) or allocation of the thermal energy based on the current market price for the output commodity. Second, the control system requires specific state estimators (temperature, pressure, etc.) to provide input to the control algorithms, recognizing that optimal placement of instrumentation necessary to provide these data is critical to reliable control system performance. A validated simulation of the integrated system provides an excellent virtual test bed for sensitivity studies associated with the control system design and optimization of the control system architecture.

Results of the dynamic simulation will identify areas of significant uncertainty or significant sensitivity that will impact the prioritization of subsystem and integrated system testing. While modeling and simulation can provide a vast amount of understanding on the potential performance of an integrated system, there is great value in translating simulations to hardware demonstrations (particularly non-nuclear, electrically heated demonstrations) prior to building an integrated nuclear hybrid system prototype.