Investigating the Functional Mock-up Interface as a Coupling Framework for the multi-fidelity analysis of nuclear reactors

This paper investigates the use of the Functional Mock-up Interface (FMI) for code coupling in nuclear engineering. The FMI is a standard that defines a container and an interface to exchange dynamic simulation models. It has been developed and refined by several actors over the past two decades. This coupling standard allows seamless integrations of independent objects called Functional Mock-up Units (FMU). Since communication among FMUs is standardized, encapsulating a simulation tool and model within an FMU allows this tool and model to be coupled with any other FMU. This approach is opposed to creating dedicated code-to-code coupling interfaces and enables a more sustainable approach to code coupling in nuclear engineering. The paper showcases the utilization of the FMI standard for the simulation of an operational load-follow scenario in a Lead-cooled Fast Reactor. The primary circuit and balance of the plant are modeled using higher and lower-fidelity codes, respectively, with a third tool employed to model a simplified control system. The paper investigates the pros and cons of the proposed approach by exercising it throughout the following workflow: incorporation of the FMI standard into an existing code; setting up of models using different codes; coupling of these codes based on different architectures; simulation and post-processing of results. As an outcome, implementing an FMI interface presents itself as a judicious long-term investment for simulation software. However, users and developers should be aware of the limited FMI capabilities for the coupling of partial differential equations. In addition, a coupling standard by itself cannot address some difficulties, such as simulation restart, that are associated with the handling of a heterogeneous set of tools.