The FLEX reactor, a graphite-moderated two-fluid molten salt reactor concept that will produce energy at a lower cost than unabated coal and gas and could be deployed in the early 2030s, is introduced. Some of the unique features of the FLEX reactor are presented, including: the reactivity control thermometers, a passive replacement for control rods; the residual heat removal system, which passively removes decay heat; and the inherent safety features of the molten salt fuel and coolant. A description of the current reactor physics modeling and validation and verification strategy is provided.

It is argued that the tools and methods selected, which already exist and have been applied to operating nuclear reactors, are appropriate for modeling all relevant physics phenomena for the FLEX reactor. It is also argued that some unique features of circulating-fuel molten salt reactors, such as delayed neutron precursor transport, are of low significance to the FLEX reactor. It is also argued that a specific zero-power reactor is not required for substantiation of the FLEX reactor design or safety case.

A combination of code-to-code comparisons, appropriate benchmarks, and a stepped commissioning procedure for the first-of-a-kind reactor is expected to provide the required level of confidence. Examples of the MoltexFLEX approach to modeling reactor physics phenomena particular to the FLEX reactor are presented. Finally, an appendix is included that contains a more detailed description of the FLEX WIMS model. In this paper reactor physics is defined as including neutronics and reactor kinetics, but not thermal hydraulics.