**4.1 Systems Engineering Lifecycle**

**Figure 6** illustrates the typical stages in the systems engineering lifecycle. While this model is intended to be used iteratively and potentially out-of-order throughout the lifecycle as design modifications occur, the left side of the V-model indicates a top-down approach moving from system to subsystem to component levels and the right side indicates a bottom-up approach through implementation, integration, and testing. This model is useful for both new builds (e.g., new reactor designs) or existing builds (e.g., engineering modifications).
