Which step is the last where changes can be made at moderate cost before equipment purchase?

The last step where changes can be made at moderate cost before equipment purchase is during the Detailed Design phase. This phase is critical because it involves finalizing the specifications, materials, and configurations of the design elements of a project. During this stage, designers and engineers can still make adjustments based on previously gathered data and insights from earlier phases, such as feedback from conceptual and preliminary design reviews.

In contrast, the prior stages, like Conceptual Design and Preliminary Design, begin the idea development and proceed to rough specifications, but they do not afford the same level of detail or the refined scope that Detailed Design entails. Once the Detailed Design is finalized, the project moves towards procurement and construction, where changes become significantly more costly and complicated. The Operational Readiness phase focuses on preparing the facility for actual operations, making it too late for cost-effective modifications to the design. Thus, the Detailed Design phase is indeed the last opportunity to implement changes efficiently before substantial investments in equipment are committed.