Selecting suitable ash lagoon cells is a critical operational and environmental challenge in coal-fired power plant ash disposal systems. Conventional selection methods often emphasize disposal capacity while overlooking hydraulic connectivity, recirculation performance, operational flexibility, and infrastructure interdependencies. This study proposes an integrated Multi-Criteria Decision-Making (MCDM) framework combining the Fuzzy Analytic Hierarchy Process (FAHP) and Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) to prioritize feasible ash lagoon cells at the Poplar River Power Station (PRPS) under baseline and post-project conditions. Five criteria were evaluated: remaining capacity, hydraulic connectivity, discharge accessibility, recirculation impact, and operational flexibility. AHP was initially applied to assess judgment consistency, followed by FAHP to determine criteria weights under uncertainty. TOPSIS was then used to rank three lagoon-cell alternatives, and sensitivity analysis was conducted to assess robustness. Results identified remaining capacity as the most influential criterion. Cell 4E consistently ranked highest due to its balanced disposal capacity and operational performance, while the planned expansion of Cell 5 improved its post-project ranking. Sensitivity analysis confirmed ranking stability, demonstrating the robustness and practical applicability of the proposed FAHP–TOPSIS framework for infrastructure decision-making under uncertainty.