The code RETRAN-3D has been extensively applied in the project STARS at PSI to perform BE analysis for a range of operational and other (non-LOCA) transients for the Swiss NPPs, which include both PWRs and BWRs. Extensive assessment employing experimental and plant data has provided confidence in the applicability and accuracy of RETRAN-3D for the analysis of transients in both types of LWRs. In this context, this paper presents an in-depth study of the performance of the code for two types of applications. First, a detailed analysis of BWR/6 recirculation pump trip tests shows that the code is able to accurately predict the coupled neutronic and thermal-hydraulic behaviour during an operational transient in which plant-specific system features (e.g. SRI insertion, response of the recirculation line valves to changes in flow, etc.) play an important role. Second, RETRAN-3D is applied to BE analysis of a PWR beyond-design basis scenario, namely the failure of the RHR system during reactor shutdown. In-depth assessment of the results obtained demonstrates the applicability of RETRAN-3D and the need for employing a BE approach to predict a more accurate, shorter "time window" for operator remedial action than that estimated using the assumption of uniform primary temperature distribution.