Employing a gallium-based passive decay heat removal system (PDHRS), which enables the use of water as an ultimate heat sink, was proposed for the UCFR-100. The gallium-based PDHRS has replaced the previous sodium-based PDHRS. To evaluate the safety and thermal performance of the gallium-based PDHRS, safety analysis and heat exchanger tests were performed. The safety analysis, using both deterministic and probabilistic analyses methods, confirmed the unique safety feature of the UCFR-100. The transient analysis for design basis accidents showed the rapid cooling characteristic of the gallium-based PDHRS of the UCFR-100. In addition, compared with the sodium-based PDHRS, the probabilistic safety analysis, using the level-1 Probabilistic Safety Assessments method, showed the significant decrease of the minimal cut sets and the significant reduction of the failure probability of the gallium-based PDHRS of the UCFR-100. Finally, compared with the sodium-to-air feature in the sodium-based PDHRS, the gallium-to-water heat exchanger in the gallium-based PDHRS enhanced the heat transfer perforniance and facilitated economic advantages to the entire system. In conclusion, the gallium-water PDHRS represented risk reduction of the entire reactor system with the effective thermal performance. Copyright (C) 2016 John Wiley & Sons, Ltd.