Comparison research on numerical modeling methods for the passive heat removal system of fast reactors based on PLANDTL benchmark experiment

The Direct Reactor Auxiliary Cooling System (DRACS) is an innovative Passive Decay Heat Removal System (PDHRS) designed for pool-type Sodium-cooled Fast Reactors (SFR). It comprises an independent heat exchanger submerged in a sodium pool, connected to an air cooling system via a sodium loop. To validate its effectiveness, several studies have been conducted. Initially, system analysis codes were employed; however, they struggled to capture the detailed 3-D thermal–hydraulic phenomena within the sodium pool. The advent of computational fluid dynamics (CFD) has enabled a more comprehensive study of thermal–hydraulic behaviors in sodium pools and reactor cores, which include multiple fuel subassemblies and bundles. Full CFD simulations require substantial computational resources. To address these challenges, alternative methods have been proposed, such as using porous media to represent fuel bundles and employing a partial system + partial CFD approach instead of full CFD. Despite the development of these modeling methods, comprehensive comparisons assessing their applicability and uncertainty remain lacking. This study conducts four types of numerical simulations based on the aforementioned method pairs—“Bundles/Porous Media” and “Full-CFD/System + CFD”—using the PLANDTL benchmark experiment to evaluate their effectiveness. The “System + CFD” coupled approach demonstrated superior accuracy in predicting DRACS operation and its variation boundaries, with an average error of less than 4.2 %. Both models successfully captured the overall thermal–hydraulic characteristics. The rod bundles model provided more detailed understanding of natural circulation flow paths within the core and yielded more accurate temperature distribution, with average error below 4.0 %. Additionally, the simulations accurately captured core outlet backflow and inter-wrapper flow paths. The analysis revealed comprehensive temperature stratification in the upper plenum, resulting in detailed 3-D temperature distributions. These findings offer valuable insights for optimizing calculation and modeling methods and elucidate critical thermal–hydraulic characteristics essential for the innovative design of DRACS in pool-type SFRs.