A Three-Dimensional Multiphysics CFD Analysis of Cobalt Adjuster Units in CANDU Reactors - In Core Operation

Adjuster rods are essential components for Cobalt-60 production through core irradiation in CANDU reactors. As operational demands increase, especially at elevated power levels and extended durations, a rigorous evaluation of their performance under heightened thermal conditions becomes imperative. This study presents a comprehensive thermal hydraulic analysis focusing on cobalt adjuster assemblies within the reactor core. Leveraging advanced 3D multi-physics computational fluid dynamics (CFD) modeling, adjusters were analyzed under normal operating conditions and during a total loss of moderator inventory (LOMI) accident. The developed model integrates a conjugate heat transfer analysis, facilitating the comprehensive consideration of heat transfer mechanisms within both fluid and solid domains. Furthermore, the CFD modeling employs a multi-dimensional mesh to accommodate the diverse length scales of various components, ranging from the adjusters' bundles to the calandria vessel. The model's predictions were found to accurately capture the flow characteristics, including the transition between momentum-driven and buoyancy-driven flows for different operational scenarios. Moreover, the temperature distribution in different components was predicted, and the critical hotspots were identified. The outcome of this analysis provides an insight into the flow pattern and heat transfer affecting the adjuster units and can be utilized as an operational, design optimization, or regulatory guideline.