Development of boron dilution model in COBRA-EN

The dilution characteristic of concentrated boron in reactor core is very important for reactivity control during the Small Break Loss of Coolant Accident (SB-LOCA). Nonuniform distribution of the boron concentration may increase the risk of reactivity accidents. However, tracking the boron concentration inside the primary coolant system meets lots of challenges. Hence the development of a thermal-hydraulic code with accurate boron tracking model is necessary. This paper presents a multidimensional boron transport model based on subchannel software COBRA-EN. The original model gives a poor prediction in tracking the boron concentration of high Re, since it only considers the mass exchange caused by pressure difference and ignores the mass exchange put forward by the turbulent diffusion. In this paper, a new tracking model was proposed and implemented in the COBRA-EN code, in which the turbulent diffusion was considered. Then a 3×3 fuel bundle model was adopted to verify the proposed model. The CFD study was also conducted to investigate the feasibility of the new tracking model. The results indicate: (1) the modified boron tracking model are more close to the actual than the original model; (2) the modified model was verified by FLUENT; (3) the model shows defects at low Re due to the ignorance of molecule diffusion.