Solution of the 4th AER Dynamic Benchmark by Use of the Code DYN3D with a Particle-in-Cell Method for the Description of Boron Transport

The results of the 4th AER dynamic benchmark obtained by the use of the code DYN3D in the Research Centre Rossendorf are presented. The benchmark comprises the modeling of a boron dilution transient caused by switching on the first main coolant pump of a loop containing a plug of partially undiluted water. Therefore, the course of the transient is significantly affected by correct modeling of the boron transport. The results of studies on the influence of numerical diffusion during boron transport are presented. Consequently, a new numerical model based on the particle­in­cell method has been developed and implemented into DYN3D. The basic features of this model are described. It allows the complete suppression of numerical diffusion and to take into account the effect of boron enrichment in the liquid water phase in the case of boiling in a simple manner. Another important feature of the benchmark task was that each participant had to use his own macroscopic cross section library. In our analysis the MAGRU library for VVER­440 was used. A few reactor­physical key parameters had been given in the benchmark definition for optional adoptation of the cross section data. The presented analysis was carried out with a minimum amount of adoptations. Finally, only the static supercriticality of the boron dilution was given an the Doppler feedback was adopted. Some interesting thermo­hydraulic effects are discussed, which can be used for a deeper comparative analysis of the thermo­hydraulics models of the codes. Due to the low mass flow rate and high heat flux densities occuring, vapour generation during the transient is significantly affected by subcooled boiling under thermodynamic non­equlibrium conditions.