Status of Development of the Reactor Dynamics Code DYN3D

The reactor dynamics code DYN3D has been developed in HZDR and its predecessor organizations over more than 20 years. Originally, the code was developed for the analysis of reactivity-initiated transients and accidents in Russian VVER type reactors and recommended by the IAEA as a reference code for the VVER-440/V213 reactor. In the meantime, the code became an advanced simulation tool for transients in Light Water Reactors with 16 users in 7 countries.
The DYN3D code comprises neutron statics and kinetics calculations, thermal hydraulics of the reactor core and modeling of transient fuel rod behavior. In the paper, an outline on the basic models is given.
3D neutronics is based on nodal expansion methods for hexagonal, square and trigonal geometry of the fuel assemblies. With respect to neutron energy resolution, two-group and multi-group versions are available. Besides of standard diffusion approximation, simplified P3 (SP3) transport approach for square and trigonal lattices is implemented. Neutronic calculations can be performed with resolution on fuel assembly or pin-wise level.
Neutronics is coupled to thermal hydraulics, where the reactor core is modeled by parallel coolant channels. One- and two-phase flow in the channels is described based on a four-equations model. The code comprises a simplified thermo-mechanical model of the transient fuel behavior.
Macroscopic neutronic cross section libraries containing the dependence of cross sections from fuel burn-up and thermal hydraulics feedback parameters like fuel temperature, moderator density and temperature or boron concentration can be linked to the code through various data interfaces.
Besides of the basic models, the code disposes of various special features like:

• Zone-wise inner-nodal neutron flux reconstruction at pin level
• Reactor poisson dynamics
• Consideration of history effects in burn-up
• Decay heat model with consideration of the power history before shut down
• Calculation of steady states with external source

Comprehensive efforts have been made on verification and validation of DYN3D against numerical benchmarks, dedicated experiments on neutron kinetics and thermal hydraulics as well as real plant data. Some examples for verification of new models like history effects and SP3 method on trigonal lattice will be given.
Last but not least, examples on the application of DYN3D for the simulation of reactivity initiated accidents like control rod ejection will be shown. An advanced graphical tools for the visualization of the results of calculations will be demonstrated.