Development of the coupled 3D neutron kinetics/thermal-hydraulics code DYN3D-HTR for the simulation of transients in block-type HTGR

At the Forschungszentrum Dresden-Rossendorf (FZD), the Light Water Reactor (LWR) dynamics code DYN3D is extended and adopted for the application to block-type High temperature gas-cooled reactor (HTGR). DYN3D is a two-group diffusion code for 3D steady-state and transient core calculations based on nodal expansion methods. In addition to the neutron kinetics, it disposes of a thermal-hydraulics model for flow in parallel coolant channels. Macroscopic cross section data libraries precalculated with variation of burn-up, reactor poisons concentrations and thermal-hydraulic feedback parameters are linked to the code. Recently, a multi-group version of the code was developed.
In this paper, we give an overview of the latest developments of DYN3D concerning block-type HTGR.
The SP3 transport approximation is implemented into the multi-group DYN3D code to take anisotropy of the neutron flux and heterogeneity of the core more precisely into account. The SP3 method previously implemented into DYN3D for square fuel element geometry of LWR is being extended for hexagonal geometry of the graphite blocks, where the hexagons are subdivided into triangular nodes to be able to perform a systematic mesh refinement.
The main challenge in cross section generation for the HTGR core calculations is the treatment of the so-called “double heterogeneity”. The Reactivity equivalent Physical Transformation (RPT) approach is applied in order to eliminate the double-heterogeneity of HTGR fuel elements in HELIOS calculations. The full core analysis of the reference simplified HTGR core is performed with DYN3D using macroscopic nodal cross sections provided by HELIOS. The DYN3D results are verified against full core Monte Carlo simulations.
A 3D heat conduction module coupled with a channel-type coolant flow model is implemented to take into account the temperature reactivity feedback to neutronics physically correctly. It is shown that there is significant redistribution of the produced heat by heat conduction between the graphite blocks.