Dr. Sören Kliem

Reactor Safety
Phone: +49 351 260 2318

Nuclear Reactor Safety

The work within the topic “Nuclear Reactor Safety” is concentrated on the research of safety aspects of currently operating nuclear reactors and new reactor types being under development in the neighboring countries. It covers the development of methods for analyses of transients and postulated accidents, the investigation of ageing phenomena in materials and integrity of components, and the safety related aspects connected with the thermal fluid dynamics of multi-phase flows. Especially after the Fukushima accidents in 2011, the efforts on the investigation of accidents management measures in nuclear reactors have been increased.

The work is organised in the following four research themes:

Safety of the reactor core

Safety of the reactor core

The reliable prediction of the coupled neutron-kinetic and thermal hydraulic processes within the reactor core during abnormal operation and postulated accidents is one of the key research areas of the reactor safety.

On that background the current work conducted at the Reactor Safety Division of the Institute of Resource Ecology is concentrated on:

  • Validation and application to light water reactors and innovative reactor concepts of the Monte Carlo code SERPENT2
  • Development, verification and application of the in-house reactor dynamics code DYN3D
  • Extension of the DYN3D code to innovative reactor concepts
  • Coupling of DYN3D to system and computational fluid dynamics (CFD) codes

Multi-phase flow model development and experiments

Multi-phase flow model development and experiments

Multi-phase flows occur in many industrial processes, but are of special importance in safety analyses of nuclear reactors. The available simulation tools still have considerable deficiencies to predict transient multi-phase flows. The reliability of prediction of the spatial and temporal flow characteristics does not yet meet the requirements for nuclear safety analyses.

Accordingly, the work within this theme conducted at the Computational Fluid Dynamics Division and the Experimental Thermal Fluid Dynamics Division of the Institute of Fluid Dynamics aims at:

Multi-phase flow model development and experiments
  • Development and validation of local, physics-based, geometry-independent closure models reflecting the mass-, momentum and energy transfer between the phases
  • Development of innovative measuring techniques for computational fluid dynamics (CFD) model qualification
    • wire-mesh sensor technology
    • ultrafast X-ray tomography
  • Conduction of industry-grade validation experiments at the TOPFLOW test facility

Structural materials

Hot cells
HZDR/Rainer Weisflog

Structural materials of nuclear reactors suffer degradation of their mechanical properties due to fast neutron irradiation. A detailed mechanistic understanding has not yet been achieved and is subject of intense worldwide research. The work of the Structural Materials Division at the Institute of Resource Ecology is focused on:

  • understanding the mechanisms of defect formation at the nm scale
  • elucidating the effect on the mechanical properties

The resulting new insight substantially contributes to the scientific background required for the safety assessment of nuclear reactors.


  • Long term irradiation effects on pressure vessel steels of running nuclear power plants
  • Irradiation effects on advanced structural materials for extreme conditions


  • Hot Cell Laboratory for the preparation and mechanical testing of radioactive samples
  • Ion irradiation at the Ion Beam Center to emulate the effect of neutron irradiation

Beyond design basis accidents

Beyond design basis accidents

This research area concentrates on safety relevant investigations of phenomena and processes for beyond design basis accidents/severe accidents. Safety analysis and assessment of the nuclear power plant behaviour and response form the basis for the topic. Collaborations within national and international research projects facilitate this research. The work performed at the Reactor Safety Division at the Institute of Resource Ecology is focused on:

  • Assessment and optimization of accident management measures (preventive domain)
  • Assessment and optimization of severe accident management measures (mitigative domain)
  • Effectiveness of the applied accident management and prediction of the time margins for operators’ interventions

The investigations contribute to the safety assessment of nuclear reactors.


  • Computer codes: ATHLET, ATHLET-CD, ASTEC, ANSYS