Dr. Sören Kliem

Reactor Safety
Phone: +49 351 260 2318

Department of Reactor Safety


Neutron physics and reactor dynamics

  • Validation and application to light water reactors and innovative reactor concepts of the Monte Carlo code SERPENT2
  • Application of the deterministic lattice code HELIOS-2
  • 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 the system code ATHLET
  • Coupling of DYN3D to the Computational Fluid Dynamics codes ANSYS CFX and TRIO_U

Plant dynamics and severe accident analysis

  • Accident analysis and analysis of plant dynamics using the ATHLET system code
  • Assessment of the impact of severe accident management measures on the progression of severe accidents in PWRs and VVERs

Monte-Carlo simulations (n- / γ-field calculations)

  • Development of the in house Monte Carlo code TRAMO
  • Fluence calculations of the of the reactor pressure vessel and internals using MCNP and TRAMO

Latest Publication

Numerical investigation on steam condensation and heat transfer in an emergency condenser tube with the thermo-hydraulic system code ATHLET: Post-test simulation of a series of COSMEA tests

Jobst, M.; Schäfer, F.; Kliem, S.


The aim of the PANAS project, sponsored by the German Federal Ministry of Education and Research, was an in-depth analysis of the energy transfer processes of condensation and evaporation in inclined pipes at high and low system pressures, as they are used in passive residual heat removal systems for GEN III/III+ nuclear power plants. Passive residual heat removal systems play an important role in controlling accidents in the design basis, but also in beyond design basis accidents. By application of innovative measurement technologies, it was possible to study condensation and evaporation processes with an unprecedented temporal and spatial resolution, providing a new and unique database for further development and validation of CFD and system codes. With the work carried out in the project the experimental database was significantly expanded and an important contribution was made to provide sufficiently validated numerical tools, which allow authorities and experts to evaluate new reactor concepts and nuclear power plants with passive
residual heat removal systems.
Based on a series of COSMEA tests conducted in 2020 and 2021, the paper presents results of post-test simulations performed with a standard version of ATHLET as well as with an improved version of the code. After an overview on the research topics, the paper discusses the heat transfer phenomena in an emergency condenser pipe, the corresponding physical models implemented in ATHLET and the input deck developed to simulate the COSMEA tests. First results of the simulations with the standard version of ATHLET showed a significant underestimation of the transferred heat flow, an overestimation of the condensate outlet temperature and, depending on the experimental conditions, an underestimation or overestimation of the condensation rate. An in-depth analysis of the results helps to identify possible reasons for the deviations. The second set of simulations with an improved version of the code shows much better agreement with the experimental data. The corresponding part of the paper describes the modifications to the source code, the physical reasons behind these modifications and the improvements obtained with the updated models.

Keywords: Condensation heat transfer; passive safety systems; system code development; ATHLET; AC2

Involved research facilities

  • TOPFLOW Facility



Foto: Gruppenbild der Abteilung


NameBld./Office+49 351 260Email
Dr. Sören Kliem250/1082318


NameBld./Office+49 351 260Email
Dr. Silvio Baier250/1113034
Dr. Yurii Bilodid250/2092020
Dr. Emil Fridman250/2092167
Jörg Konheiser250/1092416
Kerstin Kurde250/1073025
Dr. Evgeny Nikitin250/2072906
Dr. Frank Schäfer250/1172069