SIMMER Code Family

The SIMMER code family is an important tool for transient reactor analyses. It is developed by JAEA (Japan) in cooperation with CEA (France) and KIT.

In Europe the code is furthermore applied by research partners of EdF (France), PSI (Switzerland), SCK-CEN (Belgium), ENEA and University of Pisa (both Italy), and JRC (EU organization).


Code Features

SIMMER-III (SIMMER-IV) is a 2D (3D) fluid-dynamics code coupled with a structure model and a space-, time- and energy-dependent neutron dynamics model.



During a hypothetical severe accident, the basic reactor materials may exist in different physical condition. SIMMER can treat up to seven liquids/particles fields and one gas components to move independently exchanging momentum, heat and mass and interact with available solid structure. The numeric of the fluid-dynamics module utilizes a 2nd order difference scheme in space and time.

One of the features of the code is the tracking of interfacial areas (IFA) with their respective sources and sinks. Pool and channel flow regimes are treated. The code utilizes an elaborate equations-of-state (EOS) system, describing fuels, steel, various coolants (sodium, water, HLMs), absorber and gases in their different aggregate states continuously up to the critical point. The fluid-dynamic part of the code is designed in a very general way; it is not restricted to the simulation of accident phenomena in reactors and can be run independently from the neutronic module to solve many engineering problems.

The neutronics module provides nuclear heat sources based on the mass and energy distributions calculated by the other code elements. The transient neutron flux distribution is calculated based on the improved quasi-static method: the spatial shape of the neutron flux is evaluated by the PARTISN code by solving the full time-dependent neutron transport equation, while its amplitude is evaluated by point-kinetics equations. The decay heat is taken into account separately. For ADS application, a time dependent external neutron source with a space-energy distribution defined by the user has been integrated into the kinetics equations.

The fluid-dynamics and neutronics equations are usually solved on different Eulerian meshes giving the necessary flexibility and fulfilling accuracy conditions.


Application Field

The original application field of the code, the behavior of a largely degraded SFR core during a hypothetical severe accident, has been widely extended within recent years. Nowadays, the code is applicable to

  • Critical and subcritical reactor systems
  • Fast reactors cooled by sodium (SFR), heavy-liquid metal (HLM), or gas (GCR)
  • Molten-salt reactors


Meanwhile, the application range stretches from steady-state to accident Initiation Phase (IP) potentially followed by core degradation, till the Post-Disassembly Expansion (PDE) phase with assessment of mechanical energy release.

Click for a 60 s slideshow of a SIMMER-III simulation for a ULOF accident scenario of a SFR.


Code Validation/Verification

The SIMMER code has undergone an elaborate and systematic validation and verification phase with major components being thoroughly tested: fluid-convection algorithm and overall solution scheme, interfacial area and momentum exchange models, heat transfer, and phase change phenomena. The boiling pool dynamics has been studied, fuel freezing and fuel relocation problems and Fuel-Coolant Interaction (FCI) issues. The material expansion dynamics of the PDE phase was considered and analyses were performed of reactor experiments for core disruptive conditions.


KIT Code Developments

The TRANS group of KIT is the major driver for the neutronic development of SIMMER with important improvements attributed to

  • External neutron source (ADS)
  • Heterogeneity effect (water-cooled systems)
  • Core thermal expansion / control rod driveline (CRDL) expansion reactivity feedback
  • Implementation of PARTISN allowing parallelization
  • Optimization of the energy group structure of cross-section libraries


Fluid dynamic developments are related to models for reactor components (e.g. internal pump, internal heat exchanger), equations-of-states (EOS) for molten salt fuels, fuel simulants and HLM coolant, and a k-e turbulence model used for molten-salt reactors.

Other efforts are on the coupling of SIMMER with other codes: fuel performance codes for a better representation of irradiated fuel behavior; improved initialization of SIMMER on basis of SAS-SFR results; the coupling to structure codes to include elasto-plastic deformation of the reactor vessel.

KIT is engaged within a three-lateral framework with JAEA and CEA to develop the next generation of the SIMMER code.



  1. Sa. Kondo et al, “SIMMER-III: A Computer Program for LMFR Core Disruptive Accident Analysis”, JNC TN9400 2001-002, Japan Nuclear Cycle Develop. Institute (2000).
  2. W. Maschek et al, “The SIMMER Safety Code System and its Validation Efforts for Fast Reactor Application”, Proceedings of the PHYSOR 2008, Interlaken, Switzerland, September 14-19, (2008).
  3. H. Yamano et al, “First 3-D Calculation of Core Disruptive Accident in a Large-Scale Sodium-Cooled fast reactor”, Annals of Nuclear Energy, 36, pp. 337-343, (2009).