SIMMER-III is a general two-dimensional, three-velocity-field, multiphase, multicomponent, Eulerian, fluid dynamics code coupled with a space-time and energy-dependent neutron transport kinetics model. The philosophy behind the SIMMER-III development was to generate a versatile and flexible tool, applicable for the safety analysis of various reactor types with different neutron spectra and coolants including the new accelerator-driven systems for waste transmutation. Currently, a three-dimensional version is also available, coined SIMMER-IV. The main backbone for analyses, however, is still SIMMER-III.

SIMMER-III has proven especially well suited for fast spectrum systems such as the liquid-metal-cooled fast reactor where it is one of the key codes for safety analysis, including its application within licensing procedures. To serve especially the last purpose, the code must be made sufficiently robust and reliable and be tested and validated extensively. A comprehensive and systematic assessment program of the code has been conducted. This paper gives the major achievements of this assessment program.

The SIMMER-III code handles by default liquid-metal-cooled fast reactor core materials - fuel, steel, coolant, control rod, and fission gas, in solid, liquid, and vapor states. The total of 27 density and 16 energy components are modeled in three velocity fields and one structure field in order that important fluid motions in a degraded core are simulated adequately. The spatial differencing method is based on Eulerian staggered mesh with a higher-order differencing scheme to mitigate numerical diffusion. An improved analytic equation-of-state model provides good accuracy especially at high temperature and pressure. Multiple flow-regime treatment is available over the entire void fraction range. An interfacial area convection model improves the flexibility of the code by tracing transport and history of interfaces and thereby better represents physical phenomena. A generalized and flexible code framework, along with improved numerical stability and accuracy, allows us to apply it to a variety of simple and complex multiphase flow problems.

The code assessment program is an ongoing effort. Two major milestones have been achieved in the past by completing two assessment campaigns, Phase 1 and Phase 2: Phase 1 for fundamental code assessment of individual models and Phase 2 for integral code assessment for key phenomena relevant to liquid-metal-cooled fast reactor safety. Through this systematic code assessment program, comprehensive validation of the physical models has been conducted step-by-step. The assessment program has demonstrated that SIMMER-III is a state-of-the-art code with advanced models sufficiently flexible for simulating transient multiphase phenomena occurring during core disruptive accidents. This paper concentrates on the specifics of the code, mainly reflected in its application to sodium experiments related to the safety of liquid-metal-cooled fast reactors.