SIMMER-III Code Verfification

The code development has been accompanied by two extended verification/validation phases.

In a Phase-I assessment more fundamental and separate effect tests of individual code models have been performed. The 34 test problems documented cover e.g. shock-tube problems, bubble columns, liquid sloshing motions, freezing in tubes, rapid vaporization of fuel, two-phase blow-down tests. This first round of verification has set the boundary conditions for further code development and improvement.

The Phase-II assessment which dealt with more integral tests, including in-pile experiments, has now been finished and a final report is prepared. In the framework of the Phase-II assessment the neutronics part of the code has been evaluated too. This was necessary as a new neutronics model based on TWODANT had been implemented and had to be tested and compared to critical assemblies.

The SIMMER-III code development was originally mainly dedicated to fast reactor severe accident studies, but besides the investigations with sodium, most of its assessment in 1D and 2D two-phase flows was performed with air/water or vapor/water experiments. Therefore closure laws for momentum exchange functions were chosen as general as possible with regard to the materials and the original engineering correlations were not tuned to a specific experiment. The same holds for heat transfer coefficients, which of course are different for metals or water, mostly because of the difference of the Prandtl numbers. In 2D, the flow regime map is so simple (bubbly, transition and dispersed) that no material physical properties dependence is included. In 1D, it depends also on the droplet entrainment based on the Ishii and Mishima correlation. The basic strategy to utilize as general models and correlations as possible was a prerequisite of a successful treatment of very different experiments within the verification effort for the code.

In the Phase-II assessment 5 categories of code application can be defined:

  • Boiling pool dynamics
  • Fuel freezing and material relocation
  • Fuel-coolant interactions
  • Material expansion dynamics
  • Disrupted core neutronics

In the table below the different experiments and phenomena are briefly described and the experimental name or test facilities are mentioned. In case no name is entered usually a comparison with a small scale test, with a benchmark solution or a comparison with other code systems e.g. the French ERANOS neutronics system has been made.

Brief Description of Test

Facility / Test

Category 1: Boiling Pool Dynamics


Isothermal 3D bubble column


Two-phase flow with large liquid-to-gas density ratio


Void distribution of two-phase flow in a pipe


Vaporization and condensation in an internally heated boiling pool


Two phase flow: gas-jets and large bubbles

Burty + Castillejos

Vaporization and stratification of a multi-phase, multi-component material configuration


In-pile experiment of a nuclear heated fuel pool


Steady state two phase flow


Rapid depressurization of a tube initially filled with water


Category 2: Fuel Freezing and Relocation


Freezing of an Al2O3 melt in a quartz tube


Freezing of an Al2O3 melt in a quartz tube with particles


Basic freezing tests in tubes with simulation materials


Molten fuel freezing in steel tubes


Two-phase flow freezing of simulation material


In-pile post failure extended fuel motion


Fuel relocation of UO2 melt through control rod structures


In-pile structure-wall melt attack and fuel relocation


Category 3: Fuel-Coolant Interactions


Alumina thermite melt injection into sodium


Premixing and quenching of UO2/ZrO2 melt in water


Large-scale fuel-sodium interaction


FCI experiment in alumina/water system with propagation


Premixing and quenching of an alumina jet in water


Premixing and quenching of high temperature solid balls in water

QUEOS Q08, Q12

Category 4: Material Expansion Dynamics


Scaled post-disassembly expansion of nitrogen or steam in water


Post-disassembly expansion


Expansion and condensation of a large steam bubble in water


High speed two-phase flows through simulated upper core structures


Developing annular flow


Category 5: Disrupted Core Neutronics


Reactivity changes from fuel re-distributions: slumping and dispersed configurations (TWOTRAN based calculations)


Transients simulated with space-time neutron kinetics using the improved quasi-static method


Reactivity changes from fuel re-distributions: slumping and dispersed configurations (TWODANT based calculations)


Comparison of SIMMER neutronics with the ERANOS system


Table 1: List of Test Problems for SIMMER-III Phase II Assessment


The outcome of the verification/validation phase was very satisfying. SIMMER can reliably describe steady and transient two-phase flow problems and also the numerical stability is satisfactory. Fuel freezing phenomena can be simulated well reflecting also differences in fuel freezing behavior of e.g. UO2 and alumina. In the fuel coolant interaction area the code is especially suited for fuel/sodium interactions, but the satisfactory simulation of corium/water interactions gives further confidence into the code. The neutronic part has matured so far that the complicated distorted material configurations under core disruption can be described with high reliability.

A conclusion of Phase I and Phase II assessment is that the code can reliably describe phenomena and scenarios encountered in transients and severe accidents in liquid metal cooled reactors. Additionally, due to the generality and versatility of the fluid-dynamics/thermal-hydraulics modules, the code can be applied to multi-phase problems in other reactor types too.