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.

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.