Requirements and realization
Similar physical properties of melt simulant:
As simulant melt a thermite mixture of aluminum with iron oxide and the admixture of further oxide additives is used. Up to 3200 kg of melt is generated by ignition of the specific thermite mixture in a concrete crucible. After completion of the chemical reaction the melt separates into the lighter oxide melt on top and the heavier metal melt at the bottom. The oxide fraction of the produced melt is composed of a mixture of alumina, calcia, and iron oxide. The composition is selected to have a wide freezing range which should be comparable to a corium melt, however at lower temperatures. The wide freezing range is important not only for the growth and stability of the crust, but also for the viscosity of the melt, which substantially increases when by onset of crystallization in the agitated melt slurry of crystals and liquid may be generated.
Long-term heat source
The initial temperature of the melt is about 1900 °C and the melt has sufficient overheat over the liquidus temperature to show the important effects also in transient tests. The simulation of the decay heat is provided by alternating additions of thermite and Zr metal to the upper oxide layer of the stratified melt.
Proper distribution of decay heat in the oxide layer and the metallic layer
Taking the reaction enthalpies of the added Zr and thermite into account, approximately 75 % of the heating power is deposited in the oxide phase and 25 % in the metal melt. In this way a rather prototypic heating of both melt phases is achieved. A further advantage of this method for the simulation of the decay heat is that experiments with reinforced concrete can be performed.
Measurement of erosion progression
To detect the time dependent erosion front and to control the course of the experiment the crucible is instrumented with NiCr-Ni (Type K) thermocouples embedded in the concrete. In most cases, their failure indicates the arrival of the melt front. To measure the melt temperatures, tungsten-rhenium thermocouples are also implemented in the concrete wall of the tests crucibles.