Results of the SSM-SICOPS melt tests of the EU-SAFEST project

Within the EU-SAFEST project, the Swedish Radiation Safety Authority (SSM) proposed to perform tests on molten corium-concrete interaction (MCCI) with basaltic concrete and BWR-specific corium, characterized by a higher Zr/U ratio than PWR corium. These tests (denoted SSM-1a/2b/2c/3) were carried out in Framatome’s SICOPS facility in Erlangen. Once the melt initially containing ≈75 wt% UO2, ≈17 wt% ZrO2, was generated and its interaction with the concrete had started, metallic Zr was subsequently added to the melt from the top. All tests showed a fast concrete ablation by the melt, faster than in previous SICOPS experiments without Zr addition, most likely due to the higher temperature of the melt caused by the Zr oxidation. The absence of crusts and even erosion profiles at the bottom seen during post-test examination support the assumption of a homogeneously mixed melt pool during MCCI. In the presence of high melt temperatures and the vivid mixing by rising concrete decomposition gases, local crusts between melt and concrete should not be stable and no evidence for them has been found. As the formation of such crusts is seen as the main potential mechanism for anisotropic concrete ablation in the early stages of an MCCI, the investigated conditions with Zr being present in the melt can be considered capable of suppressing this effect. This is important since the core melt, after its release from the RPV, typically contains a high fraction of non-oxidized Zr. While no crusts were found at the interface, sampling in the late phase of the SSM-3 test revealed evidence for the formation of a kind of transition zone between melt and concrete. This conclusion was drawn based on tactile feedback during sampling from the bottom of the melt pool where a soft/viscous zone could be felt and from the appearance of taken samples including small solid aggregates from the transition zone. Gas measurements during SSM-3 showed a significant H2 production, due to the reaction of water from the concrete with metallic zirconium. The peaks in the H2 concentration occurred when the concrete erosion velocities were highest. This confirms that, even at high erosion and superficial gas rates, the Zr in the bulk is still capable of reducing the percolating steam.