Essential experimental results of the CORA test program on severe core damage phenomena

In the framework of the CORA program, the chemical interactions between core materials, that may occur with increasing temperature up to complete melting, were examined. The high-temperature material behavior of PWR, BWR, and VVER-1000 fuel elements was studied in large-scale integral experiments and extensive separate-effect tests. In many cases the reaction products are liquid at temperatures above 1200 C or have lower eutectic melting points than their original components. This results in a relocation of liquefied components, often far below their original melting points. Control rod materials can separate from fuel materials by a non-coherent stage-by-stage relocation process; this may cause recriticality during flooding of a partially degraded core with unborated water. Significant relocation of UO2 dissolved in molten unoxidized Zircaloy can begin at the Zircaloy melting temperature (1760 C), about 1000 K below the melting point of UO2. Quenching (flooding) of the degraded fuel element results in locally enhanced Zircaloy-steam reactions, causing renewed temperature rise, melt-down of materials, and additional strong generation of hydrogen. (orig.)