Conference Agenda

The CODEX (Core Degradation EXperiment) facility has been operating since the 1990s as a versatile experimental platform for VVER- and PWR-type fuel configurations. The CODEX programme includes both fuel damage and coolability experiments using electrically heated fuel rods.

In the fuel damage tests, Zr alloy cladding materials (Zircaloy-4, E110, Cr-coated claddings) and different pellet materials (UO₂, Al₂O₃, ZrO₂, B₄C) were used, and their degradation and interactions were analysed. Some tests were representative of design-basis loss-of-coolant accident (LOCA) conditions, while others reproduced boundary conditions typical of selected severe accident scenarios. High-temperature conditions (in some tests up to 2300 °C) led to ballooning and burst of some fuel rods, oxidation of Zr components, fragmentation, and melting of parts of the bundles.

The coolability experiments were purely thermal-hydraulic tests using stainless steel cladding materials, with a typical maximum temperature of 700 °C. On the fuel rods, special sleeves were applied to simulate the ballooned sections of the fuel rods and the resulting reduction in flow area. The coolability experiments demonstrated that even highly blocked VVER-440-type bundles remain coolable under reflood conditions, with only moderate delays in quenching due to ballooning, local power peaks, or bypass flow.

The presentation will summarise the most important results of the CODEX experimental programme, which included 20 fuel damage and 34 coolability tests between 1995 and 2025.