Modeling for Late-Phase Hydrogen Generation in a Degraded Core

Evaluations of severe accident conditions for water-cooled reactors with metallic fuel pin cladding must consider the oxidation of this material for accident sequences that could lead to high metal temperatures in a steam environment. Such representations are included in integral accident analysis computer codes. If the oxidation causes sufficiently high temperatures to melt, or liquefies the core materials, the core geometry changes as the melt drains downward and freezes on cooler structures promoting blockages and redirection of steam flowing through the fuel assemblies. Once this configuration forms, the accident condition is characterized as the late phase of core oxidation. The Phebus in-reactor experiments investigated hydrogen generation in this compacted core state and measured the generation rates over several thousand seconds. This paper investigates the role of countercurrent steam-hydrogen flow to the debris upper surface as a limit for the generation rate and finds that this provides a close description of the behavior for the Phebus experiments. Applying this mechanism to reactor accident conditions shows how this should be considered in the Severe Accident Management Guidelines.