Abstract
The QUENCH-LOCA bundle test series was launched to investigate the influence of the secondary hydriding phenomena on the applicability of the cladding embrittlement criteria. Seven out-of-pile bundle tests with different zirconium alloy-based cladding materials were performed according to a temperature/time-scenario typical for a LBLOCA in German PWRs. Each bundle contained 21 electrically heated rods with length of about 2 m. For two tests pre-hydrided claddings were used. The profilometry measurements performed over whole length of the post-test claddings showed formation of not only main ballooning area (with burst) but also additional two or three ballooning regions. Cladding wall thinning from 725 to 350 μm due to ballooning was observed at the burst side along 50 mm below and above burst opening. Oxide layer formed after the burst at the inner cladding surface around the burst opening with a thickness of about 15 μm decreasing to 3 μm at a distance of about 20 mm from the burst opening. Hydrogen enrichments (secondary hydriding) were observed for rods having been exposed to peak cladding temperatures of more than 1200 K. The average maximal hydrogen concentration inside the hydrogen bands was less than 1000 wppm and the local absolute maximal hydrogen concentration in these regions was less than 1800 wppm (neutron tomography). A part of the hydrogen absorbed inside the claddings formed the hydrides with μm-sizes, which are distributed in the matrix intra as well inter granular (EBSD). During quenching, following the high-temperature test stages, no fragmentation of claddings was observed. Tensile tests performed at room temperature after bundle tests evidenced fracture at concentrated zirconium hydride regions for several rods with local hydrogen concentrations 1500 wppm and more. Claddings with lower hydrogen concentrations fractured due to stress concentration at burst opening edges. Other tensile tested claddings failed after necking far away from burst.
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