Lessons learned from the QUENCH-LOCA experiments

Seven QUENCH-LOCA bundle tests (QUENCH-L0…-L5 and -L3HT) with different cladding materials (Zry-4, M5®, Opt. ZIRLO™) were performed according to a temperature/time-scenario typical for a LBLOCA in German PWRs. For two tests (QUENCH-L4 and -L5) pre-hydrogenated claddings (with 100 and 300 wppm H for M5® and Opt. ZIRLO™, correspondingly) were used. Generally, a peak cladding temperature of about 1350 K (1250 K for QUENCH-L5) was reached at the end of the heat-up phase at the bundle elevation of 950 mm. The maximal heat-up rate was 8 K/s (2.6 K/s for commissioning test QUENCH-L0); the cooling phase lasted about 120 s (0 s for QUENCH-L0) and was terminated by 3.3 g/s/effective rod water flooding. The tangential temperature gradient across a rod was between 30 and 70 K on the burst onset. The detailed profilometry measurements performed over whole length of the claddings showed formation of not only main ballooning area (with burst) but also, for several rods of each bundle, additional two or three ballooning regions. Another parameter calculated based on profilometry measurements is the bundle blockage. The maximal coolant channel blockage was less than 35%. Due to the moderate blockage good bundle coolability was kept for all bundles. 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. The maximal oxide thickness at the outer cladding surfaces was less than 15 µm. Surface cracks, penetrating both layers, were formed in vicinity of burst opening during ballooning. 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. The hydrogen, produced during oxidation of the inner cladding surface around the burst opening, can be absorbed by the metal with formation of hydrogen enrichments around the oxidized area (secondary hydrogenation). Such enrichments with a quite complex 3D form were observed for inner rods having been exposed to peak cladding temperatures of more than 1200 K. Except for the QUENCH-L0 test, no hydrogen bands were observed for bundle outer rods, the peak cladding temperature measured for these rods was less than 1200 K. Neutron tomography analyses showed (except the QUENCH-L0 bundle with non-prototypical long duration of the transient stage and the QUENCH-L3HT with not prototypical high temperatures) that the maximal hydrogen concentrations inside the hydrogen bands was less than 750 wppm (averaged through the cladding cross section). EBSD analysis showed that 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. Another part of the absorbed hydrogen was probably dissolved in the metallic matrix. During quenching, following the high-temperature test stages, no fragmentation of claddings was observed meaning that the residual strengths and ductility were sufficient. Tensile tests at room temperature evidenced fracture at hydrogen bands for several inner rods with local hydrogen concentrations about 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 region.