First results of the bundle test QUENCH-L5 with pre-hydrogenated opt. ZIRLO™ claddings

The QUENCH-LOCA-5 test with pre-hydrogenated optimised ZIRLO™ claddings (≈300 wppm H) was performed according to a temperature/time-scenario typical for a LBLOCA in a German PWR with the similar parameters as the QUENCH-LOCA-3 test with fresh optimised ZIRLO™ claddings: maximal heat-up rate 8 K/s, cooling phase lasted 120 s and terminated with 3.3 g/s/rod water flooding. Significant difference was the lower temperature before the start of the transient (to decrease the dissolution of hydrides during the conditioning stage of experiment). Accordingly to this, the lower maximum temperature of 1250 K was reached on the end of the heat-up phase at elevation 950 mm (1350 K for QUENCH-LOCA-3). Tangential temperature gradient across a rod was up to 30 K on the burst onset. Due to higher ballooning rate, the maximum blockage ratio of cooling channel (25% at 940 mm) was higher in comparison to QUENCH-L3 (21% at 923 mm). Due to moderate blockage good bundle coolability was kept for both bundles. Some rods have up to three ballooning regions for both tests. The reason is successive onset of ductile temperature threshold at different elevations. The cladding burst occurred at temperatures between 1027 and 1151 K (QUENCH-L3: 1064 and 1188 K). The inner rod pressure relief to the system pressure during less 30 s (similar to QUENCH-L2). The average burst temperatures of 1081 K (808°C) for QUENCH-L5 is lower than for QUENCH-L3 (1117 K or 844°C) due to lower α-Zr→β-Zr phase transition temperature. No secondary hydrogenation was indicated for the QUENCH-L5 claddings due to very short high temperature period. During quenching, following the high-temperature phase, no fragmentation of claddings was observed (residual strengths or ductility is sufficient). Tensile tests at room temperature showed cladding fracture mostly due to stress concentration in the region of burst opening