The QUENCH-L5 experiment was performed in the framework of the QUENCH-LOCA test series. The overall objective of this bundle test series is the investigation of ballooning, burst, degree of oxidation and secondary hydrogen uptake of the cladding under representative design-basis accident conditions and their influence on the mechanical properties. The various experiments of the series examine the behavior of different cladding materials and the effect of pre-hydriding. For the QUENCH L5 test, opt. ZIRLO™ claddings pre-loaded with approximately 300 wppm hydrogen and with an outside diameter of 10.75 mm have been used. Like in all experiments of the QUENCH LOCA series, the fuel rod simulators were separately pressurized with krypton to 55 bar. Bundle configuration and test protocol were similar to the reference test QUENCH L3 with as-received opt. ZIRLO claddings. Specific objectives of QUENCH L5 were to provide information about the behavior of pre-hydrided opt. ZIRLO alloy on the response to a best-estimate large-break LOCA sequence, with special focus on the impact of hydrided claddings on their ballooning and burst parameters, as ... mehrwell as cladding integrity during quenching. The test was successfully conducted at the Karlsruhe Institute of Technology (KIT) on February 10, 2016.
The experiment started by stabilizing the bundle conditions with an application of electrical bundle power of 3.25 kW (linear heat rate of approx. 0.9 W/cm) and gas flows of 6 g/s argon plus 2 g/s superheated steam resulting in maximum bundle temperatures of about 800 K. During this stabilization phase (lasting 1700 s) the rods were refilled with krypton to 55 bar. The transient stage was initiated by increasing of electrical power to 60 kW and lasted 76 s. During this period the peak cladding temperature increased from their initial values to a maximum of 1205 K. The average heatup rate at the maximum temperature location was 6 K/s. The increased ductility of the heated cladding resulted in a progressive ballooning and consequent burst of all rods. The burst temperature is 1087 ± 36 K (about 35 K lower in comparison to the QUENCH-L3 test performed without pre-hydrogenation). The experiment continued with power decrease to 3.5 kW to simulate decay heat and injection of steam at a nominal of 20 g/s (cool-down stage). In this stage mostly steady cooling to about 930 K occurred. The cooling phase was followed by up to 100 g/s (3.3 /g/s/effective rod) water injection from bundle bottom (quench stage). Complete quench was achieved at about 290 s.
Post-test videoscope inspections showed typical ballooning pictures at the hottest bundle elevations between about 850 and 1000 mm. The bundle was dismounted and geometric parameters of all rods were determined by laser scanning; the range of circumferential strains measured was between 21% and 33%. It is slightly higher as for the QUENCH-L3 claddings; as well as the axial extension of ballooning region for each cladding was larger for QUENCH-L5 in comparison to QUENCH-L3. Some rods have up to three ballooning regions for both tests QUENCH-L5 and -L3. The maximum blockage ratio of the cooling channel (25%) due to ballooning was slightly higher in comparison to QUENCH-L3 (21%). A small bending of all rods was detected in the plane going radially through the burst opening. Ultrasound measurements were used to determine thinning of cladding wall in vicinity of burst openings. Axial and radial distribution of oxidation rate was measured by eddy current methods; maximal combined thickness of ZrO2 and α-Zr(O) layers was about 15 µm (lower in comparison to QUENCH-L3 due to lower temperatures). During quenching, following the high-temperature phase, no fragmentation of claddings was observed for both QUENCH-L3 and QUENCH-L5 (residual strengths or ductility is sufficient). No secondary hydrogenation was indicated for the QUENCH-L5 claddings due to relative short high temperature period: less of 30 s above 850 °C (complete transition to β-Zr phase) in comparison to almost 100 s for QUENCH-L3. Measurement of mechanical properties and determination of residual ductility were carried out by tensile tests with cladding tube segments (about 800 mm length) at room temperature and showed fracture of claddings at engineering stress of about 540 MPa (about 40 MPa higher in comparison to the QUENCH-L3 test) mostly due to stress concentration at burst opening tips. Residual part of claddings was fractured due to necking far away from the burst.