Abstract
New out-of-pile QUENCH-LOCA bundle tests are being performed in the QUENCH facility within the Nuclear Safety Program of KIT. The overall objective of this bundle test series is the investigation of ballooning, burst and secondary hydrogen uptake of the cladding under representative design basis accident conditions as well as detailed post-test investigation of the mechanical properties of the claddings to check the embrittlement criteria. The program was started in 2010 with the QUENCH-L0 commissioning test using 21 electrically heated rods with as-received Zircaloy-4 claddings followed by the QUENCH-L1 reference test using the same material. Advanced cladding alloys (Duplex DX/D4, M5®, ZIRLO™) will be applied in further tests. For QUENCH-L0 each rod was separately pressurized with krypton with initial pressures from 35 to 55bar. The transient phase with heating from 793K to 1343K lasted 185s. The increased ductility of the heated cladding resulted in a progressive ballooning and consequent burst of all of the pressurized rods during the transient. The test was terminated by water quenching of the bundle without the usual intermediate slow cooling phase. Post-test investigations showed circumferential strain values between 20 and 35% at cladding positions with oxidation degree corresponding to 2% ECR. Neutron radiography of cladding tubes showed that at the inner rod simulator group so called secondary hydriding occurred. Maximal hydrogen concentrations up to 2500wppm on the boundary of the internal oxidized region around burst positions were determined. These hydrogen containing bands correlate with cladding bulk zones with increased micro hardness. XRD and TEM analysis indicated dissolution of hydrogen in the metal matrix without noticeable zirconium hydrides formation. Tensile tests of claddings with hydrogen containing bands showed simultaneous cladding tube ruptures at these bands below and above the burst opening.
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