Two out-of-pile bundle tests, QUENCH-L0 and QUENCH-L1, were performed recently at Karlsruhe Institute of Technology (KIT) in the framework of the QUENCH-LOCA program devoted to the investigation of the so-called secondary hydriding of the cladding. 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 cladding mechanical properties to analyze the material behavior with respect to embrittlement. The program was started in 2010 with the QUENCH-L0 commissioning test using 21 electrically heated rods with as-received Zircaloy-4 claddings followed in 2012 by the QUENCH-L1 reference test using the same material. These two tests differ in 1) heat-up rate during the first transient and 2) presence of a cool-down phase before quenching. The maximum heating rate reached during QUENCH-L0 was only 2.5 K/s, whereas the transient phase of QUENCH-L1 was performed with the maximum rate of 7 K/s. The state of the QUENCH-L0 bundle was practically “frozen” immediately after the transient phase by fast injection of two-phase fluid. The reference test QUENCH-L1, was performed with a typical cooling phase after the transient phase. It provides data on Zircaloy-4 cladding embrittlement based on more prototypical temperature history. Post-test neutron radiography and tomography revealed formation of hydrogen bands around the oxidized inner cladding surface in vicinity of the burst openings for both tests. However, the concentration of hydrogen absorbed inside these bands was different for both tests: whereas the maximum hydrogen concentration for QUENCH-L0 reached 2560 wppm, the corresponding value for QUENCH-L1 was only 1690 wppm. Complementary model calculations confirm that the differences in hydrogen concentrations are mainly related to the differences in temperature sequences. Subsequent tensile tests with tube segments at room temperature revealed the dependence of the mechanical behaviour on hydrogen concentration: tubes with hydrogen contents above 1500 wppm were simultaneously double ruptured along the hydrogen bands, whereas tubes with hydrogen concentrations below 1500 wppm failed at the middle of burst openings.

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