Influence of the Temperature History on Secondary Hydriding and Mechanical Properties of Zircaloy-4 Claddings: An Analysis of the QUENCH-LOCA Bundle Tests

Abstract

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.

Application and Development Progress of Cr-Based Surface Coatings in Nuclear Fuel Element: I. Selection, Preparation, and Characteristics of Coating Materials

“…However, their inherent water-side corrosion and accompanying embrittlement caused by hydrogen absorption are still the critical factors that cause cladding failure and limit a further increase in nuclear fuel lifespan and burnup [1,13,26,30,31,38]. Notably, under accident conditions, the rapid oxidation of zirconium alloy cladding in high-temperature steam (releasing a lot of heat and flammable hydrogen), balloon/burst (secondary hydrogenation), and the accompanying reduction in the ductility seriously threaten the integrity of the cladding and even the core [39][40][41]. Once the situation is out of control, the failure of the cladding tube causes severe accidental consequences (such as the Fukushima Nuclear Accident [2]) that pose considerable risk to the safety of the reactor.…”