High-temperature single-rod tests on the oxidation of Cr coated cladding tubes, carried out at KIT within the IAEA project ATF-TS

Twenty two single rod oxidation tests with Cr coated zirconium alloy tubes were carried out within the framework of the IAEA project ATF-TS under DBA and BDBA transient conditions in an induction furnace. Eight M5® and four ZIRLO™ claddings PVD coated to 7 µm thick Cr layer, four E110 claddings with arc ion Cr plating to 8 µm thick Cr layer, six opt. ZIRLO™ claddings PVD coated to 5, 18 µm thick Cr layer and 18+8 µm thick Cr+CrN layer were tested.
The tests performed with ZIRLO claddings coated to 7 µm Cr layer showed that for Tmax<1300 °C, only small oxidation of Cr layer with low hydrogen release (factor 15 in comparison to not coated claddings) occurred. For higher temperatures, formation of numerous local swellings at the outer cladding surface due to Zr-Cr eutectic at 1350 °C occurred and significant hydrogen release due to oxidation of Zr substrate was measured.
The tests performed with E110 claddings coated to 8 µm Cr layer showed significant decrease of hydrogen release under extended LOCA conditions (max 1250 °C) in comparison to not coated samples (factor 15). For peak cladding temperature of 1350 °C, numerous local swellings at the outer cladding surface formed and intensive hydrogen release (factor 18 in comparison to 1250 °C) was measured.
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The tests performed with opt. ZIRLO claddings coated to three different layer thicknesses showed that an increase in the thickness of the Cr layer by 3 times (from 6 to 18 μm) reduced the hydrogen release by 30% under DBA conditions (transient to 1200 °C), however, only by 3% under BDBA conditions (1400 °C). Barrier layer CrN has only moderate influence on hydrogen release under DBA, but strong reduced hydrogen release under BDBA (reduction factor 4). A local temperature escalation and formation and relocation of (Zr,Cr)+Zr melt for Cr/CrN coating occurred perhaps due to the lack of intermediate oxidation of part of the zirconium substrate, which could not react with steam due to the barrier effect of CrN layer.