Speaker
Description
Zirconium alloys have long served as fuel cladding in light water reactors due to their excellent mechanical properties and low neutron absorption cross-section. However, during loss-of-coolant accidents (LOCAs), they react violently with high-temperature steam. Chromium-coated zirconium alloys are a leading candidate for accident-tolerant fuels (ATFs), yet their protective capability fails when temperatures exceed the Cr-Zr eutectic point (~1332°C). At such temperatures, Cr rapidly diffuses into the Zr matrix, forming a ZrCr2 layer (several micrometers thick), which thins the coating, initiates cracks, and embrittles the substrate.
Simultaneously, the Cr2O3 oxide layer accelerates spallation due to liquid-phase eutectic breakdown. The synergistic effect of Cr diffusion and steam oxidation at high temperatures results in a "crocodile-skin" oxidation morphology on the cladding surface, with oxidation rates surpassing those of uncoated zirconium alloys.
This study establishes a Zr-Cr diffusion mechanism model to quantitatively reveal Cr concentration profiles, ZrCr2 layer growth kinetics, and defect formation mechanisms, providing theoretical support for failure prediction and emergency strategies in extreme conditions.
Experimental Methods & Key Findings :
(a) 1250°C: No ZrCr2 layer formed within 10min; after 60min, a 3.29μm layer emerged, with Cr coating thickness significantly reduced (~40μm diffusion depth). This indicates that low temperatures require prolonged diffusion to accumulate critical concentrations.
(b) 1350°C: A 0.64 μm ZrCr2 layer formed in 30s, increasing to 1.69μm at 90s. Cr coating thickness remained stable, demonstrating better structural integrity under high-temperature, short-duration conditions.
(c) 1400°C ~1482°C: ZrCr2 layer thickness saturated with prolonged holding (growth rate declined after 150s). Cracks and voids appeared at the coating-substrate interface due to diffusion-induced volume expansion and stress accumulation.
(d) 1482°C: The ZrCr2 layer was continuously consumed, with extended holding failing to thicken it or even causing its disappearance. This reveals a critical diffusion temperature and the dissolution mechanism of Cr atoms into the Zr matrix post-coating depletion.
(e) Driving Forces & Failure Mechanisms: ZrCr2 growth is driven by concentration gradients, thermodynamic negative free energy, and thermal stress fields. Kirkendall voids, lattice distortion, and grain boundary defects are the primary causes of coating failure.
