Eutectic Reaction and Oxidation Behavior of Cr-coated Zircaloy-4 Accident-tolerant Fuel Cladding under Various Heating Rates

The Cr-coated zirconium alloy claddings are near-term approach for accident tolerant fuel (ATF) claddings. The performance of magnetron-sputtered Cr-coated zirconium alloys is investigated under conditions simulating severe accidents. The transient oxidation experiments were performed using a thermogravimetric analyzer (TGA) with five different heating rates (2-50 K/min) up to 1380 degrees C, which is above the Zr-Cr eutectic temperature. The weight gain of the Cr-coated specimens was found to be significantly lower than that of the uncoated reference up to approximately 1330 degrees C. However, after the occurrence of the Zr-Cr eutectic reaction and failure of the coating at 1330 similar to 1380 degrees C, the oxidation rate rapidly increased and the weight gain rate of the Cr-coated specimens was 1.5 to 10 times higher than that of the uncoated specimens. The difference was greater at higher heating rates. These results suggest that the Cr-coating has a limited effect in preventing oxidation and degradation of cladding in case of eutectic reaction. The oxidation was faster at higher heating rate, and it may even cause runaway oxidation of cladding due to the temperature rise caused by oxidation heat. These findings are important for understanding the capabilities and degradation behaviors of Cr-coated cladding under severe accidental scenarios.