High-temperature interaction of oxygen-preloaded Zr1Nb alloy with nitrogen

Potential air ingress scenarios during accidents in nuclear reactors or spent fuel pools (SFP) raised the question of the influence of air, especially of nitrogen, on the oxidation of zirconium alloys which are used as fuel cladding tubes and other structure materials. In this context, the reaction of zirconium with nitrogen containing atmospheres and the formation of zirconium nitride plays an important role for the understanding of the oxidation mechanism. This paper presents the results of the interaction of the oxygen-preloaded niobium-bearing alloy M5® with nitrogen over a wide range of temperatures (800-1400°C) and oxygen contents in the metal alloy (1-7 wt.%). The samples were prepared in-situ and investigated in a thermal balance by (1) controlled pre-oxidation at 1200°C, (2) homogenization of the oxygen at 1400°C in inert atmosphere, and (3) reaction with flowing nitrogen at the defined temperature. A strongly increasing nitriding rate with raising oxygen content in the metal was found. The highest reaction rates were measured for the saturated α-Zr(O), as it is existing at the metal-oxide interface, at 1300°C. The temperature maximum of the reaction rate was approx. ... mehr100 K higher than for Zircaloy-4 already investigated in a previous study [1]. The paper presents results of the thermogravimetric experiments as well as post-test examinations by optical microscopy, SEM as well as microprobe elemental analyses. Furthermore, a comparison with results obtained with Zircaloy-4 will be made.