Air oxidation modeling
Air ingress scenarios could occur in reactor and spent fuel sequences. For example, in reactor sequences air could be admitted through the broken reactor vessel at the late phase of severe accident after reactor vessel has breached. During air oxidation, oxygen oxidizes the cladding and then exothermic heat is released. After oxygen starvation, nitrogen reacts with the cladding significantly. Currently the oxidation behaviour of nitrogen is not fully understood, and hence the modelling of nitriding during air oxidation is the subject of a recently started PhD project at PSI. A short review of current knowledge is presented.
optical micrograph of a Zircaloy-4 in air oxidation at 850°C
Many separate effect tests were performed mainly at KIT and IRSN. Also, the integral tests were conducted in the frame of QUENCH-10 and -16. Through these air oxidation tests the nitride formation was observed under oxygen starvation and two major roles of nitrogen were identified.
The first role of nitrogen is the cladding degradation by forming a micro porous oxide scale due to the differences between the molar volumes of ZrN and ZrO2. Through the pores by the ZrN inclusions in the oxide scale the oxygen could easily access and oxidizes the ZrN. During ZrN reoxidation ZrN is converted to ZrO2 and hence oxide scale experiences the stresses due to the volume expansion and it leads to macro cracked oxide.
The second role of nitrogen is the exothermic heat release from the ZrN formation and reoxidation. The heat released from ZrN formation and reoxidation is same as the heat from oxidation by oxygen. Currently some reactor system codes represent the nitrogen-driven cladding degradation by as a catalyst effect by modelling the enhanced diffusion of oxidant and hence accelerated kinetics. However most reactor system codes do not implement ZrN formation heat release, and none of them represents the ZrN reoxidation heat release.
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Simulation of air oxidation during a reactor accident sequence
Part 1 Phenomenology and model development
Part 2 Analysis of PARAMETER-SF4 air ingress experiment using RELAP5/SCDAPSIM