Breakaway Oxidation of Constrained Zirlo at 1000℃

Abstract

According to Nuclear Regulatory Commission guide line proposed in March 2014, cladding performance tests for LOCA are needed to ensure that fuel-rod cladding retains ductility following long-time oxidation in steam at temperatures in the range of 650-1,050 degrees Celsius. During high temperature oxidation of zirconium alloys, transition to breakaway stage causes the most critical degradation, since oxygen and hydrogen pick up are accelerated. There have been many researches about mechanisms of breakaway oxidation. The breakaway mechanism proposed by Leistikow and Schanz has been widely accepted, many researches have been conducted to prove LS model. LS model basically assumes that breakaway oxidation is initiated by the transition of oxide phase (tetragonal to monoclinic), which causes stress distribution change inside zirconium oxide. In this study, breakaway oxidation behavior of Zirlo is studied, concentrating on specimen holding state during the experiment, whether specimen is constrained or not. Claddings are being constrained in commercial reactors, however, there have been no researches whether constrained state of a specimen can affect breakaway oxidation or not. According to LS model, onset of breakaway oxidation is related to stress distribution inside Zirconium oxide. When a specimens is constrained, thermal stress within the specimen can cause compressive stress, and this can affect breakaway oxidation Several experiments were performed for Zirlo specimens which are constrained or in free condition for different time steps at 1000 C. It is clear that constrained specimens show different breakaway oxidation