PSI - Issue 71

Tapan K. Sawarn et al. / Procedia Structural Integrity 71 (2025) 263–270

266

analysed using inert gas fusion technique (IGF). 3. Results and Discussions

Three typical four-point-bend test load-displacement plots of clad tubes subjected to LOCA integral tests (cladding tubes were oxidised in steam at 1000, 1100, and 1200 °C temperature) are presented in Fig.2. It can be observed from the plot that strengthening occurred at 1100 °C due to interstitial solid solution strengthening by oxygen. At 1200 °C peak load and ductility both reduce significantly indicating a gross embrittlement due to oxidation.

Fig. 2. Typical load-displacement plots of 4PBT. Typical appearances of the 4PBT specimens, after testing, are shown in Fig. 3a-c. Different responses to 4 PBT could be observed for different oxidation conditions (oxidation temperature and durations), viz. 1000 °C – 12 s, 1100 °C – 12s and 1200 °C – 12 s, respectively. The responses are like; 1. Specimens did not show any cracking, 2. Crack formed at the burst tip but did not propagate and 3. Specimens underwent partial cracking where the crack propagated to some extent and finally got arrested. Microstructural examination was carried out on the samples taken either from the partially cracked location or from near the burst opening for the un-cracked specimens. Typical microstructures corresponding to each temperature are shown in Fig. 4. Ballooning and burst occurred at the localised hot spot. Therefore, a circumferential temperature gradient always existed at the burst tip and opposite to it. At 1000 °C, equiaxed recrystallized α -Zr grains were observed both near the burst tip and the region opposite to it. At 1100 °C, equiaxed and coarser α -Zr grains were noticed near the burst tip. However, a two- phase structure dominated by prior β -Zr was present in the microstructure opposite to the burst tip. It is noteworthy that α -Zr phase was retained above the α to β transformation temperature (970 °C). This can be attributed to the stabilization of α -Zr phase by the oxygen in the solid solution. The presence of oxygen in zirconium alloys leads to an increase in the αZr/βZr transition temperatures and particularly affects the upper transus i.e α+β to β -Zr transition temperature (Stern et al. 2008). Due to the combined effect of high temperature and localised clad thinning oxidation at 1200 °C temperature resulted in higher α -Zr(O) layer thickness near the burst tip which gradually decreased and became minimum in the region which was opposite to burst o pening. In Fig.4, it is evident that the burst tip region and the region opposite to it got completely occupied by α - Zr(O) and prior β -Zr layer respectively for oxidation at 1200 °C. Hence, from the microstructural examination we can correlate the crack initiation and propagation phenomena with the evolved microstructure. The central metallic part remained ductile due to the absence of α -Zr(O) brittle phase and a low oxygen concentration (0.17 wt %) corresponding to oxidation temperature of 1000 °C. Therefore, no crack initiation was seen at the burst tip. In case of oxidation at 1100°C, oxygen content in the metallic part was relatively higher (0.19 wt %) in comparison with the previous one. Due to stress concentration at the burst opening tip, crack got initiated but did not propagate through the ductile matrix of two-phase structure as discussed above. Oxidation at 1200 °C indicates a different behaviour where both crack initiation (in the brittle α -Zr(O) layer beneath the oxide) and propagation through the prior β -Zr layer rich in oxygen concentration. However, the crack finally got arrested in the oxygen lean region of prior β -Zr. This is a result of oxygen concentration gradient established by the circumferential temperature gradient as discussed earlier in this section. The average oxygen concentration of the prior β -Zr phase was observed to be 0.59 wt%.