Impact of Thermal Tests Simulating Dry Storage of VVER-1000 Fuel on the Hydride Phase Morphology in Fuel Element Claddings

Abstract

In this work, we carried out metallographic studies of the lateral and longitudinal sections of the fuel element claddings used in the fuel assemblies of VVER-1000 reactors. The average fuel burnup in these fuel elements during one and six annual campaigns was ~20 and ~70 MWd/kg U, respectively. The effect of long-term thermal tests under simulated dry storage conditions of fuel elements on the hydride morphology in the lateral and longitudinal sections of E110 zirconium alloy claddings was experimentally studied. The test fuel elements were kept in helium at 380°C for 468 days under stationary conditions and from 20 to 380°C (48 cycles, holding time from 1 to 10 days at 380°C) for 427 days during thermal cycling. Metallographic studies of the microstructure of the lateral and longitudinal sections of fuel element claddings were carried out in order to determine the effect of fuel burnup and thermal testing modes on the morphology of zirconium hydrides in the fuel elements from the regions corresponding to the middle of the fuel rod and the gas collector location. It was established that the specific length of hydrides does not exceed 4.5 × 10–3 μm–1 and correlates with a low hydrogen content in the claddings. Thermal tests did not change the hydrogen content in the claddings, but caused an increase in the specific length of hydrides in the lateral sections of the claddings by 1.5–1.9 times in fuel elements with low burnup. In the longitudinal sections of the claddings of these fuel elements, as well as in the fuel elements with a high level of fuel burnup, this coagulation of hydrides was not observed. Thermal tests under stationary conditions and in thermal cycling mode did not lead to a significant change in the hydride orientation, which was predominantly tangential with a high fraction of the chaotic component in the lateral section of the claddings, whereas the axial orientation of hydrides prevailed in the longitudinal section.