Hydrogen Migration And Mechanical Behavior Of Hydrided Zirconium Alloys

Zirconium alloys have been widely used for nuclear fuel cladding materials in light-water nuclear reactors. The cladding corrodes as a result of exposure to the coolant water and produces hydrogen as a result of the corrosion reaction. A fraction of this hydrogen can be picked up into the cladding material. Once the hydrogen content reaches the terminal solid solubility, zirconium hydride particles start to precipitate. The cladding suffers waterside corrosion in service, leading to hydrogen ingress, which can redistribute in the cladding and form hydrides. Because these zirconium hydrides are more brittle than the zirconium matrix, they can deteriorate the ductility of the cladding. Therefore, understanding hydrogen behavior in cladding is important to maintain cladding integrity. This study aims to investigate hydrogen migration under a temperature gradient and mechanical behavior of hydrided zirconium alloys. The hydrogen transport and hydride precipitation /dissolution model HNGD was implemented in the fuel performance code BISON to predict hydrogen behavior. The hydrogen is distributed inhomogeneously in the cladding as a result of Fick's law and Soret effect. The hydrogen tends to move from higher to lower concentration governed by Fick's law and higher to lower temperature based on the Soret effect. Hydrogen migration tests were designed to determine the heat of transport value (Q*) of hydrogen in Zr, a parameter needed to evaluate the Soret effect. Hydrided samples were subjected to a long annealing schedule in a temperature gradient to re-distribute the hydrogen. The annealed samples were cut into several pieces along the temperature gradient, and the hydrogen contents were analyzed using hot vacuum extraction. The hydrogen distribution along the temperature gradient was observed in this experiment, and from this data, the heat of transport value (Q*) was determined. Further, the mechanical behavior of zirconium alloys was assessed using ring compression tests. The zirconium alloy tubes were characterized by electron backscatter diffraction (EBSD) to identify the microstructure of materials. Stress relieved anneal ZIRLO (SRA) and low Sn Partially recrystallized anneal LT ZIRLO (PRXA) show different grain shapes and sizes. After characterization, the zirconium alloy tubes were hydrogen charged and cut into 8 mm length rings. The ring samples were subjected to compression at 12 o'clock following a specified thermomechanical cycle. This thermomechanical treatment caused partial precipitation of radial hydrides in certain positions of the ring samples. The radial hydride fractions were characterized and showed a difference between ZIRLO and LT ZIRLO because of their different microstructures. Finite element modeling conducted using ABAQUS could then determine the threshold stress for two materials by comparing simulation results (stress state) and hydride morphologies. In addition, the ring compression tests for assessing hydrided cladding ductility for various hydride morphologies were conducted at room temperature. Ring samples with different radial hydride continuity factors (RHCF) were tested to determine their load-displacement curves. The 1% permanent strain and 2 % offset strain criteria were chosen to assess the ductility of samples. The ductility degrades with increasing RHCF.