Hydrogenation of claddings in the new developed HOKI oven and performance of the long-term bundle test under dry storage conditions in the framework of the SPIZWURZ project

During the dry storage of spent nuclear fuel, the initially dissolved hydrogen precipitates as hydrides in the metallic matrix. The orientation of hydrides influences the crack propagation and depends mainly on the hoop stress. In the frame of the SPIZWURZ project, the reorientation of zirconium hydrides in cladding tubes is being investigated under conditions similar to dry storage. For the long-term SPIZWURZ bundle experiment, 21 zirconium alloy tubes were charged with hydrogen to 100 and 300 wppm in the special developed HOKI tube oven as homogeneously as possible along a length of 1.3 m. The developed process allows a stepwise and controlled hydrogen absorption through the specially treated inner surface of claddings placed in the oven heated to 450 °C. The hydrogenation was carried out by successively supplying fixed masses of hydrogen, initially increasing the pressure in the tube sample from the achieved vacuum level to about 0.5 bar. Because of absorption, this pressure dropped to a predetermined vacuum level within a few minutes. During hydrogenation and subsequent heat treatment processes, the outer surface of the tube was exposed to a flow of oxygen to create an outer oxide layer more than 1 μm thick, which served as a barrier to hydrogen accumulating in the tube wall. ... mehrAfter the hydrogen loading of the samples, the axial distribution of hydrogen was determined by laser scanning profilometry. This method makes it possible to use the correlation between the swelling of the rod diameter and the hydrogen concentration. This correlation was obtained by hot extraction of hydrogen from pre-hydrogenated reference samples. A long-term bundle test with hydrogenated and pressurized cladding tubes began on 12.05.2023 and will last until mid-January 2024. The peak cladding temperature decreased in steps of about 15 K from 400 to 200 °C currently (the average cooling rate is about 0.9 K/day).