Quantification of hydrogen uptake of steam-oxidized zirconium alloys by means of neutron radiography

The macroscopic total neutron cross section of zirconium alloys depends linearly on the hydrogen content in the metal. This offers the possibility of quantitative determination of the hydrogen content by means of neutron radiography. The method is applied to determine quantitatively the hydrogen content and distribution in corner rods of a VVER-type nuclear fuel bundle simulator investigated in the framework of post-test examinations of a large scale severe nuclear accident simulation experiment. Unexpectedly high hydrogen concentrations were found. The axial hydrogen distributions in the corner rods show two maxima. The reasons for these distributions and the large hydrogen content are not the temperature distribution but the appearance of oxide layers. Highest concentrations occur at positions where the oxide layers show a lot of open circumferential cracks. In these cracks large local hydrogen partial pressures can be expected. The hydrogen content at positions with compact oxide layers is much lower.