Zirconium alloys with 98% zirconium content are used as nuclear fuel cladding material because they exhibit low neutron absorption, good corrosion resistance and sufficient strength during and after reactor operation. However, hydrogen is taken up by the cladding during operation and precipitates as hydrides on cooling when the solid solubility limit is reached. These zirconium hydrides affect the mechanical properties of the metal, depending on their amount, shape, length, connectivity, and orientation. The post-operational hydrogen concentration thereby depends on several factors, e.g. the operation temperature, duration, fuel burn-up and primarily on the cladding tube’s chemical composition and fabrication. Depending on the post-storage of the spent nuclear fuel (SNF) and thus the cooling procedure, the hydrides precipitate in the zirconium matrix in a specific orientation, with a specific density and a specific length and connectivity.

In order to investigate the hydrogen behaviour in zirconium and Zircaloy cladding tubes, in-situ and ex-situ measurements of the hydrogen diffusion in dependence of its solubility and elastic and plastic tensile stresses were performed. For this purpose, cold neutrons were used to visualise hydrogen as a dark contrast within zirconium, which is possible because of the very low neutron cross section of zirconium that, contrarily to hydrogen, attenuates neutron beams only weakly. On the beamlines, we already installed various experimental settings, e.g. a tensile testing machine and different furnaces to carry out in-situ investigations at different temperatures. The hydrogen diffusion is quantified with diffusion coefficients and locally analysed in dependence of the lattice directions in the zirconium. With calibration samples of the same matrix material and known hydrogen contents, precise determinations of the hydrogen concentration can be calculated locally for the samples – either in 2D or 3D, depending on the method (radiography or tomography). For the evaluation of the experiments, the post-test hydrogen concentration is measured with carrier gas hot extraction (CGHE) and then compared to the neutron imaging results.

This paper describes various experimental settings to carry out neutron imaging experiments under different temperature-time conditions with hydrogenated Zircaloy cladding tubes and zirconium samples.