Neutrons interact with the magnetic moment of the atomic shell of an atom, as is common for X-rays, but mainly they interact directly with the nucleus. Therefore, the atomic number and the related number of electrons does not play a role in the strength of an interaction. Instead, hydrogen that is nearly invisible for X-rays has a higher attenuation for neutrons than most of the metals, e.g., zirconium, and thus would be visible through dark contrast in neutron images. Consequently, neutron imaging is a precise, non-destructive method to quantify the amount of hydrogen in materials with low attenuation. Because nuclear fuel cladding tubes of light water reactors are made of zirconium (98%), the hydrogen amount and distribution in metallic claddings can be investigated. Even hydrogen concentrations smaller than 10 wt.ppm can be determined locally with a spatial resolution of less than 10 μm (with a high-resolution neutron microscope). All in all, neutron imaging is a very fast and precise method for several applications. This article explains the basics of neutron imaging and provides samples of investigation possibilities, e.g., for hydrogen in zirconium alloy cladding tubes or in situ investigations of hydrogen diffusion in metals.

During operation in light-water reactors, Zircaloy cladding tubes take up hydrogen that precipitates under specific temperature–pressure conditions after operation in the form of zirconium hydrides. These zirconium hydrides have a detrimental effect on the mechanical stability of the cladding tubes. The conditions for their formation and their orientation need to be forecasted in order to determine that the spent nuclear fuel remains shielded during interim dry storage. Internal and external stresses that affect the cladding tubes throughout their slow cooling process during interim dry storage may worsen and accelerate the cladding tubes' embrittlement with regard to possible reorientation processes of zirconium hydrides. Generally, hydrogen in solid solution follows gradients in temperature, concentration, and stress. Consequently, hydrogen moves from higher to lower temperatures and from lower to higher stresses due to the thermodynamically more favourable conditions. In order to investigate elastic stress conditions similar to those during interim dry storage of hydrogenated cladding tubes, a modified mobile tensile testing machine was designed to elongate zirconium samples under defined temperatures of up to 500 ^∘C. During the experiments, hydrogen movements within the samples are monitored by in situ neutron radiography. Because of the very low neutron cross section of zirconium, the metal is nearly invisible for neutrons, and the contrarily behaving hydrogen that scatters neutrons strongly appears as dark contrast in neutron images. Due to the sample's different cross section profile, the tensile stress created by the tensile testing machine is amplified and thus should lead to visible hydrogen movements. This paper describes neutron radiography experiments under different temperature–time conditions with hydrogenated tensile zirconium samples that simulate stress conditions of cladding tubes during interim dry storage.

With regard to delays in finding and preparing final repositories, e.g. in Germany, the prolongated interim dry storage phase for spent fuel may exceed several decades. Therefore, further evidence for the integrity of spent nuclear fuel for longer storage periods needs to be gathered. The integrity, and thus the state of degradation in connection with e.g. hydride embrittlement of cladding tubes, needs to be evaluated for these prolongated storage periods. Adequate experimental data with dry storage related test conditions are rare. Instead, modelling is the only approach to predict the long-term storage behaviour of spent nuclear fuel during dry storage. However, the codes and tools for these modelling activities need to be based on realistic parameters and correlations, which only can be gained and improved experimentally. In this context, the worldwide first long-term large-scale bundle experiment LICAS-01 was conducted in the framework of the SPIZWURZ project with conditions very similar to dry storage conditions, as e.g. a very slow cooling phase.Using the LICAS facility at the Karlsruhe Institute of Technology, a bundle of 21 fuel rod simulators with a length of 2.5 m was heated up to about 400°C and then cooled down uniformly in order to simulate the slow cooling process that is characteristic for the dry storage phase. Three cladding tube materials (Zircaloy-4, optimized ZIRLO™, DX D4), each with two different hydrogen contents (100 and 300 wt.ppm) and two different inner pressures (106 and 146 bar) were used and subjected to a cooling transient of 250 days. Post-test measurements were conducted non-destructively and destructively, concerning the post-test creep behaviour, the hydrogen diffusion behaviour in the cladding tubes and the hydride reorientation process. Mutually and after the analysis, a blind and open benchmark simulation showed the usability of different fuel rod performance codes with regard to the hydrogen related aspects and the cladding creep behaviour. The cladding creep and the reorientation of hydrides showed a different behaviour between Zircaloy-4and DX-D4 compared to optimized ZIRLO™. The latter was hardly affected by a reorientation of hydrides, which was not predicted by computer codes in a corresponding blind benchmark exercise. These results show that further data is crucial to improve and to extend the database used for modelling purposes.This paper presents the LICAS-01 bundle experiment and its results, focussing on the cladding creep and the behaviour of hydrogen in Zr-based cladding tubes.