Ensuring a secure and reliable execution of the International Fusion Materials Irradiation Facility (IFMIF) and the DEMO-Oriented Neutron Source (DONES) requires their liquid lithium loops to be purified from hydrogen isotopes which are generated during operation. For this purpose, an yttrium pebble-bed will serve as a hydrogen hot trap. Former intentions to predict the retention behavior of an yttrium pebble-bed are based on the consideration of the trap as a black box with a predetermined trap efficiency. Disregarding the internal physical mechanisms of the gettering process these models are built on simplified assumptions and should be extended to allow reliable trap designs for IFMIF/DONES. Therefore, a detailed numerical model describing the hydrogen transport from flowing liquid lithium into an yttrium pebble-bed has been developed from scratch within the scope of this work. It enables simulating the hydrogen retention process into an arbitrarily dimensioned getter bed for the low concentration regime by solving a system of differential equations with a finite-difference approach. The model is used to calculate the time evolution of the hydrogen concentrations in a simplified loop system which is connected in line with an yttrium pebble-bed. Special focus is placed on the observation of a case relevant for IFMIF/DONES considering a constant generation of tritium in the loop. Simulation results reveal that the trap efficiency decreases with time and that lower system temperatures significantly improve the trap efficiency. It is found that for heavier pebble-beds the tritium inventory build-up in the lithium is slowed down more efficiently. These findings are of great importance for the design of the hot traps for IFMIF/DONES. To demonstrate the reliability of the model experimental data of a previous deuterium retention experiment are successfully reproduced.

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Compliance with imposed hydrogen concentration limits in the lithium loop of DONES requires the installation of an yttrium-based hydrogen trap. To determine an appropriate H-trap design, it is essential to have access to a numerical tool capable of simulating hydrogen transport in the DONES lithium loop connected to an yttrium pebble-bed. In the past, a simplified model was created that allows such calculations when hydrogen concentrations in lithium are low. However, in certain DONES operating phases, the concentration in the lithium is high and in a range where yttrium dihydride (YH2) formation is likely. Due to the anticipated great impact of YH2 formation on the H-trap performance a new model is developed that includes the mechanism of hydride formation. It is based on a mathematical reproduction of complete pressure composition-isotherms of the Li-H and Y-H systems. Thus, the conditions that trigger YH2 formation are determined and the variation of hydrogen solubility in different yttrium hydride phases is deduced. An approximate concentration-dependent relationship of hydrogen diffusivity in yttrium is derived and incorporated into the model. Simulations are performed to analyze the dynamics of the concentration decrease during purification of the lithium circuit prior to the experimental DONES phase by varying design parameters of the trap. It is found that hydride formation greatly increases the hydrogen gettering capacity of the H-trap and limits the maximum concentration in the lithium. Indeed, YH2 formation may be purposefully triggered to exploit its beneficial properties for DONES. The simulations show that the H-trap must be replaced at least every 28 days during the DONES experimental phase to meet tritium limits. This work sets the conditions for the required pebble-bed mass of the H-trap at a given temperature to comply with DONES safety requirements. Finally, the model is validated by an accurate numerical reproduction of experimental results.