Impacts of mechanical fields on hydrogen transport in ITER divertor monoblocks
Résumé
In nuclear fusion devices, plasma facing components are submitted to intense hydrogen isotopes and heat fluxes. The most exposed component of tokamak is the divertor, which in ITER is composed of tungsten monoblocks. These are drilled in their centre for the cooling pipe, made of CuCrZr alloys. Between the pipe and the tungsten part, a copper interlayer is added. Monoblocks are thus subjected to thermal gradients and induced thermomechanical fields, that affect hydrogen retention in its volume: for safety reason, the retention must be low and it is undesirable that hydrogen isotopes permeate through the cooling pipe, contaminating the complete cooling system. The present study aims to perform a parametric study on hydrogen isotope retention in the monoblocks, as a function of the exposure conditions (thermal and hydrogen fluxes) with a focus on mechanical field effects. Based on specific developments under Abaqus software, a fully coupled finite element model is used, combining transient heat transfer, thermally activated transient hydrogen transport and trapping, and thermomechanical behaviour, including thermal expansion. Hydrogen diffusion across bi-material interfaces is based on chemical potential continuity, assumed to only depend on the ratio between hydrogen concentration and its solubility. Cycling pulsed exposure is considered for the ITER plasma operations phase (16 months) and a constant temperature is imposed for the long maintenance phase (8 months). The developed tools are first validated by comparing the results obtained on the one hand for heat transfer and hydrogen transport and trapping only, and on the other hand for heat transfer and thermomechanical behaviour only. Next, simulations coupling thermomechanical behaviour, heat transfer and hydrogen transport are performed to highlight the effect of mechanical field on diffusion and trapping.
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