Numerical investigation of hydrogen isotope retention by an yttrium pebble-bed from flowing liquid lithium

Abstract

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.

MHD R&D Activities for Liquid Metal Blankets

“…Because of this experimental limitation, tritium transport codes have been validated using experiments under hydrodynamic conditions or verified against analytical solutions [120][121][122]. These exercises do not ensure code validation in MHD flows, but once the hydrodynamic transport of dissolved species has been verified, there is high confidence that the transport equations for passive scalars are implemented in a correct way, taking into account all relevant physical phenomena and applying them to MHD velocity distributions as well.…”

Impact of yttrium hydride formation on multi-isotopic hydrogen retention by a getter trap for the DONES lithium loop

“…Parameter Sh is the Sherwood number of the Li flow through the trap. The expression for the mass transfer coefficient in pebble-beds [23] which is used in this model is equivalent to the one derived in the equations ( 20)-( 24) of the article [12]. Assuming a linear concentration decrease in the boundary layer allows writing an approximated expression for the retention flux occurring on the Li side of the Li-Y interface [24]:…”

“…The model considers the diffusivity relation of deuterium in Li for all three hydrogen isotopes. It has the unit (m 2 s −1 ) and is determined from the tritium diffusivity measured in [25] by considering the isotope effect of hydrogen diffusion in metals [12,26,27]:…”

“…In contrast to the previous model presented in [12], this work considers a more accurate numerical description of hydrogen isotope transport inside of each Y pebble. From now on, a pebble is regarded as a sphere consisting of N + 2 spherical shells i.…”

“…The determination of appropriate trap designs and operating conditions to meet these safety limits requires the development of a simulation tool capable of realistically modeling the transport of hydrogen isotopes from flowing liquid Li into an arbitrarily sized Y getter bed. This was the purpose of a previous numerical study [12] using the simulation software EcosimPro © [13]. It allows performing dynamic simulations of the hydrogen isotope concentrations and fluxes in the trap and in the surrounding loop system by continuously solving a system of coupled differential transport equations and algebraic boundary conditions.…”