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Description
Plasma-wall interactions in tokamaks lead to transport and retention of hydrogen isotopes, especially tritium, in the plasma exposed material causing safety issues. Cleaning procedures are envisaged in ITER to recover the tritium [1] using pure deuterium (D) operation to trigger isotope exchange. This work presents isotope exchange simulations and experimental validation of the model, which can be used to evaluate the efficiency of such procedure.
To mimic damage caused by neutrons, tungsten is irradiated with MeV W ions (self-damage), creating multi vacancies and self-interstitial atoms which are the kind of defects created by neutron induced cascades. This material is exposed to D to study fuel retention in damaged W. Isotope exchange experiments has been done in self-damaged W by a D/H exposure [2]. To model this data, a single occupancy trap model is compared to a multi-occupancy trap model based on energies data from DFT calculations [3] [4], which has been used to simulate isotopic exchange in undamaged tungsten (W) [5]. This physics is here implemented in the FESTIM transport code [6]. One model includes two traps with single occupancy and the other includes one trap with two occupancy levels, allowing for another isotope exchange process. Simulated D profiles are compared to experimental depth profiles to quantify the effects of the multi-level trap.
The simulations show for both models good accordance with the experimental results at the temperature of 600K. The multi-occupancy model shows a slightly faster isotopic exchange than the single-occupancy traps model. The difference between both models increases with decreasing temperature. This is the expected behavior since higher temperature helps detrapping in the single occupancy model. This difference is also being investigated at higher temperature to see the effects on the model.
[1] Loarte, ITER Physics chapter 4: power and particle exhaust, Nuclear Fusion (2007)
[2] S. Markelj et al., Journal of Nuclear Materials, vol. 469, p. 133‑144, febr. 2016
[3] N. Fernandez et al., Acta Materialia, vol. 94, p. 307‑318, aug. 2015
[4] J. Hou et al., Acta Materialia, vol. 211, p. 116860, june 2021
[5] K. Schmid et al., Journal of Applied Physics, vol. 116, nᵒ 13, p. 134901, oct. 2014
[6] R. Delaporte-Mathurin et al., International Journal Of Hydrogen Energy, vol. 63, p. 786-802, apr. 2024