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Hydrogen (Tritium) retention in plasma facing materials and its removal are critical safety issues for ITER and DT reactors. Hence hydrogen retention characteristics, i.e. where, how, and how much hydrogen is retained in torous, have been extensively studied. Here, we summarize present knowledge on them and suggest future necessary works.
In present tokamaks, most of hydrogen is retained in redeposited carbon layers on either plasma facing surface or remote area from plasma with concentrations of 1 - 0.01 in H/C atomic ratio depending on temperatures at the deposition. Hydrogen release characteristics and/or properties of hydrogen retaining in the redeposited carbon layers are quite dependent on their hydrogen concentration, which was determined by post mortem analysis. Owing to strong bonding nature between C-H, most of H in the redeposited carbon layers is bounded to C as confirmed by an infrared absorption study. Nevertheless thermal desorption spectrum of hydrogen changes significantly with hydrogen concentration. For those having lower H/C need higher temperature to desorb hydrogen compared to those having higher H/C. To remove T in the carbon, combustion or air oxidation is one of powerful techniques. However those containing low H deposited at high temperatures are found to be very difficult to burn and hydrogen release starts earlier than oxidation of carbon during the heating in air. For the combustion, micro-structure of the layers plays critically important role and TEM and Laser Raman analysis showed that those layers hardly burn were consisted of well graphitized but very fine grains.
Until now those above described knowledge were given by post mortem analysis of samples taken from the torous after long operation of plasma discharges including integrated results and missed shot-by shot changes. In addition, D and T retained in the samples could be influenced by oxidation or isotopic exchange with moisture (H2O) in the atmosphere when they were taken from the torous. It is urgent to develop in situ analyzing techniques to observe where and how much tritium retained. The possible application of reflection and absorption of visible and infrared light or lasers such as Laser Raman and infrared absorption spectroscopies will be discussed.
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