4.19.6.2.1 Safety issues in ITER

4.19.6.2.1.1 In-vessel tritium inventory

Estimates of the tritium inventory and of permeation in the PFCs of a magnetic fusion device are impor­tant for assessing the radiological hazards from rou­tine operation and from potential accidents, for the design of the water detritiation system, and for pre­dicting the tritium supply requirements. In addition, these estimates have contributed to the decisions involving the choice of different armor materials in ITER options, which have a strong impact on tritium retention. In spite of the experimental and modeling progress which has taken place in the recent past, understanding of the subject of tritium-wall interac­tions is still far from complete and quantification of the tritium inventory in ITER is highly uncertain. The retention and permeation of implanted tritium in ITER PFCs have been widely studied in the past (see Section 4.19.3 and the example of calculations found elsewhere).9,187-189 On the basis of the results of these calculations, it can be concluded that the inventory of tritium in the beryllium first wall of a device like ITER, due to implantation, diffusion, trapping, and neutron-induced transmutation, will be on the order of 100 g rather than the kilogram quantities estimated previously70,100 and most of that will come from neutron-induced transmutations in the Be itself.

The dominant process for long-term retention of tritium in beryllium for ITER is expected to be
codeposition (see Section 4.19.3.2.2) with eroded wall material (i. e., the incorporation of tritium in the deposited layers where impurity atoms or mole­cules are deposited together with eroded material and a flux of energetic or thermal atoms). The inven­tory of this potentially volatile tritium must be kept as low as reasonably achievable (~ 1 kg tritium), in order to minimize the impact on the environment in case of an accidental release, in particular to avoid the evacuation of the neighboring population.

The rate of formation of the codeposited material depends on the energy of the incident particles and on the substrate temperature during the deposition. In ITER, the total amount of tritium trapped in the codeposited layers will strongly depend on whether carbon is retained in the divertor during DT opera­tion. But even in a full metal ITER configuration (e. g., with Be wall and W divertor) there is evidence for potential tritium accumulation for ITER in deposited Be layers.9 In contrast to carbon, tritium codeposition in beryllium layers is expected to be released at relatively low temperature and there are provisions to periodically bake the divertor in ITER at 350 °C to release tritium trapped in the codepos­ited Be layers (see Section 4.19.3.2.2).