Speaker
Description
Tritium accumulation is going to be a serious safety issue in future fusion devices. Accumulation in co-deposited layers is one of the main channels of hydrogen isotope accumulation in tokamaks, at least for low Z-materials [1]. Even W layers can contain up to 5-10 at.% of hydrogen, with some works reporting up to 20 at.% D content [2]. Their properties vary greatly with deposition conditions, such as deposition temperature, deposition rate, hydrogen particle energy and flux during deposition. The range of parameters encountered in fusion devices complicate experimental study in laboratory conditions. This makes it necessary to develop predictive models and scaling equations for hydrogen content in co-deposited layers [3,4]. There are still a lot of questions remaining, such as the difference between retention of different hydrogen isotopes (H vs. D vs. T) and their mixtures, the effect of He on hydrogen accumulation, the role of hydrogen implantation energy on the structure of the co-deposited layer and hydrogen accumulation.
In this work, a review of available experimental and theoretical data is presented for hydrogen isotope co-deposition with a number of fusion-relevant materials, including W, B, and Li. The commonly used methodologies for laboratory studies of co-deposition, and their pitfalls are discussed, such as the differences between post-mortem and in-vacuo analysis, the difficulties in setting up and interpreting experiments with varied hydrogen implantation energies, etc. Applicability of both empirical scaling [3] and theoretical diffusion-advection model [4] are discussed and compared, based on available experimental data, both from literature sources and newly produced data. Experimentally observed difference between H, D and H-D mixture co-deposition with W and B is shown, with an eye towards possible implications for co-deposition in future D-T using fusion devices, including ITER.
Based on the available data, the rate of possible tritium accumulation in ITER is estimated and compared with other predictions. Possible ways of removing tritium from co-deposited layers are demonstrated, such as exposure to light hydrogen isotopes at elevated temperatures up to ~200 °C, which is shown to have > 98% effectiveness in removing deuterium from W-D and Li-D co-deposits.