UC San Diego

Tritium accumulation in nuclear fusion reactor materials is a major concern for practical and safe fusion energy. This work examines hydrogen isotope exchange as a tritium removal technique, analyzes the effects of neutron damage using high energy copper ion beams, and introduces a diffusion coefficient that is a function of the concentration of trapped atoms.

Tungsten samples were irradiated with high energy (0.5 - 5 MeV) copper ions for controlled levels of damage - 10^{-3} to 10^{-1} displacements per atom (dpa) - at room temperature. Samples were then exposed to deuterium plasma at constant temperature (~ 380 K) to a high fluence of 10^{24} ions/m^2, where retention is at is maximized (i.e. saturated). By then subsequently exposing these samples to fractions of this fluence with hydrogen plasma, isotope exchange rates were observed. The resulting deuterium still trapped in the tungsten is then measured post mortem. Nuclear reaction analysis (NRA) gives the depth resolved deuterium retention profile with the 3He(D,p)4He reaction, and thermal desorption spectroscopy (TDS) gives the total amount of deuterium trapped in the tungsten by heating a sample in vacuum up to 1200 K and measuring the evaporated gas molecules with a residual gas analyzer.

Isotope exchange data show that hydrogen atoms can displace trapped deuterium atoms efficiently only up to the first few microns, but does not affect the atoms trapped at greater depths. In ion damaged tungsten, measurements showed a significant increase in retention in the damage region proportional to dpa^{0.66}, which results in a significant spike in total retention, and isotope exchange in damaged samples is still ineffective at depths greater than a few microns. Thus, isotope exchange is not an affective tritium removal technique; however, these experiments have shown that trapping in material defects greatly affects diffusion.

These experiments lead to a simplified diffusion model with defect densities as the only free parameter. After examining the rate limiting processes, it is observed that trapped and solute atoms reach equilibrium concentrations before atoms diffuse further. This ultimately leads to the derivation of a diffusion coefficient that has a non-linear dependence on the concentration of trapped atoms, and this new coefficient can resolve discrepancies of diffusivity measurements in the literature.