Engineering challenges about hydrogen

• Hydrogen has been attracting considerable attention as a carbon-free energy resource. However, its explosive nature causes difficulty in its widespread use in society.

• In fusion reactors, tritium, the radioactive hydrogen isotope, is used as fuel. Tritium accumulation in fusion power plants should be minimized for radiation safety and fuel-cycle sustainability.

• Hydrogen is also known to damage the mechanical integrity of materials, i.e., hydrogen embrittlement. 

Three basic properties of hydrogen behavior

• Solubility is about how much hydrogen is dissolved in a material. High solubility (= high SC) means more hydrogen is uptaken.

• Diffusivity is about the mobility of hydrogen in a material. High diffusivity (= high DC) means hydrogen can move more distance on average for a given time. 

• Permeability is about the permeation rate of hydrogen from a high-concentration region (e.g., plasma side) to a low-concentration region (e.g., vacuum). High permeability (= high PC) means more hydrogen leaks.

Effects of lattice defects/imperfections

• Lattice imperfections, namely defects, often act as traps for hydrogen.

• Traps largely affect solubility, diffusivity, and permeability.

• The trap capacity, as well as the trap strength, depend on the types of traps. 

  • A vacancy acts as a strong trap, which may capture multiple hydrogen atoms simultaneously. 

  • Grain boundaries may serve not only as traps but also as new pathways for hydrogen in a material.

Nucleasr quantum effects & isotope effects

• Because of hydrogen's low mass, hydrogen atoms have the property of exhibiting pronounced quantum properties under certain conditions. It is known as nuclear quantum effects (NQEs).

• Most of the experimental data for hydrogen is obtained for H, while data for D and T are scarce, even though they are needed in fusion engineering. The difference in the properties between H, D, and T is called the isotope effect.

• In experiments, NQE makes the determination and interpretation of the properties of hydrogen and its isotope effects difficult. Also, in computer simulations, dealing with isotope effects is still challenging.