A paper recently published in the journal Nuclear Engineering and Technology demonstrated the feasibility of using graphene to control hydrogen isotopes, specifically tritium.
Study: Adsorption of Hydrogen Isotopes on Graphene. Image Credit: Rost9/Shutterstock.com
Tritium, a fast-decaying radioelement of hydrogen with one proton and two neutrons, is produced by nuclear reactors and by the interaction between cosmic rays and atmospheric particles. Tritium beta can diffuse through several materials, has 12.35 years half-life, and decays into helium-3.
Tritium does not pose any significant risk to health as beta particles cannot infiltrate the epidermis when they are outside the body. However, tritium can replace hydrogen in the biological molecules before emitting radiation or decaying after it is absorbed into the body, leading to several adverse health effects such as carcinogenesis.
Several next-generation fusion reactors are fueled by deuterium-tritium (D-T) reactions owing to their high reactivity and large cross-section. Although deuterium is naturally abundant, the supply of tritium is limited. Thus, tritium reactors are designed to produce their tritium in a lithium blanket.
Additionally, producing large amounts of tritium is necessary for self-sufficient fusion reactors. Tritium is also produced in certain fission reactors, such as salt-cooled fission reactors and Canada deuterium uranium (CANDU) heavy-water reactors. Liquid salts used to cool these reactors can breed tritium with neutron activation.
Tritium control methods are required in fuel reactors to maintain their efficiency and prevent negative health effects as tritium can escape from the reactor systems since it can diffuse through several materials, including metals.
Graphene has gained considerable attention as a suitable material for tritium control due to its exceptional thermal conductivity, strength, and large surface area. Several studies have demonstrated the feasibility of using graphene to control hydrogen isotopes.
For instance, functionalized graphene was used to separate certain hydrogen isotopes, while graphene coating on copper catalyst significantly reduced deuterium permeation compared to uncoated copper.
In this study, researchers used molecular dynamics (MD) simulations to evaluate the possibility of using graphene to control hydrogen isotopes. Researchers investigated the penetration, reflection, and adsorption of hydrogen isotopes on graphene during the collision of tritium atoms with various graphene structures using a large-scale atomic/molecular massively parallel simulator (LAMMPS).
The second-generation reactive empirical bond order (REBO) potential was employed throughout all simulations as the REBO model can accurately predict the covalent bond potential energy and simulate the hydrogen-carbon interactions and their bond formations.
The simulation structure comprised 100 hydrogen isotopes and a single graphene layer with 61,490 atoms. The graphene layer length was 40 nm in both y and x directions, while the y and x coordinates of 100 hydrogen isotopes were evenly distributed over the graphene layer, with the hydrogen isotope center set directly above the graphene center.
The z coordinates of the isotopes were set to 10 nm when the graphene layer was located at z=0. The hydrogen isotopes traveled and collided with the graphene layer/layers at a constant velocity. Simulation structures were equilibrated for 30 ps with one fs timestep.
The sizes of the simulation box in the y and x directions were determined carefully to ensure that the system can mimic an infinite-length graphene structure through the periodic boundary conditions. However, the atomic vibration wavelengths in graphene were still limited by the simulation structure size.
Graphene was equilibrated using isothermal, isothermal-isobaric, and microcanonical ensembles to reach 900 K, 300 K, and 10 K target temperatures. The hydrogen isotopes possessed incident energies of 500, 200, 100, 50, 20, 10, 5, 2, 1, 0.5, 0.2, or 0.1 eV. The penetration, adsorption, and reflection rates were determined by computing the number of hydrogen isotopes.
The impact of the number of graphene layers on hydrogen isotope reflection, adsorption, and penetration was also investigated using the simulation structure. Graphene layers were set parallel to the y-x plane at 0.335 nm intervals, and the bottom-most graphene layer was located at z=0.
Isotopic effects were investigated using the same simulation procedure by varying the hydrogen isotope masses. Additionally, the effect of incident angle was examined by performing another simulation where the tritium atoms traveled to the graphene floor with 30 and 60o incident angles.
Wrinkled graphene was generated by randomly indenting a single graphene layer, while crumpled graphene was created more severely indenting a single graphene layer. After the generation of crumpled and wrinkled graphene structures, simulation structures were equilibrated at 300 K and tritium atoms were then directed toward the structures.
Tritium atoms were mostly reflected from the graphene layer at low incident energies. The tritium adsorption rate was highest on a single graphene layer at five eV incident energy. The adsorption rates gradually increased until five eV incident energy and then decreased when the incident energy was raised further.
The incident energy required for the highest adsorption of tritium was shifted to higher values when the number of graphene layers was increased. In five layers of graphene, the tritium adsorption rate reached the highest value when the incident energy was 20 eV.
The adsorption rates were also influenced by the hydrogen isotope mass. The adsorption rates of hydrogen atoms were lowest compared to tritium and deuterium atoms due to the lower momentum of hydrogen atoms.
The adsorption rates of tritium atoms impacting graphene at greater incident angles were lower as the incident energy normal component was quickly decreased with the increasing incident angle.
The flexibility of crumpled graphene improved the tritium atom adsorption rate. At higher incident energies of more than five eV, crumpled graphene demonstrated lower penetration and higher adsorption rates compared to unwrinkled or wrinkled graphene.
No isotopic effects on carbon-hydrogen interactions were observed at incident energies below one eV, as the predominant interaction was derived from the force of π electrons. Between one eV and 50 eV incident energies, heavier isotopes demonstrated lower reflection rates and adsorption rates than lighter isotopes owing to their greater momentum than lighter isotopes.
Higher adsorption rates were observed consistently when the incident angle of the impacting tritium atoms with 0.5-5 eV incident energies was smaller. However, tritium atoms with incident energies greater than five eV impacting graphene at larger incident angles led to lower penetration and higher reflection rates.
This study's findings demonstrated the effectiveness of graphene as an efficient material for controlling hydrogen isotopes, specifically tritium. Moreover, the findings can be utilized to create novel nanomaterials for tritium control. Graphene can be used in several applications for hydrogen isotope control.
Wu, E., Walz, R., Park, J. et al. Adsorption of Hydrogen Isotopes on Graphene. Nuclear Engineering and Technology 2022. https://www.sciencedirect.com/science/article/pii/S1738573322002960?via%3Dihub.