Tritium Permeation Modeling

Predicting how tritium moves through hot metal walls, a loss path and a containment concern the fuel cycle must account for.

Why tritium moves through walls

Tritium is a small, mobile hydrogen isotope. At the high temperatures of a fusion plant it dissolves into metal walls, diffuses through them, and can escape on the far side. This permeation is both a loss from the fuel cycle and a containment concern. Modeling it accurately is essential to closing the tritium balance and to demonstrating that release stays within limits.

The physics

• Surface uptake: tritium dissolving into the wall from the plasma or coolant side
• Diffusion: transport through the metal, faster at higher temperature
• Trapping: defects and impurities that hold tritium and delay its transit
• Recombination and release: tritium leaving the far surface as molecules

Temperature dependence

Permeation is strongly temperature-dependent: both solubility and diffusivity follow Arrhenius laws, rising sharply with temperature. This couples permeation to the thermal design, hotter walls permeate more, so managing tritium loss and thermal performance are linked problems, not separate ones.

import math

def permeability(P0, activation_energy, T, R=8.314):
    # Arrhenius: permeability rises with temperature
    return P0 * math.exp(-activation_energy/(R*T))

Traps complicate the picture

Real materials contain traps, defects and impurities that temporarily hold tritium. Traps delay permeation and increase the retained inventory, which matters for both loss accounting and safety. Neutron irradiation creates more traps over time, so permeation behavior changes as the material degrades, linking this to degradation modeling.

Kronos framing

Permeation is one of the loss terms in the Hyperion breeder's tritium fuel-cycle accounting, which targets a breeding ratio near 1.8 to carry margin against exactly these losses. Modeling it, with honest treatment of temperature and trapping, is part of showing the cycle can close in simulation ahead of first-of-a-kind first tritium around 2030.