Permeability of hydrogen and its isotopes is generally defined as the steady-state diffusional transport of atoms through a material that supports a differential pressure of the hydrogen isotope. Assuming steady state, semi-infinite plate, and Fick’s first law for dif­fusion J — —D(dc/dx), we can express the steady — state diffusional flux of tritium as

Ji = — D(Cxi — Cxi) [15]

x2 — x1

where cx is the concentration at position x within the thickness of the plate. Using chemical equilibrium (eqn [7]) and assuming that the tritium partial pres­sure is negligible on one side of the plate of thickness t, the steady-state diffusional flux can be expressed as

Ji — ^ pTT [16]

and the permeability, F, is defined as:

F = DK [17]

Substituting eqns [2] and [5] into eqn [17], the per­meability can be expressed as a function of tempera­ture in the usual manner:

F = K0D0 exp[-(AHs + Ed)/RT [18]

Permeability is a material property that charac­terizes diffusional transport through a bulk material, that is, it is a relative measure of the transport of tritium when diffusion-limited transport domi­nates; see LeClaire21 for an extensive discussion of permeation. By definition, the permeability (as well as diffusivity and solubility) of hydrogen isotopes through metals is independent of surface condition, since it is related to diffusion of hydrogen through the material lattice (diffusivity) and the thermody­namic equilibrium between the gas and the metal (solubility).

In practice, experimental measurements are strongly influenced by surface condition, such that the measured transport properties may not reflect diffusion-limited transport. Under some conditions (such as low pressure or due to the presence of resid­ual oxygen/moisture in the measurement system), the theoretical proportionality between the square root of pressure and hydrogen isotope flux does not describe the transport;21,22 thus, studies that do not verify diffusion-limited transport should be viewed critically. In particular, determination of the diffusivity of hydrogen and its isotopes is partic­ularly influenced by the surface condition of the specimen, since diffusivity is determined from tran­sient measurements. While permeation measurements (being steady-state measurements) are relatively less sensitive to experimental details, the quality of reported solubility relationships depends directly on the quality of diffusion, since solubility is typically determined from the measured permeability and dif — fusivity.1 In addition, trapping affects diffusivity and must, therefore, be mitigated in order to produce solubility relationships that reflect the lattice disso­lution of hydrogen and its isotopes in the metal. These characteristics of the actual measurements explain the fidelity of permeation measurements between studies in comparison with the much larger variation in the reported diffusivity and solubility.