Hydrogen and its isotopes behave similarly in many regards. Gaseous protium, deuterium, and tritium are all diatomic gases that dissociate, especially on metal surfaces, and dissolve into the metal lattice in their atomic form (in some materials, such as polymers and some ceramics, the molecules may retain their diatomic character during penetration ofthe material). The isotope atoms readily recombine on the free surfaces, resulting in permeation ofthe gaseous hydro­gen isotopes through metals that support a gradient in hydrogen concentration from one side to the other. In order to understand this process, it is necessary to characterize the source of the hydrogen isotope as well as its transport within the materials.

For the purposes of the presentation in this sec­tion, we focus on tritium and its transport through materials. Much of the discussion is equally valid for the deuterium and protium as well (and subsequent sections normalize data to protium). In this section, we provide background on the diffusivity and solu­bility of tritium in metals and relate these thermody­namic parameters to the permeability. In addition, we discuss the role of trapping of hydrogen isotopes on transport of these isotopes, as well as kinetically limited transport phenomena such as recombination.

4.16.2.1 Equation of State of Gases

In the case of gaseous exposure, the ideal gas equation of state characterizes the thermodynamic state of the gas:

V0 = RT/p [1]

where V® is the molar volume of the ideal gas, T is the temperature of the system in Kelvin, p is the partial pressure of the gaseous species of interest, and R is the universal gas constant equal to 8.31447 J mol-1 K-1. The ideal gas equation of state provides a good estimate for most gases, particularly at low pressures (near ambient) and elevated temperatures (greater than room temperature). In the context of materials exposed to hydrogen isotopes in fusion technologies, the assumption of ideal gas behavior is a reasonable estimate for gaseous hydrogen and its isotopes. More details about the equation of state for real gaseous hydrogen and its isotopes can be found in San Marchi et a/.1