PyRK has the Material() class with specific solids such as graphite being subclasses inheriting all the attributes of the class.

Additionally, it has the Liquid_Material() class which inherits everything from Material() but with additional attributes like viscosity (solids are not viscous). Additional subclasses of Liquid_Material() exist like FLiBe since it is a liquid coolant.

To model HGTRs, a new subclass Gasous_Material() should be created which inherits attributes from Material() but has additional attributes specific to gasses, like compressibility, or anything else necessary for reactor Kinetics.

These features might require temperature dependent models as well, like those for conductivity, viscosity, specific heat, density.