- 1.52 -


                 dKR
        The term d- in equation (8.51) is evaluated by numerically differentiating KR(T), where KR(T) is an empirical expression based on measured or calculated opacities.

        Krascella and Kesten have made theoretical calculations of the temperature dependent Rosseland mean opacities of uranium (68), hydrogen (69), and possible seed materials (70). The calculated uranium and hydrogen opacities at several pressures are shown in Figures 17 and 18. The opacities were calculated for a plasma consisting entirely of uranium, hydrogen, or seed material rather than some mixed composition. In order to use these values in the composite opacity expression given by (8.56), the fuel, coolant, and seed opacities are weighted by the relative amount of each material. The composite opacity based on this weighting is


          K    KU  U +KH   H    S S                 (8.57)
          R     R  --   R R-    R N
where Ni is the particle density of material i and N =   N.


is the total material density. The seed particle density will be determined by the amount of seed material that is required to reduce the temperature at the inside face of the containment wall below the graphite melting point. By assuming that the seed material is uniformly mixed and