An approximate calculation of advective gas-phase transport of 14C at Yucca Mountain, Nevada

Abstract A quasilinear partial differential equation, which describes gas-phase transport of a 14 C kinematic wave through a porous medium, is derived, its sensitivity to system variables is analyzed and it is applied to one possible release scenarion at the porposed Yucca Mountain, Nevada high-level radioactive waste repository. Advection, isotope exchange between CO 2 in a flowing gas phase and HCO 3 − in a static aqueous phase, and radioactive decay are incorporated. The governing equation is solved analytically by the method of characteristics. The mass fraction of 14 C in the gas phase,X 14 g , is controlled by radioactive decay. The relatively long half-line of 14 C, about 5720 years, and the relatively shallow proposed burial depth of the radioactive waste, about 350m, requires significant retardation of the 14 C wave velocity for significant reduction in X 14 g . 14 C wave velocity is most sensitive to temperature and pH which control the distribution of total carbon between gas and liquid phase; the greater the partitioning of carbon into the liquid phase, the greater the retardation of the 14 C wave velocity and the greater the ultimate reduction in X 14 g from initial conditions. Partitioning of total carbon into the liquid phase is greatest at low temperatures, C , and high pH values, > 8. Increasing water saturation also tends to retard 14 C wave velocity but to a lesser extent. The governing equation has been applied using conditions that may possibly occur at the proposed Yucca Mountain repository. Calculations indicate that the 14 C wave takes about 5900 years to reach the surface with a X 14 g equal to 25 ppm. Diffusion and dispersion are not of major importance for these conditions. These calculations are approximate due to the number of assumptions involved. Discharge of 14 C into the gas before the selected time would accelerate wave arrival and increase the amount of 14 C reaching the surface.