USE OF A 1-D NUMERICAL MODEL TO EVALUATE THE EMANATION OF RN-222 AT THE LAND SURFACE ABOVE A LOW-LEVEL RADIOACTIVE WASTE DISPOSAL FACILITY

A 1-D numerical model was developed to evaluate the emanation of radon gas from a low-level radioactive waste disposal facility over the 1000-year performance period. The model was used to evaluate radon flux at the land surface so that it could be compared to the established instantaneous flux limit of 20 pCi/m2-s. Rn-222 was the only radon isotope considered since it is the only one with a sufficiently long half-life long that it can diffuse through from the waste disposal zone through air-filled pores to the land surface above the facility before subsequent decay occurs. The numerical model PORFLOW (ACRI, Inc.) was selected to perform the simulations because of its ability to incorporate the radioactive decay chains leading to the in-growth of Rn-222 in addition its ability to simulate multi-phase contaminant transport.

The model was gridded to allow an accurate representation of both the waste disposal zone and the closure cap that will cover the facility following permanent closure. Since Rn-222 exists in the gaseous phase, transport of Rn-222 was attributed entirely to diffusion through the air-filled pore space of both the waste disposal zone and the overlying soil cover. Porous media material properties were selected from the range of plausible values such that simulation results were both reasonable and conservative with respect to predicting whether the established radon emanation limit would be exceeded. Likewise, boundary conditions were selected to insure that reasonable but conservative results were obtained. Each radioisotope that eventually leads to the formation of Rn-222 (Ra-226, Th-230, U-234, U-238 and Pu-238) was evaluated separately using a unit source term such that the results could be utilized to establish reasonable and somewhat conservative facility disposal limits for each parent.