11.52. The hazards from deep geologic disposal of high-level radioactive wastes can best be evaluated by modeling the transport of radionuclides of concern through the barriers. In this model, consideration must be given to the decay taking place during the time required for transport. In other words, a risk model might assume that ground water would intrude upon the buried waste package; then dissolution of the various radionuclides into the water would be described. Finally, the various pathways by which the decaying radionuclides can reach people would be modeled.

11.53. One measure[19] of relative hazard from a given radionuclide is based on the concept of water-dilution rate. The calculated rate of future discharge of the radionuclide into the biosphere (“people environment”), after decay, when divided by the maximum permissible concentration of the specific radionuclide in drinking water, yields the water-dilution rate.

11.54. The starting point for the transport model is the dissolving of the radionuclides into the groundwater. However, ceramic fuel pellets discharged from water-cooled reactors are not only insoluble in water, but are encased in clad intended to resist dissolving. Such stored assemblies are then encased in a metal canister. As another option, high-level repro­cessed waste would be incorporated into a solid matrix, such as glass, then encased in a metal canister. For modeling purposes, it is customary to assume a constant level of solution over a period of 10,000 years, although some specific elements are so insoluble that little of such components will actually dissolve.

11.55. The rate at which the dissolved elements are transported through the geologic medium is considerably reduced by “sorption,” a combination of physical and chemical adsorption and absorption, with the soil or rock. However, the sorption tendency varies from element to element, resulting in a different mix leaving the medium than that entering. In general, the heavy elements, such as uranium and plutonium, are highly sorped, while lighter elements are not strongly affected. The very low migration of ura­nium and plutonium has been confirmed by actual experience with the debris from weapons tests and leaked waste from DOE facilities. Thus, the model to determine water-dilution rates for individual radionuclides must take into consideration the input rate from dissolution, the isolation distance, the groundwater velocity, a sorption equilibrium constant, the half-life, and the maximum permissible concentration in drinking water.

11.56. Groundwater is expected to move at a very low velocity in the neighborhood of a geological repository. Also, physical chemical solubility and mass transfer principles apply to the determination of the concentration of the radionuclide in the liquid. A lower concentration value results than if such principles are not considered.

11.57. Since a realistic model is complex and involves many assump­tions, it is premature to cite results. However, a “feeling” for relative risks has been obtained by comparing water dilution rates from buried high — level wastes with those from uranium ore, uranium mill tailings, and coal ash, using consistent modeling assumptions. The buried waste proved to be two orders of magnitude safer than the ore and mill tailings and slightly safer than the coal ash over a period of 1 million years. Of course, another comparison could be made with the lethal doses from various poisonous common chemicals of commerce (i. e., cyanides, arsenides, pesticide com­pounds, etc.) which are not subject to radioactive decay.