Retardation of radionuclides by rock units along the path to the accessible environment

The process that can retard the movement of radionuclides in water are sorption, precipitation, and diffusion. The most important retardation mechanism in the tuffs of Yucca Mountain is sorption. Based on information from the mineralogy-petrology data there is a 100-m-thick zeolited zone in the unsaturated Calico Hills and Prow Pass units below the repository horizon in the Topopah Spring Member. These tuffs are 60% zeolited and exist under the entire 8 x 106 m2 of the repository. If an average cation-exchange capacity of the zeolites of 2.5 meg/g is assumed, the unsaturated zeolitized tuff volumes are capable of sorbing by ion exchange 75 x 105 metric tons of waste elements if the relatively unfractured rock of these zeolitized tuffs all comes into contact with the waste elements. 9 references