Geochemistry of Important Radionuclides in a Geological Repository

1 Thermodynamic Stability and Geochemistry of L Tc, Am, Np, and Pu Aqueous Species and Solids 

Previous discussion indicated that for both spent fuel and reprocessed waste, the chief sources of radioactivity for the first 10 to 100 y are the fission products °Sr and Cs. For later times out to 10 y, isotopes of Am, Pu, and Np are successively the dominant contributors to radioactivity. Assuming, as is likely, that corrosion failure of metal-waste packages in a geologic repository will not be significant before 10 to lO y, both Sr and - Cs with 30 y half-lives, will have decayed to insignificance. The actinides and long-lived fission products and Tc remain important at later times (cf. Krauskopf 1988). The U.S. program assumes that 1, Tc, and the actinides and 

The probability of release and transport of radionuclides from the vicinity of the waste is largely determined by (1) their solubilities in the groundwater (2) their tendency to be adsorbed 

Actinides and Their Daughter and Fission Products Chap. 13 

Thermodynamic data for U and its adsorption reactions have already been discussed. Among other important radioisotopes, the behavior of iodine in is relatively simple and well understood. Iodine occurs as iodate (lOj) in highly oxidized waters and iodide (I ) under more reducing conditions, including in most groundwaters (cf. Pourbaix 1966). At 25°C and below pH = 5, the redox boundary defining equal concentrations of these species for = 10 M is given by 

---