Professor Peter Santschi discusses the high-risk radioisotope Iodine-129 and issues relating to its safe disposal
Iodine-129 (129I), with a half-life of half-life of 16 million years, is commonly considered the single greatest risk driver in high-level and low-level nuclear repositories. This risk stems from several basic properties of 129I, and under many geochemical conditions, it can move as an anion at nearly the rate of water through the subsurface environment. 129I is also extremely radiologically toxic because over 90% of body burden accumulates in the thyroid, which weighs only about 14g in an adult. There is also a large worldwide inventory of radioiodine as a result of its high fission yield and this inventory is rapidly increasing as a result of nuclear energy production. Radioiodine is produced at a rate of 40 GBq (1 Ci) per gigawatt of electricity produced by nuclear power. To illustrate how the properties of 129I magnify its risk, 129I accounts for only 0.00002% of the radiation released from the Savannah River Site in Aiken, South Carolina, but contributes 13% of the population dose, a six orders of magnitude magnification of risk with respect to its radioactivity.
Iodine-129 from low-level waste is commonly disposed of in cementitious materials. Grout, a dense cementitious fluid, mixed with a reducing slag, is often used to immobilise radionuclides. However, the reducing environment might not be conducive to immobilise iodine. In earlier studies to test this hypothesis, iodine speciation in grout samples with slag (Grout+slag) and without slag (Grout–slag) , and its impact on iodine immobilisation, were determined. Irrespective of whether iodide (I–) or iodate (IO3–) was amended to the aqueous phase, there were no significant differences in iodine uptake Kd values as defined by the iodine ratio in the grout versus in solution (Kd values, [Isolid]/[Iaq]; Kd = 3 mL/g). Desorption Kd values (6.1 to 121.8 L/kg) were significantly greater than uptake Kd values, and the grout-amended iodine speciation (I–, IO3–, organo-I) and grout formulation had a significant impact on desorption-Kd values. Rankings of desorption Kd values by iodine species were I– <<organo-I ≤IO3– and by grout formulation were Grout+slag < Grout–slag. After 28 days of equilibration, organo-I comprised >40% of the iodine in the leachate. The organo-I formed from organic carbon (OC) that originated from the grout material (~0.1% OC). Iodine in the solid and aqueous phases were found to be not in equilibrium. More than 50% of the solid phase iodine was strongly bound and could not be extracted with a strong acid (0.1M HCl), suggesting that a mobile and much less mobile iodine grout fraction were present.
If one would want to immobilise iodine more effectively, different engineering approaches would need to be used to promote binding of I–, IO3–, or organo-I. For example, the silver based immobilisation technologies (e.g., AgCl, Ag-impregnated granular activated carbon, Ag-mordenite) remove iodine from the aqueous phase by promoting the formation of Ag-iodide precipitates. The solubility of AgI is eight orders of magnitude lower than it is for AgIO3. Similarly, coprecipitation of iodine into calcium carbonate phases occurs only with IO3– and not with I– and org-I. It is anticipated that increased attention directed at understanding and quantifying the speciation of radioiodine, as opposed to simply total radioiodine, will lead to improved remediation results to be used for long-term radioiodine disposal in cementitious waste forms.
Department of Marine Sciences
Texas A&M University – Galveston
Galveston, TX 77554, USA