The potential effectiveness of mercury minerals in decreasing the level of iodine-129 in a nuclear fuel waste disposal vault

Abstract

Mercury forms sparingly soluble phases with I under certain conditions. Therefore, Hg minerals have been suggested as potential additives to a clay-based buffer material in a nuclear fuel waste disposal vault for the selective removal of 129I from solution. In oxidizing systems containing HgS, clay (bentonite or kaolinite), and either a synthetic groundwater solution (SGW) or deionized, distilled water, it was found that Hg lowers the concentration of I- in solution to ∼10-6 mol/L. X-ray diffraction analysis indicated that Hg2I2(s) is formed in these systems and is the phase controlling the level of I- in solution. At levels of I- below ∼10-6 mol/L, all the I- is consumed in the formation of HgI20(aq) and no solid HgI phase can form. Bentonite has only a minor effect on the Hg/I system, and kaolinite has little or no effect. Theoretical calculations of the Hg system indicate that, under oxidizing conditions and when the levels of Cl- or Br- are ∼1 and 0.01 mol/L, respectively (values typical for deep groundwaters in the Canadian Shield), Hg2I2(s) is not able to decrease the I- activity below ∼10-5 mol/L, because of the greater stability of H92Cl2(s) and Hg2Br2(s) phases (in the experimental study, Hg2I2(s) is able to lower the level of I- below 10-5 mol/L because the Cl-concentration in the SGW was lower (0.18 moVL) and there was no Br in the system). Furthermore, Hg-I minerals are not stable under reducing to mildly oxidizing conditions. The maximum level of 129I in a disposal vault in Canada is likely to be much less than 10-3.5 mol/L. Therefore, Hg can not significantly decrease the level of 129I in solution. The results of this study indicate that Hg minerals would not be effective additives to a buffer material in a disposal vault in the Canadian Shield for the removal of '2'I from solution. © 1986.