Evaluation of radioiodine (129I) dissolution from sediment of a brackish lake beside a spent nuclear fuel reprocessing plant in Japan

Abstract

Dissolution fluxes of stable (127I) and radioactive (129I) isotopes of iodine from a brackish lake sediment beside a spent nuclear fuel reprocessing plant in Japan were evaluated through two kinds of experiments: incubation using a sediment core sample for 24 h, and observation of 127I and 129I concentrations in sedimentary pore water. For 127I, the dissolution flux evaluated in the incubation experiment was comparable with that obtained from the vertical gradient of 127I concentration in pore water in the observation experiment. This suggests that degradation of organic matter in the surface sediment is an important source of dissolved 127I found in the water. For 129I, the dissolution flux estimated in the incubation experiment showed negative values, indicating the transfer of 129I to the sediment from the overlying water (i.e., absorption). Moreover, the flux evaluated from the observation experiment was positive. This result suggests that degradation of organic matter in the surface sediment is scarcely important to the supply of 129I from the sediment to the water in the studied lake. The dissolution flux of 129I estimated in the observation experiment was smaller than the absorption flux of 129I in the incubation experiment. This potentially indicates that the dissolution of sedimentary 129I does not significantly change 129I concentrations in the water and sediment of the lake. This hypothesis was consistent with previous research conducted for the studied lake.