Effect of hydrothermal treatment on plutonium retention in deep liquid radioactive waste disposal

Plutonium (Pu)-containing acidic liquid radioactive waste was injected into a deep sandy aquifer disposal (314-386 m) at the Seversk site, Tomsk Region, Russia, over several decades. Herein, laboratory simulation of the near-field conditions of the injection well was conducted, including the waste zone (acetic acid, hydrothermal conditions at 150 °C, pH 2.4), the zone of displacement solutions (nitric acid, pH 1.9, low-level waste, decreasing temperature) and the remote zone with unaltered disposal sands and neutral pH. A study of Pu behavior in the waste zone during 1 and 3 injection cycles (for 50 h) and an additional 3 months of hydrothermal conditioning revealed Pu(IV) sorption on the surface of secondary precipitates, emphasizing the main role of pH in Pu retention and mobility. X-ray absorption fine structure (XAFS) spectroscopy and high-resolution transmission electron microscopy (HRTEM) were used to determine Pu speciation and preferential phases responsible for Pu retention. Long-term leaching of sorbed Pu proved effective but slow reversible Pu sorption, while multiple injection cycles and additional hydrothermal conditioning reduced the mobility of dissolved Pu species by stabilizing solids containing Pu. Pu(V), partly flowing from the nitric acid zone, is largely retained in the remote zone with neutral pH and fresh sands, serving as a natural migration barrier.