EMSP Project 70070: Reactivity of Primary Soil Minerals and Secondary Precipitates Beneath Leaking Hanford Waste Tanks - Final Report

Abstract

Since the late 1950s, leaks from 67 single-shell tanks at the Hanford Site have released about 1 million curies to the underlying sediments. The radioactive material was contained in water-based solutions generally characterized as having high pH values (basic solutions), high nitrate and nitrite concentrations, and high aluminum concentrations. The solutions were also hot, in some cases at or near boiling, as well as complex and highly variable in composition reflecting solutions obtained from multiple methods of reprocessing spent nuclear fuel. In order to understand the observed and probable distribution of radionuclides in the ground at Hanford, major reactions that likely occurred between the leaked fluids and the sediment minerals were investigated in laboratory experiments simulating environmental conditions. Reactions involving the dissolution of quartz and biotite and the simultaneous formation of new minerals were quantified at controlled pH values and temperature. Result s show that the dissolution of quartz and formation of new zeolite-like minerals could have altered the flow path of ground water and contaminant plumes and provided an uptake mechanism for positively-charged soluble radionuclides, such as cesium. The dissolution of biotite, a layered-iron-aluminum-silicate mineral, provided iron in a reduced form that could have reacted with negatively-charged soluble chromium, a toxic component of the wastes, to cause its reduction and precipitation as a new reduced-chromium mineral. The quantity of iron released in the experiments is sufficient to explain observations of reductions in dissolved chromium concentration in a plume beneath one Hanford tank. Fundamental data obtained in the project are the rates of the reactions at variable temperatures and pHs. Fundamental data were also obtained on aspects of the surface reactivity of clay or layered-silicate minerals, a small proportion of the total mass of the sediment minerals, but a large proportion of the number of sites where reactions can occur. Results were also finalized on a component of a previous project related to the Hanford waste tanks that had the goal of measuring the incorporation of rhenium, an analogue of radioactive technetium, in iron and aluminum-oxides minerals as they aged in tank sludges at higher temperatures. Small amounts of rhenium were occluded in the iron-rich solids and the amount increased with aging time. Results from the quartz and biotite experiments are in a form that can be used in models of fluid flow in the Hanford subsurface. Results from the rhenium experiments can be used to understand aspects of closing certain of the Hanford tanks.