Hanford Science and Technology Program: Reaction Transport Experiments Investigating the Migration of 137Cs in Sediments Beneath the Hanford SX Tank Farm

摘要

Over one million gallons of high-level-waste with more than a million curies of {sup 137}Cs have leaked from Hanford tank farms to the sediments beneath the tanks. Early on, it was assumed that cesium migration would be limited because laboratory experiments had shown that cesium strongly sorbs to phyllosilicate minerals common in soils [1-5]. Additionally, minimal cesium desorption has been observed in contaminated Hanford sediments [6]. However, recent observations beneath the Hanford tank farms show that cesium has migrated to greater depths than expected [7]. Various explanations for enhanced cesium migration include (1) physical processes such as fast flow pathways or bypassing of exchange sites in immobile zones, and (2) chemical processes associated with the very high salt contents and high pH of the tank fluids. Ion exchange processes are clearly indicated in the depth profiles of {sup 137}Cs, and potassium, sodium, calcium, and nitrate (acting as a tracer) from the bore holes beneath tank SX-108 and tank SX-115. Below both tanks, cesium concentration peaks are retarded with respect to potassium and sodium concentration peaks. The importance of cation concentration on ion exchange is illustrated by comparing the sodium and tracer profiles beneath the tanks. Pore water with high sodium concentrations at SX-108 show little or no retardation of sodium, as is indicated by superimposed sodium and nitrate peaks. In contrast, at SX-115 sodium is significantly retarded relative to tracers (nitrate and Tc), presumably due to the lower sodium concentrations of the SX-115 leaks compared to SX-108 leaks. Calcium and magnesium form very distinct peaks at the leading edge of the sodium front under both SX-108 and SX-115. Observations such as these, led Zachara and his co-workers [8] to conduct a series of systematic cesium experiments over a wide range of cesium and salt concentrations to develop an ion exchange model that could be used to predict cesium migration beneath the Hanford tank farms. We report reactive transport experiments of cesium sorption to Hanford sediments, with the specific objective of testing the applicability of cation exchange models derived from batch experiments to calculate cesium mobility beneath the Hanford tank farms. These experiments couple cesium exchange thermodynamics determined for Hanford sediments based on the batch experiments by Zachara and co-workers [8] and flow and transport using a reactive transport model [9]. Our experiments include (a) binary sodium-cesium exchange experiments at two different high salt and cesium concentrations, (b) a ternary potassium-sodium-cesium exchange experiment, and (c) a high base experiment in which mineral dissolution and precipitation affect cesium exchange. These experiments provide a more direct measure of retardation of cesium in Hanford sediments, which can be used in field-scale analyses of transport in addition to testing the applicability of the batch-derived exchange model. Our experiments investigate the reversibility of cesium exchange dominated by sorption to high affinity, frayed-edge sites on micas (FES sites) and to basal layers of expandable clay minerals (clay sites). Results of the high salt reactive transport experiments form the building blocks from which we determine the validity of extending the cesium exchange model to the very reactive tank wastes containing concentrated base solutions at elevated temperatures.