Chemical Evolution of Leaked High-Level Liquid Wastes in Hanford Soils

Abstract

A number of Hanford tanks have leaked high level radioactive wastes (HLW) into the surrounding unconsolidated sediments. The disequilibrium between atmospheric CO₂or silica-rich soils and the highly caustic (pH > 13) fluids is a driving force for numerous reactions. Hazardous dissolved components such as ¹³³Cs, ⁷⁹Se, ⁹⁹Tc may be adsorbed or sequestered by alteration phases, or released in the vadose zone for further transport by surface water. Additionally, it is likely that precipitation and alteration reactions will change the soil permeability and consequently the fluid flow path in the sediments. In order to ascertain the location and mobility/immobility of the radionuclides from leaked solutions within the vadose zone, we are currently studying the chemical reactions between: 1) tank simulant solutions and Hanford soil fill minerals; and 2) tank simulant solutions and CO₂.

We are investigating soil-solution reactions at: 1) elevated temperatures (60 - 200 °C) to simulate reactions which occur immediately adjacent a radiogenically heated tank; and 2) ambient temperature (25 °C) to simulate reactions which take place further from the tanks. Our studies show that reactions at elevated temperature result in dissolution of silicate minerals and precipitation of zeolitic phases. At 25 °C, silicate dissolution is not significant except where smectite clays are involved. However, at this temperature CO₂ uptake by the solution results in precipitation of Al(OH)₃ (bayerite). In these studies, radionuclide analogues (Cs, Se and Re - for Tc) were partially removed from the test solutions both during high-temperature fluid-soil interactions and during room temperature bayerite precipitation. Altered soils would permanently retain a fraction of the Cs but essentially all of the Se and Re would be released once the plume was past and normal groundwater came in contact with the contaminated soil. Bayerite, however, will retain significant amounts of all three radionuclides.