Enhanced Sludge Processing of HLW: Hydrothermal Oxidation of Chromium, Technetium, and Complexants by Nitrate

Abstract

Executive SummaryThe Office of Economic Management has identified treatment of High Level Waste (HLW) as the second most costly environmental problem facing the DOE. In order to minimize costs of disposal, the volume of High Level Waste (HLW) requiring vitrification and long term storage must be reduced. Methods for efficient separation of chromium from waste sludges, such as the Hanford Tank Wastes (HTW), are key to achieving this goal since the allowed levels of chromium in the high level glass control waste loading. At concentrations above 0.5 to 1.0 wt.% chromium prevents proper vitrification of the waste. Chromium in sludges most likely exists as the extremely insoluble oxides and minerals, with chromium in the +3 oxidation state. In order to solubilize and separate it from other sludge components, Cr(III) must be oxidized to the more soluble Cr(VI) state. Efficient separation of chromium from HLW could produce significant savings.This project sought to lay the foundation for the application of hydrothermal processing for enhanced chromium separation from HLW sludges. Experiments were conducted which examined four areas:a. Oxidation of insoluble Cr(III) solids to soluble Cr(VI) using oxygen, b. Oxidation of insoluble Cr(III) solids to soluble Cr(VI) using nitrates, c. Transport properties and speciation of reducing and oxidizing agents under hydrothermal conditions, d. Reactions of plutonium and americium under oxidizing hydrothermal conditions. From the results of our experiments in these areas, we have developed a fundamental understanding of chromium speciation, oxidation/reduction and dissolution kinetics, reaction mechanisms, and transport properties under hydrothermal conditions in both simple and complex salt solutions.Experimental results show that at moderate temperatures (125-200°C), insoluble Cr(III) hydroxide can rapidly be oxidized to soluble Cr(VI). The rate of reaction increases with increasing oxygen and hydroxide concentration and increasing temperature. Experiments also show that sludge components such as aluminate can slow the oxidation/dissolution rate, and that at higher temperatures, organic components in the waste can consume the oxygen before it reacts with the chromium. Reactions of chromium with nitrate only become important at temperatures above 350°C. At lower temperatures the rates of dissolution are not controlled by transport properties. At higher temperatures, particularly near phase transition points, reagent transport could begin to slow and effect dissolution rates. A global rate expression was developed for the reactions involving oxygen and nitrates. The expression agrees well with the experimental results.Future experiments should examine the reactions of HLW sludges with oxygen at temperatures between 100°C and 200°C at hydroxide concentrations between 1M and 6M. This work should be performed at the Hanford site. Work at Los Alamos will continue, supported by the Nuclear Weapons Program. It will focus on high temperature hydrothermal processes including the oxidation o...