Carl A. Alexander

The environmental biogeochemistry of chelating agents and recommendations for the disposal of chelated radioactive wastes

Abstract Chelating agents are used in nuclear decontamination operations because they form very selective and strong complexes with numerous radionuclides. However, if environmentally-persistent chelated wastes are disposed of without pretreatment to eliminate the chelating agents, increased radionuclide migration rates from the disposal sites may occur. The environmental chemistry of the three most common aminopolycarboxylic acid chelating agents, NTA (nitrilotriacetic acid), EDTA (ethylenediaminetetraacetic acid), and DTPA (diethylenetriaminepentaacetic acid) is reviewed. This review includes information on their persistence in the environment, as well as their tendency to form complexes with actinides. Data on the sorption of chelated actinides by geologic substrates and on the uptake of chelated actinides by plants are also presented. Increased solubility and/or migration of radionuclides by chelating agents used in decontamination operations have been observed at two different radioactive waste burial grounds. EDTA was found to be promoting the migration of 6O Co and possibly other radionuclides from liquid waste disposal sites at Oak Ridge National Laboratory (1). Recently EDTA has again been identified in radioactive wastes-this time in trench waters containing from 600–16,100 pCi 238 Pu per liter from solid waste burial grounds in Maxey Flats, Kentucky (2). These observations at Oak Ridge and Maxey Flats suggest that the practice of disposing chelated radioactive wastes should be reevaluated. Three different technical options for disposing chelated low-level radioactive wastes are proposed: 1. [1] Bind the solidified chelated waste in some kind of solid matrix that has a slow leach rate and bury the waste in a “dry” disposal site. 2. [2] Substitute biodegradable chelating agents in the decontamination reagent for the chelating agents that are persistent in the environment. 3. [3] Chemically or thermally degrade the chelating agents in the waste prior to disposal. The relative advantages and disadvantages of each of these options are discussed. We feel that surprisingly little attention has been given to an obvious procedure for the disposal of chelated radioactive wastes: chemically or thermally degrading the chelating agent prior to disposal. Any of the above three options might in fact be a satisfactory approach to the disposal of chelated wastes. However, we suggest that the burial of chelating agents such as EDTA be avoided and that option [3] be given more consideration.

VOLATILIZATION CHARACTERISTICS OF URANIUM MONONITRIDE.

Abstract Experiments have been performed by transpiration, vacuum microbalance, effusate collection, and mass spectrometry to determine mass transport in uranium mononitride. It is shown that under equilibrium conditions uranium mononitride will preferentially lose nitrogen, and molten uranium will be a product at all temperatures above the melting point of uranium. Under dynamic vacuum conditions, however, nitrogen vaporization is markedly curtailed probably as a result of the slow rate of diffusion of nitrogen through uranium mononitride. Under dynamic conditions, it is possible to vaporize UN without the accumulation of molten uranium on the surface. The transpiration determinations of vaporization of UN have been utilized in a thermodynamic cycle to establish a value of 126 kcal/mole as the standard heat of vaporization of uranium. This value was further substantiated by effusate collection which yielded a value of 126.5 kcal/mole for the heat of vaporization of uranium. It is indicated that the UN homogeneity range extends to UN 0.97 at 1600°C and approaches UN 0.96 at 1700°C. Across the portion of the homogeneity range of UN that was examined the activity of uranium changes by a factor of about a thousand at these temperatures.

Thermodynamic activities in Zircaloy-4 by mass spectrometry

Abstract Thermodynamic activity of tin in Zircaloy-4 has been determined over a temperature range from 1062 to 2315 K. At the lower end of the temperatures, the activities were determined from equilibria between the Zircaloy-4 and tellurium vapor. At the high temperature end, activities were determined by direct vaporization. Chromium and iron activities were determined at two temperatures. Activity coefficients were computed for tin and also the iron and chromium. Activity coefficients for chromium and iron are of the order of unity (2 and 3, respectively), indicative of near ideal behavior in the Zircaloy-4. The activity coefficient of tin, however, is of the order of 10 −2 which is indicative of strong attractive forces between the zirconium and tin in Zircaloy-4.

Use of simple thermodynamic and structural parameters to predict self-reactivity hazard ratings of chemicals☆

Abstract A major problem in the transportation, transfer and storage of bulk chemicals is the problem of catastrophic instability under unforeseen situations. N