ABSTRACT

Equilibrium, Kinetic and Reactive Transport Models for Plutonium

The purpose of this project is to develop computer-based models to predict the behavior of plutonium in natural and engineering systems and to apply these models to important problems related to plutonium at the Pantex Plant and other DOE facilities. Successful completion of the project will result in tools that can better predict the environmental impact of plutonium and to develop better engineered systems to minimize these impacts.

 The behavior of plutonium in natural and engineered systems depends on the chemical species in which it exists and the rates at which it is transformed among different species. Some species are much more mobile in the environment and therefore they have much more impact on human health and environmental systems. It is important to understand what are the stable species that can exist under given conditions (equilibrium), how fast transformations among species occurs (kinetics) and how fast the species move through the environment considering their potential reactions (reactive transport).

Research has been conducted in many places to understand the speciation and transformations of plutonium and under varying conditions. This research can be integrated through general chemical models that speciation under equilibrium conditions and the kinetics of the transformations as they approach equilibrium. These models leverage the value of individual research efforts by making the results more general and thereby more applicable to different environmental systems. They facilitate additional research by providing a means to predict behavior so that experiments can be planned more effectively, by identifying major areas of uncertainty that need additional research and providing a direct way to compare recent results with existing data. Combining knowledge of the equilibrium and kinetics of plutonium chemistry in reactive transport models allows prediction of the movement of plutonium in natural systems which is a key element in evaluating environmental risk of unintentional releases. These models can also aid design of engineered systems to limit environmental exposure. These include systems to reduce waste production by extracting plutonium from lightly contaminated materials and systems that immobilize plutonium in waste forms before disposal.

A four part research plan is proposed. The first task will be to develop equilibrium models of plutonium that are applicable to time scales of relevant natural and engineered systems. Existing models will be evaluated for their ease of use and ease of modification in order to choose the basis of the models to be developed. The model will be linked to a non-linear regression routine so that optimal values of equilibrium coefficients can be calculated from experimental data. A second task will be to develop a kinetic model for important transformations of plutonium. This model will be linked with the equilibrium model so that the approach of a system to equilibrium conditions can be described. It will also be linked to a non-linear regression routine so the model can be used to determine optimal values of kinetic coefficients. The third task will be to combine the equilibrium and kinetic models with a model of advective-diffusive transport to provide a reactive transport model. The fourth task will be to apply the model to natural and engineered systems of importance to the Amarillo National Resource Center for Plutonium.