Pourbaix Diagrams for Uranium, Molybdenum and Technetium

Abstract

Pourbaix diagrams represent in redox potential - pH space the isothermal phase equilibrium of a particular element in contact with water. The phase equilibrium involving aqueous ions or complex ions potentially coexisting with solid oxides or hydrated oxides is essential in understanding fuel behaviour in direct contact with water. The treatment will describe a method of constructing the diagrams by Gibbs energy minimization, highlight the significant features of the diagrams, and show how the data may be used in support of a mass transport model.

Recent modelling activity in our laboratory has put emphasis on high temperature equilibrium involving UO₂ with noble metal fission products. Under lower temperature conditions, defective fuel may come into direct contact with the water phase. The chemical consequences require the introduction of aqueous ions into the computations. The data must be consistent with that for the solid oxide phases used in the U-O temperature-composition phase diagram development. A good test of self-consistency is the generation of the Pourbaix diagram for that element. The presentation will show how these diagrams may be developed by means that do not require an a priori knowledge of adjacent phases or domains. The technique of Gibbs energy minimization will be illustrated with graphical and tabular displays of the steps in this versatile approach. The presentation will conclude by showing how the data may be blended together to understand the boundary condition in the transport of Mo and Tc from defective fuel into the primary heat transfer system.