On the tenuous nature of passivity and its role in the isolation of HLNW

The use of reactive metals and their alloys (e.g., Ni-Cr-Mo-W-Fe and Fe-Cr-Ni alloys) for isolating high level nuclear waste (HLNW) from the biosphere relies upon a continuing state of kinetic passivity of the metal surface. Without this state, which is due to the formation and continued existence of a 'passivating' oxide film, the alloy would react rapidly with components of the ambient environment (oxygen, water) and the structural integrity of the container would be compromised. The stability of the barrier oxide layers of bilayer passive films that form on metal and alloy surfaces, when in contact with oxidizing aqueous environments, is explored within the framework of the point defect model (PDM) using phase-space analysis (PSA), in which the rate of growth of the barrier layer into the metal, (dL(+)/dt), and the barrier layer dissolution rate, (dL(-)/dt), are plotted simultaneously against the barrier layer thickness, assuming that both processes are irreversible. A point of intersection of dL(-)/dt with dL(+)/dt indicates the existence of a metastable barrier layer with a steady state thickness that is greater than zero. If dL(-)/dt > (dL(+)/dt)(L=0), where the latter quantity is the barrier layer growth rate at zero barrier layer thickness, the barrier layer cannot exist, even as a metastable phase, as the resulting thickness would be negative. In any event, phase space analysis of the PDM permits specification of the conditions over which reactive metals will remain passive in contact with aqueous systems and hence of the conditions that must be met for the viable use of reactive metals and alloys for the isolation of HLNW. (C) 2008 Elsevier B.V. All rights reserved.