Assessing Engineered Barrier Preservation - One Step at a Time

So-called ‘engineered barriers’ (wasteform, canister/overpack, buffer/backfill) perform a critical role in the multibarrier concept for the geological disposal of radioactive wastes.

It is important to be able to assess their behaviour over the time periods for which containment of wastes is sought (up to 1 million years in some cases). Consequently, barrier degradation processes such as wasteform dissolution, canister corrosion, and interaction between barriers (e.g. iron-clay, cement-clay reactions) need to be assessed quantitatively in predictive models. In many cases, these degradation processes proceed through what is known as an ‘Ostwald Step Sequence’. This type of materials transformation is named after the pioneering Latvian/German chemist, Wilhelm Ostwald (see Figure), whose eponymous rule states:

"if a reaction can result in several products, it is not the stablest state with the least amount of free energy that is initially obtained, but the least stable one, lying nearest to the original state of free energy"

Clays and zeolites, for example, tend to form poorly ordered precursors prior to the development of more crystalline forms. Growth of these metastable products may delay the attainment of true equilibrium for hundreds of thousands, or millions of years, in many examples.

Quintessa has used Ostwald’s approach to describe engineered barrier degradation and has incorporated relevant processes in its ‘QPAC’ multi-physics reaction-transport computer code. Through funding by the European Commission and the Nuclear Decommissioning Authority, we have applied this to the understanding of the potential degradation of bentonite clay in contact with steel waste packages ("Modelling Iron-Bentonite Interactions", by David Savage, Claire Watson, Steven Benbow, and James Wilson, Applied Clay Science, in press). Examination of natural systems shows that alteration of clay by iron-rich solutions proceeds via an Ostwald step sequence, where the formation of thermodynamically stable non-swelling sheet silicates such as chlorite is delayed by the formation of metastable products, such as berthierine and cronstedtite. This knowledge has been transferred through modelling with QPAC to derive a better, more realistic understanding of engineered barrier degradation processes.

"Picture to yourselves a friendly enthusiastic man, with penetrating eyes, fresh colour and reddish hair, moustache and beard, going the round of the research laboratories every day. If you had a difficulty, Ostwald had a solution to offer. If you had no difficulties, you probably got some new ideas." - F. J. Donnan, "Ostwald Memorial Lecture", J. Chem. Soc., 1933, 326.