Sorption is a combination of physical and chemical processes by which the migration of radionuclides in the geosphere (and possibly from a radioactive waste repository) can be slowed down under certain conditions. The NEA Sorption Project was launched to study the potential of chemical thermodynamic models for improving representation of sorption phenomena in the long-term safety analysis of radioactive waste repositories.

Sorption Project phases

First phase (1997-1998)

The first phase of the project ran from 1997 to 1998 and focused on a workshop held in Oxford, UK in May 1997. Presentations at the Oxford workshop highlighted the advances that have been made in the field of sorption modelling with many illustrations of the successful interpretation of sorption phenomena in natural samples. A report on using thermodynamic sorption models  for guiding the selection of radioelement distribution coefficient (Kd) was published in 2001.

Second phase (2000-2004)

The second phase of the project was launched in September 2000. It took the form of a benchmarking exercise to interpret well-characterised datasets for the sorption of radionuclides onto complex materials using several modelling approaches employed by the various organisations participating in the project.

The following systems, of interest to the safety assessment of radioactive waste repositories, were considered:

1. neptunium sorption onto hematite
2. selenium sorption onto goethite
3. uranium sorption onto quartz
4. nickel sorption onto clays
5. neptunium sorption onto montmorillonite
6. uranium sorption onto Koongarra weathered schist and component minerals
7. cobalt sorption onto a number of specific soil types.

These cases were selected to allow a number of aspects of the sorption models used by the participants to be tested:

1. sorption of radioelements of importance to radioactive waste disposal
2. modelling of both single mineral systems and complex mineral assemblages
3. sorption onto clays and oxide minerals
4. modelling over a narrow well-defined range of chemical conditions and over a wider range
5. modelling at a range of ionic strengths
6. modelling of competition effects.

The main outcomes and lessons of the Phase II Sorption Project were discussed by a wider audience of implementers, regulators and modellers at an international workshop in October 2005. The workshop provided an overview of the main project results, with emphasis on the merits and limitations of thermodynamic sorption models (TSMs) as well as recommendations on their use. A report on the main findings of the second phase of the Sorption Project had also been released, along with several publications.

On the basis of the main results obtained and lessons learnt during Phase II, alongside the consensus reached during the workshop, the following needs were identified:

• developing guidelines for model development
• elaborating up-scaling criteria (moving from a laboratory scale to a field scale)
• educating modellers and those involved in the development of safety cases on the proper application of thermodynamic sorption modelling to relevant systems,

Third phase (2007-2010)

The third phase of the project was launched in November 2007 to address the needs identified during Phase II. At the end of the phase, a report was issued in which the critical issues were treated in a way that would facilitate communication with waste management organisations and regulatory authorities. A workshop to present and discuss the report was also held.