Radioactive waste and sorption in natural systems

The safety authorities responsible for licensing the construction of waste repositories require that the operator perform a detailed analysis of what might happen, if the integrity of the containment system were to be breached. There are two major processes that need to be taken into account in the event of water entering into contact with waste products, namely, the leaching of radioactive products and the subsequent transport of radionuclides by the water through the various barriers to the biosphere. The leaching of radioactive products into the water would be an extremely slow process since wastes are immobilised in a form that is, in principle, insoluble; the barriers that the products would subsequently have to penetrate would considerably delay the physical movement of contaminated water; and, in addition, these barriers would in many cases be capable of partly removing the contamination from the water. This decontamination capability is attributable to a number of processes, the most important of which is referred to by scientists as 'sorption'. Sorption is a major factor in the assessment of the safety of a radioactive waste repository. It is therefore important to develop mathematical models capable of predicting the level of sorption, as well as the degree to which it might vary, in the event of leakage from a repository. Several modelling approaches, and even a number of models, have been proposed as part of an initial description of sorption in natural complex systems. Under the aegis of the NEA, twelve organisations from ten NEA Member countries have decided to take part in an international comparative exercise of sorption models. Once the exercise has been completed a performance assessment will be made of the various modelling approaches proposed, in which the degree of accuracy of each approach will be compared with its intrinsic complexity, and the results published in a report. (author)