Radioactive Contamination of Concrete: Uptake and Release of Radionuclides

Concrete in nuclear installations may become contaminated by various radionuclides. Consequently, decommissioning and dismantling produce considerable quantities of potentially contaminated materials that must be managed safely and cost-effectively. In this paper we present preliminary results from a research project that aims to improve knowledge about release behaviour of radionuclides from contaminated concrete, and that proposes a scientific approach to calculating the source term for radiological dose assessment for the various management options (e.g. direct reuse, recycling, disposal of rubble). The first step is to consider which nuclides are likely to have contaminated concrete, where they might be located in concrete, and the extent to which they are chemically bound to concrete constituents. Relevant radionuclides include ⁶⁰Co, ⁶³Ni, ⁹⁰Sr, ¹³⁷Cs, ¹²⁹I, U, Pu, Am and other actinide elements. Some nuclides are likely to be bound in specific solid phases and others are sorbed to greater or lesser degrees. The proposed modelling of releases from concrete takes into account the chemical behaviour (speciation, sorption and solubility) of the individual radionuclide contaminants and their binding to concrete phases. Other important factors that will influence release are mechanical and chemical condition of concrete, including cracking, carbonation, sulfate attack and degree of water saturation. Model calculations illustrate the potential release processes of desorption-diffusion, leaching (shrinking core model) and dissolution of discrete solid phases. For example, a scoping calculation suggests that 50-year old concrete may be contaminated with ¹²⁹I to about 1 cm depth from the surface or more if the concrete is degraded, and that subsequent release will occur slowly by diffusion. Strongly sorbed or particulate nuclides such as Pu are likely to remain at the surface. Predicting the behaviour of some nuclides (e.g. Ni, U) is more uncertain because of uncertainty in the key parameters and their dependence on the local chemical conditions. Release models and source terms have been developed as the starting point for (i) the modelling of radiological consequences (i.e. dose assessments) of disposal options for building materials from nuclear installations and the optimisation of the disposal process (i.e. selection of cost-effective and reasonable disposal options), and (ii) the assessment of recycling/reuse options of slightly contaminated materials in order to reduce the amount of waste for disposal.