Internal Report 262
leGras C, Klessa D & Hunt C
Supervising Scientist Division
Department of the Environment, 1997
About the report
Experiments were performed whereby fresh Ranger tailings samples were mixed with high purity water to assess the quantity of contained radionuclides that were solubilised by this process, as a function of time. It is proposed that the gypsum (CaS04.2H20) content of tailings, produced by liming, may entrain a substantial fraction of the radionuclide burden. Because gypsum is sparingly soluble in water (about 2 g/L), coprecipitated radionuclides may be mobilised if gypsum were subsequently dissolved, for example if exposed tailings had egress to the environment at some future time.
To assess whether remobilisation by dissolution would be transient rather than permanent because of rapid readsorption to the mineral matrix of the tailings, two dissolution experiments were performed where sufficient tailings were added to water to provide a saturated solution of gypsum. When this was achieved, no further net dissolution of gypsum (and hence net mobilisation of radionuclides) would occur, and the subsequent solution-phase radioactivity would be determined by adsorption rather than dissolution.
The present results imply that net mobilisation of radionuclides occurs over a two hour period in the case of progressive dissolution of gypsum (undersaturation) whereas in the case of saturation being rapidly achieved, net loss of soluble radionuclides is observed over a two hour period.