Low-level radioactive river sediment particles originating from the Chalk river nuclear site carry a mixture of radionuclides and metals

The Chalk River Laboratory of Atomic Energy of Canada Ltd., site is located on the Ottawa River approximately 200 km northwest of Ottawa, Canada. The site has two large research reactors: NRX, which operated from 1947 to 1991 and NRU, which continues to operate and is used to produce a significant fraction of the world's supply of medical isotopes. During the course of the operation of the NRX reactor small quantities of radioactive particles were discharged to the Ottawa River through a process sewer discharge pipe. These are now located in river bed sediments within a 0.08 km2 area close to the discharge pipe. In the present study, selected particles were isolated from riverbed sediments. These were then characterized by environmental scanning electron microscopy with energy dispersive micro X-ray analysis (ESEM-EDX). This was undertaken to obtain information on particle size, structure and the distribution of elements across particle surfaces. Based on the results of ESEM-EDX, particles were selected for X-ray absorption nano-tomography analysis, which provides videos showing the 3D density distribution of the particles. Furthermore, 2D and 3D Synchrotron Radiation based X-ray techniques (micro-X-ray fluorescence; micro-XRF, micro-X-ray absorption near edge spectroscopy; micro-XANES and micro-X-ray diffraction; micro-XRD) with submicron resolution (beam size 0.5 μm) were employed to investigate the elemental and phase composition (micro-XRF/XRD) and oxidation states (micro-XANES) of matrix elements with high spatial resolution and sensitivity. Results show that the particles investigated so far varied according to: 1) <∼40 μm diameter sized U fuel particles similar in structure to particles observed from Chernobyl and Krasnoyarsk-26 and 2) larger particles with diameters up to several hundred μm. The larger particles comprised a matrix of low density, sediment material with high density inclusions that contained a range of metals including Cu, Cr, As, Zr, Mo, Hf, Hg, Pb and Th in addition to U. Solid state speciation by means of micro-XRF/XRD tomography showed that the low-density sediment matrix was composed of kaolinite (Al2Si2O5(OH)4), anatase / rutile (TiO2) and calcite (CaCO3). Metal species in the inclusions identified by micro-XRF/XRD tomography included molybdenite (MoS2), meta-cinnabar (β-HgS) and UO2. However, the white line of the U L3micro-XANES profiles was slightly shifted to the left compared to the UO2 standard indicating that some U may be present in an inter-metallic form. To conclude, we have successfully isolated and characterised particles containing radionuclides and a suite of toxic metals from the Ottawa River utilizing state-of-the-art X-ray techniques. (authors)