36Cl production and mobility in the Cigar Lake uranium deposit

Can accelerator mass spectrometry (AMS) studies validate risk assessments of the long-term behaviour of contaminants such as radionuclides? AMS measurements on samples from the 1.3 billion-year-old Cigar Lake uranium ore deposit provide one approach to address this question. In Cigar Lake, elevated concentrations of uranium enhance the in situ neutron flux that produces 36Cl and other radionuclides. We calculated the production of 36Cl using a Monte Carlo neutron transport code. We then tested for the loss of 36Cl from ore samples collected from an 8 m stratigraphic section through the deposit by comparing the predicted values (assuming equilibrium between production and decay) with the concentrations measured by AMS. The 36Cl:Cl atom ratios within the ore were more than two orders of magnitude higher than in the surrounding host rock and ranged from 4 to 64 x 10-12. The 36Cl concentrations in the ore, rock, clay and fracture infilling minerals all agree with the values predicted by the Monte Carlo simulations. We conclude that 36Cl has very limited mobility. Even in matrix adjacent to more permeable fractures, there is no evidence that the measured isotopic ratios deviate significantly from the predicted values.