The activation of stable isotopes, their migration in soil and groundwater at a radionuclide production facility : a case study in Faure, Cape Town, South Africa

Abstract

The iThemba Laboratory for Accelerator Based Sciences (iThemba LABS or iTL), a facility governed by the National Research Foundation (NRF) based in Cape Town has embarked on a project called the South African Isotope Facility (SAIF). Part of the SAIF will be a project called the Low Energy Radioactive Ion Beam (LERIB) for the production of radioactive beams with the Separated Sector Cyclotron (SSC). The LERIB project is anticipated to generate significant prompt sources of ionising radiation such as neutrons displaced by protons during bombardment of targets. At present, the facility has 3 - 5 m thick concrete walls designed to shield against 200 MeV neutrons. The floor thickness varies from each vault according to the behaviour and characteristics of the produced radiation. The LERIB facility concrete floor is envisioned to be 1 m thick. Insufficient floor shielding might expose the sub-surface strata below the floor with ionising radiation. Some matter in the exposed strata might undergo activation and consequently produce undesirable radioactive isotopes. Exposure to radioactive isotopes can pose potential risks to the general public and the receiving environment. In the sub-surface, the activated isotopes can spread through the soil and groundwater bodies. Groundwater being mobile in nature might act as conduit for activated isotopes to migrate in the subsurface. The half-lives of the formed radionuclides were used to estimate the distance the radionuclides can migrate in the sub-surface before decay. The X-Ray Fluorescence (XRF) and Inductive Coupled Plasma - Mass Spectrometry (ICP-MS) were used to quantify the mass fraction of elements from soil and groundwater samples. The Inductive Coupled Plasma - Optical Emission Spectrometry (ICP-OES) was used to measure dissolved metals of major and minor oxides in groundwater samples. These scientific techniques were applied in order to measure mass fractions of elements present in the sub-surface anticipated to be exposed with neutron radiation. Results revealed that the sub-surface in the study site constitutes Si, Al, Na and Fe in abundance. This suggests that, these elements might have a high probability to undergo activation compared to those occurring at lower concentrations, such as minor and trace elements. However the neutron capture cross-section for these elements varies and hence some elements such as Si and Al were not activated. To investigate elements measured from the ICP-MS, ICP-OES and XRF which might undergo activation, soil and water samples were irradiated for an hour with neutrons from a Beryllium target from a range of 0 – 66 MeV energy. The results revealed that activating the sub-surface with neutron radiation will result to the formation of short and long-lived unstable isotopes. However, isotopes with long half-lives such as 22Na, 52Mn, 54Mn and 46Sc only occurred in soil samples and none measured from the water samples except a short-lived isotope 24Na. These results agrees with the reviewed literature that, water being a product of two hydrogens and one oxygen, with 1H from H2O considered to be a good neutron moderator should have absorbed the neutrons so as to reduce neutron capture cross-sections.