The content and behaviour of natural radionuclides in basement-hosted groundwater from Vaalputs, Namaqualand, South Africa

[摘要] English: Vaalputs, the South African radioactive waste disposal facility, is currently licensed to dispose only low and intermediate level radioactive waste. The repository has been disposing radioactive waste since 1986; however, up until May 2011 no long-lived uranium containing waste has been delivered to Vaalputs. The Nuclear Energy Corporation of South Africa (Necsa) has foreseen this disposal and so ordered this study to establish a baseline for the behaviour of naturally occurring radionuclides from the uranium and thorium decay chains in the groundwater of Vaalputs. This baseline will be used to monitor the groundwater below Vaalputs for possible anthropogenic additions to the environmental radioactivity. This baseline was established by studying a unique dataset of 25 years of analysis of activities of man-made and naturally occurring radionuclides as well as cation and anion concentrations in the groundwater at Vaalputs. This database is the result of annual monitoring of the groundwater from a confined set of boreholes on and around the facility as part of the regulatory requirements of radioactive disposal. The analytical results of radionuclides in groundwater from 13 annually and 3 quarterly sampled boreholes have been evaluated during this study. Cation and anion concentrations were measured respectively by inductively coupled plasma optical emission spectrometry and ion chromatography. In routine analyses the activities of the long-lived radionuclides 238U and 232Th were measured by instrumental neutron activation analysis, while the short-lived radionuclide 226Ra was detected by γ-ray spectrometry. The overall radioactivity hazard from total α and β radiation levels were measured by gas flow proportional counting. On occasion groundwater samples have been analysed by α-spectrometry to determine the activities of α-emitting radionuclides from the decay chains of 238U, 235U and 232Th. These analytical results have been integrated in order to evaluate trends in activities of radionuclides, the relative contributions of individual radionuclides to total radiation levels and how these are influenced by groundwater conditions. 238U levels show a natural anomaly in the near-field of the disposal site, attributed to basement rocks anomalously enriched in uranium located close to the disposal trenches. This should be taken into consideration when monitoring the groundwater for possible additions of uranium. One set of duplicate samples from 2009 has been analysed by alpha spectrometry, determining an average activity ratio of 234U/238U in the groundwater of Vaalputs as 4.1. This average ratio has been used in the rest of the study for comparison of the contribution of 234U to total α radiation with the contribution of the other α-emitting radionuclides. However, it is suggested that a more extensive experiment should be carried out to determine a statistically representative activity ratio for the different areas at Vaalputs. High levels of 226Ra, unsupported by 238U, have been found in groundwater from certain boreholes, mostly boreholes lying closely together on the western side of the property. This groundwater also yielded low activity ratios for 234U/238U, lower pH and stronger oxidizing conditions than that of the rest of the area. The distinct host of Namaqualand rocks with the absence of overlaying sedimentary rocks has been suggested as the key to the different geochemical conditions of the groundwater of these boreholes. 232Th and its daughter radionuclides yielded levels far below the guideline of 1 Bq.l-1 given by the World Health Organisation, as is expected from the known low mobility of thorium. A peak in total α radiation levels was seen in 2000 in the near-field area. Assessing the cumulative contributions of the various radionuclides it was very clear that the greatest factor in producing α radiation is 234U. However, no data is available for the levels of 234U in 2000. It is suggested that future occurrences of elevated total α radiation levels should be investigated either by performing alpha spectrometry on a duplicate sample or on a sample collected as soon as possible after the original sampling. Analysis of total β radiation levels were found to be unreliable up to 2005, and since the analysis of β-emitting radionuclides was not part of the scope of this study, no conclusions with regard to the contributors to total β radiation could be made. It is suggested that the elevation of total β radiation levels of specific beta-emitting radionuclides, especially 40K, should be determined.

[发布机构] University of the Free State