Technical Report NTB 90-25

Natural radionuclide and stable element studies of rock samples from the Osamu Utsumi mine and Morro do Ferro analogue study sites, Poços de Caldas, Brazil.

This report describes a study of the distribution and behaviour of natural radionuclides and selected stable elements at the Poços de Caldas natural analogue study sites. At the Osamu Utsumi mine, the study was focussed upon investigation of the behaviour of natural decay series radionuclides and stable elements at the redox fronts which exist in the mine. Uranium nodules from the mine were also analyzed for natural decay series radionuclides in order to characterize their ages and growth rates, and for natural plutonium. At Morro do Ferro, the objective was to provide additional information on the geochemical behaviour of thorium, uranium and the light rare-earth elements and to relate this to the groundwater flow pattern in an attempt to evaluate the degree of mobilization of these species.

A summary review of those aspects of the geochemistry of natural decay series radionuclides relevant to the interpretation of radioactive disequilibrium is provided along with a detailed treatment of mathematical modelling of natural decay series disequilibria in rock-water interactions. The overall study comprised a number of discrete subprojects carried out in five different laboratories and each of these is described in detail. In addition to constituting a self-contained study, the results and conclusions of this work were also used as an input for modelling studies and other aspects of this integrated research programme.

The natural decay series studies at the Osamu Utsumi mine confirmed the generally greater mobility of U(VI) and Ra than of U(IV), Th and Pa in groundwater. The results further confirmed that, at the position of the drillcore studied, the long-term direction of groundwater flow had been downwards along the line of the main fissure system, rather than upwards as is presently observed.

Dissolution of uranium and other elements at the redox fronts followed by diffusive movement into both the oxidized and the reduced rock is identified as the mechanism giving rise to the observed concentration profiles about the redox fronts. Deposition of uranium, either as thin, dispersed coatings on other minerals or as discrete nodules, occurs in the reduced rock as a consequence of the reduction of U(VI) to U(IV). Deposition of uranium, almost certainly by uptake on iron oxides, is also identified as a significant retardation process in the oxidized rock Some of the uranium nodules are young (on a 105 – 106 year timescale) and exhibit growth rates of 1.8 – 2.6 cm in 106 a, whereas others are old on this timescale and exhibit equilibrium within the natural decay series. The time required for growth of nodules in the reduced rock is estimated to be of the order of 10a at least, while a time of the order of 104 – 105 a is required for dissolution of micronodules stranded in the oxidized rock following the passage of the redox front. The natural 239Pu content of a nodule from the reduced rock was measured as 2.3 ± 0.7 × 108 atoms per gram, consistent with a state of secular equilibrium between 238U and 239PU. The study thus reveals that uranium (plus daughters) and plutonium are chemically stable in the form of nodules in the reduced rock for a time of at least 105 years.

One oft he redox fronts studied was concluded to have been effectively static (on a cm scale) for a period of at least 7 × 105a, while the natural decay series data for the other fronts were consistent with rates of movement in the range 2 – 20 m in 106a, in good general agreement with the estimated rate of regional erosion.

Redistribution of thorium was observed at the redox fronts, with preferential deposition on the reduced sides of the fronts. The degree of this redistribution of thorium is estimated to be at least two orders of magnitude less than the corresponding redistribution of uranium. Separation of uranium from thorium is observed as the redox fronts move downwards and the degree of separation increases with increased length of flow path of the groundwater.

Summary data are provided for a range of stable elements, including the rare-earths, which support the observations made above for uranium and indicate that the zone around the redox fronts contains generally elevated concentrations of most elements in conjunction with active dissolution at the front. This results in a variety of distributions of different elements about the redox fronts. The redox fronts thus represent an initial zone of retardation which would be a positive factor in far-field radionuclide migration considerations. However, a negative aspect of this situation is that the zone of enhanced concentrations moves in response to movement of the redox front and, given a sufficient distance of travel, could result in a breakthrough of high concentrations of radionuclides into the near-surface environment.