Release rates of radium and polonium from NORM discharges
The work reported in this study looked at the effect of particle size on the release of radium with leaching time to see if changes should be made to those set in the authorisations to discharge. A more accurate understanding of site-specific processes that affect reconcentration is needed to better model the fate of radionuclides and this will assist SEPA in determining whether changes in the NORM disposal regulations are needed. The first part was to determine the distribution of radionuclides with NORM contaminated sulphate scales by analysing not only bulk samples for specific activities of radionuclides, but also in different sized scale fractions. Considering the radionuclide detection technique of LSA scale, a simple calibration model (experimental and theoretical) was developed for photopeak efficiency calibration of a Perspex holder geometry with HPGe detectors for oil field sulphate scale. T he model has advantages such as its simplicity and ease of use and has been validated successfully for gamma photons up to 1000 keV. The laboratory results indicate that the distribution of Ra and Ac favoured the larger particles in 3 of samples but in one sample it was equally distributed between all particle sizes. The influence of particle size, particle sizes distribution on packing and packing density during sample measurement was considered. The second part of the study was to establish the factors affecting the mechanism controlling the release of radium from the samples. Experimental results demonstrated that the LR values are more related to the specific surface area of a scale particle than to its weight or activity; the LR values generally became less dissimilar, when compared on a surface area basis. The experimental results also suggested that the ‘solid concentration’ effect has significant control over the leach present rate for radium into sea and its dissolved rate was proportional to liquid-to-solid ratio present. Measurements made in the laboratory on the influence of different cation and anion on the release of radium would indicate that scales solubility increase with concentration of chloride ions and decrease with increasing the concentration of sulphate ion. The polonium release study does not reflect any significant change in the environment or its vicinity according to its nature. Considering the chemistry of polonium in discharge waste, a field based polonium uptake experiment has been undertaken with shellfish species and other environmental samples taken from a NORM discharge point in Aberdeen. These have been used as bio-indicators for the levels of Po and Pb with particular emphasis on the seasonal variation in Mytilus edulis at one site of Ythan estuary.