Physicochemical Forms Of Radionuclides Research Articles

ДИНАМІКА ФІЗИКО-ХІМІЧНИХ ФОРМ ЗНАХОДЖЕННЯ РАДІОНУКЛІДІВ У ДОННИХ ВІДКЛАДЕННЯХ ВОДОЙМИ-ОХОЛОДЖУВАЧА ЧАЕС ПІСЛЯ ЇХНЬОГО ОСУШЕННЯ: 1. МОДЕЛЬНИЙ ЕКСПЕРИМЕНТ

ДИНАМІКА ФІЗИКО-ХІМІЧНИХ ФОРМ ЗНАХОДЖЕННЯ РАДІОНУКЛІДІВ У ДОННИХ ВІДКЛАДЕННЯХ ВОДОЙМИ-ОХОЛОДЖУВАЧА ЧАЕС ПІСЛЯ ЇХНЬОГО ОСУШЕННЯ: 1. МОДЕЛЬНИЙ ЕКСПЕРИМЕНТ

Migration ability of plutonium and americium in the soils of Polessie State Radiation-Ecological Reserve

The physicochemical forms of radionuclides in soils determine the processes of their entry into the soil solutions, redistribution in the soil profile, soil–plant and soil–ground or surface waters transfer as well as spreading outside the contaminated area. The vertical distribution of plutonium and americium and their physicochemical forms in soils of Polessie State Radiation-Ecological Reserve (PSRER) were studied with the aim of establishing the potential for radionuclide migration. Samples of alluvial soddy-podzolic and peaty soils with a low (1–3%) and relatively high (~80% of dry sample mass) content of organic matter have been selected for investigation. A method employing sequential selective extraction has been used for analysis of radionuclide physicochemical forms in the soils. Activity concentrations of 238Pu, 239,240Pu and 241Am in the samples were determined via radiochemical analysis with alpha-spectrometric identification of radionuclides. The results indicate that the main proportion of plutonium and americium remains in the 0–20 cm soil layer. The inventories of mobile and biologically available forms of plutonium and americium, expressed as a percentage of the total radionuclide content in soil, lie in the ranges of 1.1–9.4 and 2.7–29% respectively. Greater proportions of mobile and biologically available forms of radionuclides appear to be associated with mineral soil as compared to organic soil. In both mineral and organic soils, the portion of mobile americium is higher than plutonium. The inventories of mobile forms of plutonium and americium increase with the depth of soils.

Radionuclide speciation in effluent from La Hague reprocessing plant in France.

Effluent from the La Hague nuclear fuel reprocessing plant was mixed with seawater in order to investigate the fate of the various radionuclides. Thus, a major objective of the present work is to characterize the effluent from La Hague reprocessing plant and to study how the radionuclide speciation changes with time when discharged into the marine environment. Discharges from the La Hague nuclear reprocessing plant represent an important source of artificially produced radionuclides to the North Sea. The transport, distribution, and biological uptake of radionuclides in the marine environment depends, however, on the physicochemical forms of radionuclides in the discharged effluents and on transformation processes that occur after entering the coastal waters. Information of these processes is needed to understand the transport and long-term distribution of the radionuclides. In the present work, a weekly discharged effluent from the nuclear fuel reprocessing plant at Cap La Hague in France was mixed with coastal water and fractionated with respect to particle size and charged species using ultra centrifugation and hollow fiber ultrafiltration with on line ion exchange. The size distribution pattern of gamma-emitting radionuclides was followed during a 62-h period after mixing the effluent with seawater. 54Mn was present as particulate material in the effluent, while other investigated radionuclides were discharged in a more mobile form or were mobilized after mixing with sea water (e.g., 60Co) and can be transported long distances in the sea. Sediments can act as a sink for less mobile discharged radionuclides (Skipperud et al. 2000). A kinetic model experiment was performed to provide information of the time-dependent distribution coefficients, Kd (t). The retention of the effluent radionuclides in sediments was surprisingly low (Kd 20-50), and the sediments acted as a poor sink for the released radionuclides. Due to the presence of non-reacting radionuclide species in the effluent, a major fraction of the radionuclides, such as Cs-isotopes, 106Ru and 125Sb, in the effluent will be subjected to marine transport to the Northern Seas (i.e., the North Sea, Norwegian Sea and the Barents Sea). The La Hague effluent may, therefore, contribute to enriched levels of radionuclides found in the English Channel, including 90Sr, 60Co and Pu-isotopes, and also 106Ru and 125Sb.

Estimation of apparent rate coefficients for radionuclides interacting with marine sediments from Novaya Zemlya

To assess the impact of radionuclides entering the marine environment from dumped nuclear waste, information on the physico-chemical forms of radionuclides and their mobility in seawater–sediment systems is essential. Due to interactions with sediment components, sediments may act as a sink, reducing the mobility of radionuclides in seawater. Due to remobilisation, however, contaminated sediments may also act as a potential source of radionuclides to the water phase. In the present work, time-dependent interactions of low molecular mass (LMM, i.e. species<10 kDa) radionuclides with sediments from the Stepovogo Fjord, Novaya Zemlya and their influence on the distribution coefficients ( K d values) have been studied in tracer experiments using 109Cd 2+ and 60Co 2+ as gamma tracers. Sorption of the LMM tracers occurred rapidly and the estimated equilibrium K d(eq)-values for 109Cd and 60Co were 500 and 20 000 ml/g, respectively. Remobilisation of 109Cd and 60Co from contaminated sediment fractions as a function of contact time was studied using sequential extraction procedures. Due to redistribution, the reversibly bound fraction of the gamma tracers decreased with time, while the irreversibly (or slowly reversibly) associated fraction of the gamma tracers increased. Two different three-compartment models, one consecutive and one parallel, were applied to describe the time-dependent interaction of the LMM tracers with operationally defined reversible and irreversible (or slowly reversible) sediment fractions. The interactions between these fractions were described using first order differential equations. By fitting the models to the experimental data, apparent rate constants were obtained using numerical optimisation software. The model optimisations showed that the interactions of LMM 60Co were well described by the consecutive model, while the parallel model was more suitable to describe the interactions of LMM 109Cd with the sediments, when the squared sum of residuals were compared. The rate of sorption of the irreversibly (or slowly reversibly) associated fraction was greater than the rate of desorption of the reversibly bound fractions (i.e. k 3> k 2) for both radionuclides. Thus, the Novaya Zemlya sediment are supposed to act as a sink for the radionuclides under oxic conditions, and transport to the water phase should mainly be attributed to resuspended particles.

Size distribution of radionuclides in nuclear fuel reprocessing liquids after mixing with seawater

Discharges from the Sellafield nuclear site are a major source of artificially produced radionuclides detected in the North Sea. The transport, distribution, and biological uptake of radionuclides in the marine environment depends, however, on the physico-chemical forms of radionuclides in the discharged effluent and on the transformation processes which occur after entry into the coastal waters. Information on these processes is needed to understand the transport and long-term distribution of the radionuclides. In the present work, waters from two separate waste tanks at the nuclear fuel reprocessing site at Sellafield were mixed with coastal waters. The diluted radioactive material was fractionated with respect to particle size by using cross flow ultrafiltration. The size distribution patterns of α-emitters, (Pu-isotopes), β-emitters, (90Sr, 99Tc), and γ-emitters, (137Cs, 95Nb, 95Zr, 125Sb), were determined over 48 h. Except for 137Cs, the radionuclides from the two sources exhibit different size distribution patterns. The association of 90Sr and 239,240Pu with colloids is particularly evident. 137Cs is present in a presumably colloidal form, with a mol. wt. < 10 kDa. The distribution patterns do not appear to be significantly influenced by transformation processes taking place in the mixing zone during 48 h.