Radionuclide solubility, temperature

An important consideration in preparing liquid samples is assurance of radionuclide solubility. Radionuclides subject to hydrolysis and radiocoiloidal behavior (see Section 4.2) require a suitably acidic medium. Other radionuclides may require specific reagents to prevent reactions that lead to insolubility or volatility, or a stable environment of temperature or darkness. 

The principal abiotic processes affecting americium in water is the precipitation and complex formation. In natural waters, americium solubility is limited by the formation of hydroxyl-carbonate (AmOHC03) precipitates. Solubility is unaffected by redox condition. Increased solubility at higher temperatures may be relevant in the environment of radionuclide repositories. In environmental waters, americium occurs in the +3 oxidation state oxidation-reduction reactions are not significant (Toran 1994). 

Field measurements of radionuclide migration can be used to help substantiate laboratory measurements of sorption, solubility, and identification of important chemical species. The fourth section describes three field investigations that provide information on the effects of organics, colloids and environmental conditions (Eh, pH, and temperature) on radionuclide transport. The chemical species of radionuclides that are mobile under specific field conditions are identified. 

Temperature. The estimated solubilities presented in this paper are strictly applicable only at 25°C. At elevated temperatures, thermodynamic data exist for only a few radionuclides and much of the data is estimated. This limitation should not significantly affect the applicability of present calculations to a repository in basalt because measured temperatures in the candidate horizons, the Rocky Coulee, Cohassett, McCoy Canyon, and Umtanum flows, are 49°C 1°C, 51°C 2°C, 56°C 1°C, and 58°C 2°C, respectively. 

In liquid-liquid extraction, an immiscible liquid—usually an organic solution—is combined with the sample in aqueous solution in an extraction flask and shaken to achieve good contact between the liquids. A reagent that functions as an extractant may have been added to one phase or the other. Information on the distribution ratio D and the extraction yield E that indicate the extent of purification from specified contaminants is available from many studies (Sekine and Hasegaw 1977). The information should describe the extractant, the organic solvent and the conditions of purity, reagent concentrations, volumes, required time, and temperature. The value of D reflects the ratio of the radioelement solubility in the organic phase to that in the aqueous phase, hence the type of solvent and the chemical form of the radionuclide to be extracted may be inferred from radioelement solubility data. If the initial conditions of the extraction procedure are not identical to those for reported extractions, the extent of extraction must be tested. 

Most fission products (including cesium and iodine) and all actinides escaping the solid AHTR fuel are soluble in the molten salt and will remain in the molten salt at very high temperatures. Fluoride salts were chosen for the liquid-fueled molten-salt reactor because actinides and fission products dissolve in the molten salt at very high temperatures. This same characteristic applies to the AHTR and provides the reactor with a second, independent beyond-design-basis-accident mitigation system to prevent radionuclide release to the environment. 

The interpretation of the results of experiments performed in recent years has yielded contradictory conclusions as to the sources and the mechanisms of contamination buildup. A th one exception, the measures taken on the basis of these results have not resulted in a clear success on the contrary, in some cases a deterioration of the situation has resulted. The question as to the reasons for such consequences emerges and it seems that the failure of many attempts has mainly been due to the fact that only macrochemical aspects (e. g. effect of pH and temperature on the solubility of the corrosion product oxides) have been taken into consideration. In reality, because of the very low mass concentrations of the essential radionuclides and their mother elements in the coolant, severe deviations in behavior from that of macroamounts are to be expected, an effect which is well known in radiochemistry. In particular, in the behavior of Co and Co trace-chemical mechanisms such as surface adsorption onto oxide particles, co-precipitation together with other elements, as well as ion exchange and isotope exchange with other constituents of the corrosion product oxides can be assumed to play an important role, but in most of the investigations performed up to now these have not been considered. 

The carry-over of corrosion product radionuclides with the main steam in the direction of the turbine is effected, on the one hand, by droplet entrainment with the residual moisture content of the steam and, on the other, by steam volatility. Usually, droplet carry-over is the most significant transport mechanism however, the oxides of the primary system metals show a measurable solubility in steam even at BWR operating conditions. At different plants, concentrations of dissolved cobalt on the order of 60 ng/kg were measured in condensed samples of main steam, i. e. significantly higher than could be explained by droplet entrainment (e. g. Hepp et al., 1986). These observations are consistent with the fundamental results on steam volatility of weakly dissociated compounds under BWR operating conditions which were reported by Styrikovich and Martynova (1963). Since only non-dissociated substances are volatile with steam, it has to be assumed that a fraction of the cobalt present as dissolved ions in the reactor water at ambient temperature is converted to non-dissociated oxide, hydroxide or ferrite at the plant operating temperature. 

Some similarities with the Murchison meteorite can be noticed. The Murchison mineral structure is dominated with a phyllosilicate (serpentine) matrix which contains minerals such as olivine, pyroxenes, calcium carbonates, iron oxides (magnetite), iron-nickel sulfides and sulfates [23-25]. It has been altered by water, by heat, by pressure shock waves, by short-lived radionuclides [26,27]. The transformation of olivine and pyroxene chondrules seems to grow with the extent of mineral hydrolysis and the formation of water-soluble organic compounds is described at temperatures below -125 C [28,29]. Aside from any terrestrial contamination, all the classes of organic molecules considered of biological relevance are identified [30-32 and Ref therein] and also non-terrestrial amino acids and enantiomeric excesses [33-35]. 

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