Radionuclide solubility, temperature effects

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. 

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. 

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