Studies on gadolinium precipitation in moderator system of nuclear reactor

Gadolinium is used in the moderator system of many Pressurised Heavy Water Reactors (PHWRs) for start-up, shut-down and reactivity control during operation. It is very much essential to maintain gadolinium concentration in the system as desired. It has been reported that gadolinium gets precipitated in as oxalate in carbonated water under the influence of γ-radiation. Hence, studies were carried out to investigate the effect of dose, presence of other metal ions and metal surfaces on the precipitation of gadolinium. The results showed that the amount of carboxylic acids viz., formic acid and oxalic acid, formed due to radiolysis is dependent on the dose and that the curve passes though a maxima. Gadolinium is added in higher concentration in Advanced Heavy Water Reactor. So, experiments with high concentration of gadolinium were also carried out. Ultra pure water saturated with high purity CO₂ containing gadolinium and desired ion/surface was irradiated with γ-radiation from ⁶⁰Co source at 25°C to doses ranging from 2.5-16.6 Mrad. At lower doses, formation of carboxylic acids takes place but as the dose increases, decomposition of these acids starts and hence the concentration Vs dose passes through a maximum. It was found that precipitation of gadolinium as oxalate occurred at lower doses. At higher doses, it was seen that pH of the solution decreases and hence solubility of gadolinium oxalate increases. It was also observed that the amount of gadolinium precipitated varied linearly with the initial concentration of gadolinium varying from 2 ppm to 20 ppm. While for gadolinium concentration from 20 ppm to 400 ppm, gadolinium in particulate form was observed. The amount of carboxylic acids formed depends on the nature of cations present in solution. It was found that the amount of oxalic acid formed in the case of gadolinium was more than that formed in the case of sodium. Presence of metal oxides such as ZrO₂ formed over zircoloy surfaces was found to enhance the precipitation of gadolinium. This was confirmed from the stronger XPS peak of gadolinium in presence of ZrO₂ compared to that in absence of ZrO₂ showed the enhanced precipitation of gadolinium in presence of ZrO₂. Further, the precipitation of gadolinium was also found to be influenced in the presence of metal surfaces such as zircoloy. (author)