Optimized coolant chemistry with regard to dose rate reduction: calculations with Optimi code

Crud dissolution and transportation from in-core to out-of-core surfaces is one of the main factors causing dose rate build-up. Investigations have shown that this process can be influenced by adjusting the pH of the coolant. Most PWR suppliers recommend application of a co-ordinated coolant chemistry with a specified constant pH of 6.9 at 300oC. However, increasing dose rate levels at many PWR plants indicate that this type of chemistry might not represent the optimum chemistry conditions for these plants. With respect to controlling the dose rate build-up the main objective of the coolant chemistry is to achieve the environmental conditions for the minimum solubility of the surface oxide layers i.e. corrosion products. A coolant chemistry with a constant pH (300oC) at 6.9 considers that the oxide layers mainly consist of magnetite, because magnetite reaches its minimum solubility at this pH. However, as is known today, the chemical composition of the corrosion products on the out-of-core surfaces differs significantly from magnetite. Depending on plant-specific conditions such as structural materials used, operating temperature and applied chemistry these oxides contain different amounts of nickel, cobalt and chromium building ferrites of different compositions. Such kinds of ferrites have their solubility minimum at a pH (300oC) higher than 6.9. The optimum pH is dependent on the corrosion product/oxide layer composition at high temperatures, which is difficult to determine experimentally. Field experience showed that the optimum chemistry conditions can be predicted using theoretical solubility data for spinels by comparing them with operating chemistry data and experienced dose rate build-up. For this purpose an empirical computer calculation code - called OPTIMI Code - has been developed in order to calculate the optimized primary coolant chemistry for each specific plant. (author)