Predictive models for D20 isotope effects on ionization and metal hydrolysis under CANDU reactor coolant conditions

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CANDU nuclear reactors are a uniquely Canadian technology in that their design is based on the use of heavy water in a closed loop to transfer heat from the reactor core to the steam generator. The pH of the heavy water in this "primary coolant system" is tightly controlled to minimize the effects of corrosion and radioactive transport. Optimizing primary coolant chemistry requires detailed models for the chemical behaviour of metal oxides, dissolved gases and pH-control additives at temperatures as high as 300 °C, using data determined in light-water systems. The methods now used to correct these models for the differences between light-water and heavy-water systems are based entirely on room temperature studies. This proposal is for the final phase of a definitive laboratory study to provide fundamental data and understanding for the difference in ionization constants between H2O and D2O, for simple acids and bases at the extreme temperatures and pressures encountered in nuclear reactors (250 to 300 °C and 10 MPa). The first two phases developed high precision AC conductance, densimetry, and UV-visible methods to measure the deuterium isotope effect on acid-base ionization. These state-of-the-art instruments, constructed of inert materials to withstand the corrosive conditions that exist in high temperature water, allow us to measure differences in the chemical equilibrium constants in H2O and D2O under identical conditions, directly. The final phase will use these instruments, and a custom-made Raman spectrometry system, to measure data for a number of model systems and to develop an improved, practical model for estimating the magnitude of D2O isotope effects on metal hydrolysis and metal oxide solubility, under CANDU operating conditions. The project will contribute to research aimed at extending the lifetime of existing reactors by providing criteria for optimizing primary circuit pH to reduce feeder tube thinning. It will make a long term contribution to Canada's leadership role in heavy water technology by providing a fundamental understanding of D2O isotope effects on chemical equilibria under extreme conditions of temperature and pressure.