Carbon Dioxide Contamination of Aqueous Morpholine Solutions and Effects on Secondary Coolant Chemistry Under CANDU Conditions
Creators: name: Fandiño, O.
name: Cox, J. S.
name: McGregor, C.
name: Conrad, J.
name: Liao, K.
name: Tremaine, P. R.
Publisher: name: Taylor & Francis
Year: 2021
Subjects: subject: Biophysics
subject: Biochemistry
subject: Neuroscience
subject: Pharmacology
subject: Biotechnology
subject: 59999 Environmental Sciences not elsewhere classified
subject: FOS: Earth and related environmental sciences
subject: 39999 Chemical Sciences not elsewhere classified
subject: FOS: Chemical sciences
subject: Ecology
subject: FOS: Biological sciences
subject: Inorganic Chemistry

description: Exposure to air can cause amine solutions in CANada Deuterium Uranium (CANDU) reactor secondary coolant circuit feed tanks to absorb carbon dioxide (CO2). Likewise, carbon dioxide can be absorbed directly into the amine-containing secondary coolant by air ingress during shutdown, lay-up, and startup. Sampling operations, including transferring the sample to the laboratory and subsequent analyses, can also provide opportunities for CO<sub>2</sub> contamination. This paper reports the results of laboratory and chemical modeling studies to examine the effects of CO<sub>2</sub> contamination on aqueous morpholine solutions. The chemistry of CO<sub>2</sub> uptake by feed tanks containing up to 50 wt% (11.5 mol·kg<sup>−1</sup>) morpholine at 25°C was modeled using the OLI Studio 9.5.2 chemical equilibrium model, and the speciation was confirmed by <sup>13</sup>C nuclear magnetic resonance spectroscopy (NMR) measurements. The effects of CO<sub>2</sub> contamination on the pH of the secondary coolant containing 60 ppm (0.006 wt%, 7.00 × 10<sup>−4</sup> mol·kg<sup>−1</sup>) morpholine and the resulting effects on the solubility of magnetite and nickel oxide from 25°C and 250°C at steam saturation were modeled as a function of CO<sub>2</sub> loading using the Electrical Power Research Institute chemical modeling software MULTEQ v.8. The chemical modeling calculations show that concentrated alkaline morpholine solutions at room temperature and pressure would be expected to have a strong tendency to absorb CO<sub>2</sub> and have additional uptake abilities due to the formation of morpholine carbamates. For dilute morpholine solutions at room temperature and pressure, the solutions are still sufficiently alkaline to absorb enough CO<sub>2</sub> to cause a measurable change in the pH of the secondary coolant. This effect was shown to be negligible under reactor operating conditions. The absorption of CO<sub>2</sub> would potentially have the most effect on either unprotected feed tanks or during lay-up conditions in the steam generators, as it could depress the pH of the lay-up solution and adversely affect the rate of corrosion in the internal components of the steam generators (e.g., carbon steel materials). The <sup>13</sup>C NMR measurements on samples of 50 wt% aqueous morpholine solutions from feed tanks at the Ontario Power Generation’s Pickering Nuclear Generating Station found that CO<sub>2</sub> was below the 0.02 wt% detection limit, and suggest that the procedures used to avoid CO<sub>2</sub> contamination in feed tanks are effective. The <sup>13</sup>C NMR was shown to be an effective tool for monitoring CO<sub>2</sub> uptake by morpholine solution in the feed tanks under conditions in which they may have undergone abnormal exposure to air.