Boiling points and speciation of aqueous electrolyte solutions under 'hideout' conditions in supercritical water-cooled reactor coolant by Raman spectroscopy

The Canadian Supercritical Water-Cooled Reactor ('SCWR') is a conceptual design for 'Generation IV' direct-cycle pressure-tube reactors. Material selection for the CANDU-SCWR design requires the designers to predict water chemistry, and chemistry control procedures, in the range from 275 to 450 ⁰C and 25 MPa. It is not generally appreciated that the presence of dissolved electrolytes can raise the critical temperature of aqueous solutions above that of pure water at T_c = 373.95 ^oC, by as much as 100 ^oC, even at concentrations as low as 2 mol/kg. This study presents a report of the use of small scale capillary cells ('high pressure optical cells', HPOC) to determine the boiling point rise of aqueous electrolyte solutions at pressures of 15, 20, 25 and 30 MPa, and to determine the speciation at temperatures just below the boiling point using Raman spectroscopy. Methods have been developed for obtaining quantitative time dependent reduced isotropic Raman spectra with quartz HPOC capillary cells under these SCWR 'hideout' conditions. Examples of boiling point rise and speciation for monovalent salts, and oxyacid contaminants are reported. (author)