Ion-pair formation in strontium chloride and strontium hydroxyde solutions under supercritical water reactor operating conditions

The proposed Generation IV nuclear reactor design is a 'combined cycle' that would have supercritical water flowing through the reactor core and directly into the turbines. To model the behaviour of fission products in the event of a fuel failure, we are examining the aqueous chemistry of strontium salts in high temperature water. We have measured the molar conductivity of solutions of strontium chloride and strontium hydroxide as low as 10^-5 mol kg^-1 from 100 ^oC to 350 ^oC and 20 MPa with a unique high-precision flow-through AC electrical conductance instrument to determine the ion-association constants in these solutions. The results from measurements at the same conditions show that the formation of SrOH⁺ and Sr(OH)₂⁰ ion-pairs is greater than for SrCl⁺ and SrCl₂⁰ ion pairs, and that the neutral species are substantial at concentrations above 10^-3 mol kg^-1 at 350 ^oC. These new formation constant data are required to determine whether the neutral Sr(OH)₂⁰ may be sufficiently soluble in supercritical steam to be carried to the high pressure turbines. The molar conductivities at infinite dilution,λ^o, were found to be a simple exponential function of the solvent viscosity, which can be used to estimate ionic mobilities and diffusion in crevices under supercritical water reactor conditions. (author)