PHASE BEHAVIOUR AND THERMODYNAMIC SOLUBILITY CONSTANTS FOR NOVEL NICKEL SULFATE PHASES FORMED UNDER SECONDARY COOLANT HIDEOUT CONDITIONS

Abstract

Inorganic ions such as aqueous sulfate, and sodium enter the secondary coolant loops of nuclear reactors through condenser leaks and can participate in hideout reactions under steam generator deposits. The equilibrium reaction products required to model hideout product formation and release under operating conditions are largely uncharacterized due to difficulties in isolating the solids upon system cooling. This project is measuring the phase behaviour and equilibrium solubility of the sodium – nickel – sulfate – water system at temperatures required to model hideout conditions. Three previously unreported nickel – sulfate phases were isolated by an in situ filtration technique at reaction temperatures between 200-250°C, and aqueous sodium/sulfate ratios between 0.5 to 2.0. These are Ni₃(OH)₂(SO₄)₂·2H₂O – trinickel dihydroxide bis(sulfate(VI)) dihydrate, Na₂SO₄·NiSO₄·4H₂O – nickel blödite, and NaHSO₄·Ni₂SO₄·2H₂O – nickel natrochalcite. Equilibrium experiments were carried out in a stirred reactor at temperatures between 225-300°C to measure the thermodynamic solubility of the phases in equilibrium with an excess of NiO. Solubility constants for the hideout phases have been determined, consistent with the Meissner activity coefficient model.