Thermodynamic Study in Support of Pressurized Water Reactor Specific Reactor Water Chemistry

A theoretical approach is discussed that regards the kinetically determined pressurized water reactor (PWR) primary system as a set of thermodynamically defined metastable states that the related high-temperature aqueous system containing a combination of possible oxide phases (Ni_xFe_3−xO₄, Fe₃O₄, and metallic nickel or NiO) and corresponding dissolution products may undergo under specified initial conditions. The study shows that stability zones of those metastable states, particularly M1 (Ni_xFe_3−xO₄) and M₃ [Ni(m) + Ni_xFe_3−xO₄], cover practically the entire PWR operational range and depend on specific plant conditions and applied chemistry control. The thermodynamic analysis is predicated on the belief that defining the stability transition boundary between those states — found as a function of temperature, coolant pH, dissolved hydrogen (DH), and ferrite stoichiometry (x value) — is of primary importance for corrosion product behavior. Such a stability change influences both the particulate and ionic levels and the related activity transport and should be regarded as an important factor in optimizing PWR primary chemistry. The study offers an original approach to reassessing such important issues as thermodynamic data and the solubility of spinel oxides, the role of transport of particulates and soluble species, “optimum” pH and DH, and the chemistry effect on crud burst.