Thermodynamic evaluation of PWR crud chemical form

The optimum pH control is being carried out to suppress corrosion and to reduce radiation source by maintaining adequate Li concentration in coolant at PWR plants. On the other hand, hydrogen concentration in the coolant has not been optimized from the standpoint of radiation source reduction. 58Co, as well as 60Co, is a major radiation source. Ni is the parent element of 58Co and has three chemical forms: spinel (NixFe3-xO4 : 0 < x < 1 so on), metallic nickel(Ni(metal)) and nickel oxide (NiO). Some plant data have shown that changes in the dissolved hydrogen concentration (DH) influence the crud concentration in the coolant. This phenomenon is considered to result from change in the chemical form of the crud. This paper describes a study to investigate the relation between crud composition and DH, based on the Gibbs free energy of formation (ΔGfo), the Gibbs free energy change (ΔG) of the chemical reaction, the chemical equilibrium constant (K), and so on (J.P.N.)