Abstract: The deposition of corrosion products in the core causes CRUD (corrosion related unidentified deposit) induced power shift and CRUD induced localized corrosion, threatening the safety and economy of the reactor. Zinc addition is regarded as a widely used method to effectively alleviate corrosion product deposition in reactor core. Despite the wide spread use of zinc, the understanding of mass balance, thermodynamic equilibrium and kinetic substitution of zinc in the primary circuit is still not clear. This paper focuses on the balance and thermodynamic kinetic behaviors of zinc in the primary circuit from the perspectives of theoretical mechanism and numerical calculation analysis. A mass balance model of zinc in the primary circuit was established to evaluate the addition, removal, deposition, precipitation and substitution of zinc in the primary circuit. The thermodynamic equilibrium and substitution kinetics of zinc in coolant were also evaluated. Theoretical analysis shows that zinc in the coolant is involved in hydrolysis and chemical reactions with iron, nickel, silicon and other elements. Zinc mainly exists in the form of zinc hydroxide under the prototypical conditions of coolant water chemistry. When zinc replaces the iron and nickel elements in the oxide corrosion layer, it tends to replace the divalent ions at the tetrahedral positions in the normal spinel, and the ability of zinc to replace the divalent ions in the inverse spinel needs to be evaluated in combination with the hydrolysis and chemical equilibrium of zinc. Numerical analysis shows that the mass balance prediction results of the model are in good agreement with the observation value from Diablo Canyon 1 reactor plant. Gradually reducing zinc addition amount is an effective means to maintain the zinc concentration in the specific optimal range. Zinc addition can effectively alleviate corrosion release up to about 50% and the thickness of CRUD deposition after zinc addition is 42% of that before zinc addition. This study can provide theoretical support to develop the optimal zinc addition technique.