Abstract: In contrast to a steam pressure regulator, a nitrogen pressurizer offers several advantages, including enhanced maneuverability, a straightforward design, and a compact size. Within nuclear reactors utilizing nitrogen pressurization, nitrogen gas in the reactor core area undergoes decomposition under radiation and interacts with radiation-induced byproducts of water, resulting in the formation of nitrogen hydrides and nitrogen oxides, which impacts the management of water quality within the primary cooling circuit and may leading corrosion intensification. The present numerical model of nitrogen decomposition product yields in the primary circuit is predominantly dependent on a computational model rooted in the kinetics of reactions. This approach involves tackling an extensive set of non-linear equations, which, in turn, hinges on unknown experimental factors. As a result, the current methodology leans heavily on a multitude of conjectures, and the condition number of its Jacobi matrix is notably high, giving rise to substantial calculation inaccuracies, spanning from −50% to 100%. Therefore, this paper aims to establish an accurate and well-convergent numerical computational model to guide the water chemistry control of nitrogen-pressurized reactor systems. From a thermodynamic perspective, the paper employed the radiation chemical yield (g-value) and equilibrium constant method to construct a decomposition calculation model for water dissolving nitrogen in a radiation filed. Specifically, the work employed point-by-point steady state hypothesis, considering that the chemical state reaches equilibrium at each time point. The model solved ten chemical equilibrium equations, an atom conservation equation and an electronic conservation equation at each time point, and simulated the time effect of radiation by changing the primary conditions at each step. Within the temperature range of 288 K to 473 K, this model exhibited relative errors from reported experimental data ranging from −60% to 10%. Employing this model, calculations to ascertain changes in coolant ion concentrations were conducted under reactor operating conditions. Furthermore, the paper presents an in-depth analysis of how calculation parameters influence the obtained results. The research reveals a consistent trend in the pH value, showing a gradual decrease under irradiation until it reaches a slightly acidic state, which comes from a relatively high concentration of NO_2^- ions, emphasizing the need for the addition of alkalizing agents during practical reactor operation to maintain suitable water quality. Additionally, the paper delves into an analysis of how different reactor parameters affect the equilibrium state. Notably, a linear relationship is identified between temperature/nitrogen concentration and the concentrations of typical ions. In contrast, the radiation dose rate does not exert a substantial influence on the final outcomes. This work carries significant reference value for the design of nitrogen pressurized reactors.