An approach to simulating the water-chemistry regime for transient and stationary operating states of a nuclear power system is proposed. The approach is based on the designer's philosophy. The first loop of the nuclear power system is divided into standard units. The special features of the reactor design and the crux of the problem being solved determine the number of units and the interaction between them. The units can be structural components of the loop as well as coolant components. The thermochemical and radiation-chemical transformations of the components of the water coolant are simulated at each unit. The interaction between the physicochemical processes at the units is expressed in the flow of the coolant components between them. The approach developed is illustrated on the analysis of the water-chemistry regime in implementing a passive safety system -a promising solution geared toward increasing the operational safety of the first loop of a nuclear power system.The development of innovative nuclear power systems with two-phase coolants entails the development of models, methods, and programs for validating the water chemistry with prescribed parameters. This requires describing the following:• the influence of the topology and noncondensing gases on interphase mass transfer and transport of coolant components along the first loop of the nuclear power system; • the radiation-chemical transformations of the cooling water in the first loop, taking account of the composition of the reactor radiation and the thermophysical characteristics of the liquid and vapor phases; • the boundary processes in the system water coolant-structural material of the first loop with prescribed radiation, thermophysical, and hydrodynamic conditions in the loop, composition of the coolant, and properties of the materials. The simulation of the chemical transformations occurring with the substances dissolved in the water under the action of fast neutrons, γ rays, and charged particles is based on the currently generally accepted theory of indirect action of radiation [1]. A system of differential equations, whose coefficients are the rate constants and activation energies of the elementary reactions, the primary yield of the products of radiolysis of water, the transfer of energy per unit length, the absorbed

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