## 3 Research Results and Discussion
### Modeling of Chemical Radiation-Induced Corrosion Processes Using the COMSOL Software Package  
The first step of the research is the analysis of an extensive list of factors affecting the process of reinforcement corrosion in concrete, mainly under the effect of gamma radiation. Given the fact that the physics of the phenomenon and the mechanism of corrosion under gamma radiation conditions are not implemented by any of the finite element modeling platforms or other proven software numerical analysis methods for building structures [161; 162; 163; 164; 165], this paper proposes a mechanism for taking into account the effect of ionizing radiation in the form of oxygen diffusion. To implement this effect, oxygen concentration was increased in the software package by 1.1 times. This allows us to get as close as possible to the characteristic parameters of reinforcement exposed to gamma quanta, taking into account the accompanying processes occurring in the concrete of the structure [69]. In this case, gamma radiation can be replaced by oxygen diffusion in the concrete structure, which leads to similar values of current density observed during irradiation. This approach ensures more accurate results, close to the experimental data reported in [60; 69]. However, it is noteworthy that at different periods of time conditions affecting the process of reinforcement corrosion are different. In general, they depend on the oxygen input into a structure, residual water in concrete, the external environment, damping conditions (the number and presence of empty pores), carbonization, gamma radiation (gamma heating), as well as the resistance of concrete and the boundary of the passivating layer. The review of open-access sources of research information reported in Section 3.1 of this paper suggests that the following mechanism is implemented at different periods of operation of reinforced concrete structures. This mechanism serves as the basis for modeling performed in the COMSOL software package, and it encompasses the following stages (see Figure 1):  
- _Initial period (the onset of operation)_: the structure contains residual water (chemically bound and in pores); this period is characterized by high (relative to the carbonization period) diffusion of external oxygen together with carbon dioxide in the air; electrical resis  
Figure 1: Conceptual scheme of mechanisms evolving in time (during different periods of operation of a reinforced concrete structure) and relevant for corrosion of the reinforcement exposed to gamma radiation, which serves as the basis for simulation in the COMSOL software package, for a segment of a reinforced concrete wall, where (**a**) is the initial time period (the onset of operation), and (**b**) is the period of substantial carbonization. tance of concrete is low; corrosion currents are low; a reliable passivation layer is formed around reinforcement bars. At this time, some residual water continues to participate in the hydration reaction in concrete; it is also subjected to accompanying radiolysis under the action of gamma radiation, which leads to the release of oxygen in the structure. Therefore, oxygen saturation of the structure is high at this stage. Calcium hydroxide gradually enters into a reaction with carbon dioxide, which leads to the formation of calcium carbonate and water. Hence, calcium carbonate fills the pores and reduces the permeability of the structure, but the water resulting from the reaction is decomposed due to radiolysis, and the structure, including its inner layers, becomes saturated with oxygen, while its permeability goes down (refer to [88] for a comparison between oxygen diffusion in carbonized and non-carbonized concrete), while the electrical resistance of concrete increases, the passivating layer of reinforcement degenerates, and corrosion currents intensify.