## 1 Introduction  
Together with hydropower, nuclear energy offers considerable potential to reduce greenhouse gas emissions and air pollutants during operation compared to coal or gas-powered plants. This positions it as an essential alternative to burning fossil fuels in the fight against climate change and its consequences. The importance of this perspective was highlighted at the _IAEA International Conference on Climate Change and the Role of Nuclear Power_, held in Vienna, Austria, from 7 to 11 October 2019 [1]. Beyond its environmental benefits, nuclear power presents a solution to the increasing energy needs spurred by worldwide population growth and economic development. This has sparked heightened interest in the advancement and implementation of novel nuclear technologies, including'small modular reactors' (SMRs) [2; 3; 4]. In contrast to their traditional, larger counterparts, SMRs are characterized by their compact size, lower power output, and modular construction. Currently, more than 70 reactor designs are under development worldwide, with commercial deployment anticipated between 2030 and 2040. These designs often incorporate cooling systems from existing, conventional nuclear reactors, showcasing an innovative approach to nuclear energy generation. Our study focuses on SMRs cooled by supercritical water (SCW), which operate under a pressure of 25 MPa and temperatures ranging from 300 to 600 \({}^{\circ}\)C from core inlet to outlet. These systems are scaled-down, lower-power, modular versions of the larger'supercritical water-cooled reactor' (SCWR) concept, functioning under similar temperature and pressure conditions. The benefits of using SCW systems for power generation stem from the increased thermodynamic efficiency achieved through higher operating temperatures. These systems can attain efficiencies of over \(\sim\)45%, compared to \(\sim\)33% for currently operational light water reactors, resulting in significant capital cost savings. Significantly, the SCWR has been recognized by the 'Generation IV International Forum' (GIF) as one of six innovative nuclear energy designs selected for further research and development [4; 5; 6; 7; 8; 9]. The long-term viability of SCWRs and their SCW-SMR variants critically hinges on precise water chemistry management. A key challenge for all SCWR designs is understanding how water radiolysis affects material performance. As the coolant circulates through the SCWR reactor core, the combined effects of neutron and \(\gamma\)-radiation fields induce the formation of various free radicals and molecular products. These include the hydrated electron (e\({}^{-}\)aq), the hydrogen atom (H\({}^{\bullet}\)), molecular hydrogen (H\({}_{2}\)), the hydroxyl radical (\({}^{\bullet}\)OH), hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) and its decomposition product O\({}_{2}\), the hydronium ion (H\({}_{3}\)O\({}^{+}\)), the hydroxide ion (OH\({}^{-}\)), and the hydroperoxyl/superoxide anion radicals (HO\({}_{2}\)\({}^{\bullet}\)/O\({}_{2}\)\({}^{\bullet-}\), depending on pH) [10; 11; 12; 13]. Oxygen (O\({}_{2}\)) is also produced as a secondary radiolytic product through the following reactions [13; 14]:  
\[{}^{\bullet}\text{OH}+\text{H}_{2}\text{O}_{2}\rightarrow\text{HO}_{2}{}^{ \bullet}+\text{H}_{2}\text{O},k_{1}=4.2\times 10^{8}\text{ M}^{-1}\text{ s}^{-1}\text{ at 300 }^{\circ}\text{C} \tag{1}\]  
followed by  
\[{}^{\bullet}\text{OH}+\text{HO}_{2}{}^{\bullet}\rightarrow\text{O}_{2}+\text {H}_{2}\text{O} \tag{2}\]  
and  
\[\text{HO}_{2}{}^{\bullet}+\text{HO}_{2}{}^{\bullet}\rightarrow\text{O}_{2}+ \text{H}_{2}\text{O}_{2}. \tag{3}\]Under the conditions proposed for SCWRs, the physical and chemical properties of water undergo dramatic changes as it transitions from subcritical to supercritical states (see, e.g., [15]). The chemical species formed through radiolysis can aggressively interact with metal alloys in this highly reactive environment, potentially accelerating corrosion processes.