## 4 Results and Discussion
As illustrated in Figure 3, our simulations operate under the assumption that the initial yields of radiolytic species, which form within the first picosecond, are not dependent on temperature [53]. Therefore, any observed variations in these species' yields across different temperatures stem solely from the chemical reactions that govern their formation and decay during track expansion by diffusion. It is crucial to recognize, however, that temperature significantly impacts the initial spatial distribution of these species within the tracks, which in turn influences their reaction kinetics [38]. Compared to low-LET radiation data from \({}^{60}\)Co \(\gamma\) rays, fast electrons, or \(\sim\)300-MeV protons (see [19] and references therein), our computed \(G\)(e\({}^{-}\)aq) and \(G\)(\({}^{\bullet}\)OH) values for fast neutrons show a similar temperature dependence across the studied range but with reduced amplitude. This pattern underscores the high-LET nature of fast neutrons. Indeed, with higher LET radiation, there is an increased occurrence of radical-radical reactions due to the greater local concentration of radicals along the radiation tracks. Many of these reactions occur before the radicals can diffuse into the bulk solution, resulting in fewer radicals escaping through combination or recombination reactions during track expansion [10; 31; 54; 55]. A notable outcome of our simulations is the marked increase in \(G\)(\({}^{\bullet}\)OH) and \(G\)(H\({}_{2}\)), coupled with a corresponding decrease in \(G\)(H\({}^{\bullet}\)), observed at times greater than \(\sim\)0.1 \(\upmu\)s. As discussed earlier, this significant change is attributed to the oxidation of water by the H\({}^{\bullet}\) atom during the homogeneous chemical stage of radiolysis, as described by the following reaction [56; 57; 58; 59]:  
\[\mathrm{H}^{\bullet}+\mathrm{H}_{2}\mathrm{O}\rightarrow\mathrm{\SIUnitSymbol{ \SIUnitSymbolC}}\mathrm{OH}+\mathrm{H}_{2},\,k=(3.2\pm 0.4)\times 10^{4}\; \mathrm{M}^{-1}\;\mathrm{s}^{-1}\;\mathrm{at}\;300\;^{\circ}\mathrm{C} \tag{9}\]  
This reaction is normally insignificant at room temperature; however, it rapidly accelerates at higher temperatures, as demonstrated by its increasing rate constants: 7.7 \(\times\) 10\({}^{4}\) M\({}^{-1}\) s\({}^{-1}\) at 400 \({}^{\circ}\)C, 3.7 \(\times\) 10\({}^{5}\) M\({}^{-1}\) s\({}^{-1}\) at 500 \({}^{\circ}\)C, and 1.3 \(\times\) 10\({}^{6}\) M\({}^{-1}\) s\({}^{-1}\) at 600 \({}^{\circ}\)C, under a pressure of 25 MPa [39]. As illustrated in Figures 2 and 3, the reaction notably boosts the production of \({}^{\bullet}\)OH and H\({}_{2}\) at temperatures exceeding 300 \({}^{\circ}\)C. Specifically, there is an increase of \(\sim\)6 \(G\)-units in the radiolytic formation of \({}^{\bullet}\)OH radicals and molecular hydrogen in SCW between 400 and 600 \({}^{\circ}\)C.  
To enhance the presentation of these findings, Figure 4a-e displays the time-dependent cumulative yield variations, \(\Delta G\)(H\({}_{2}\)), for each reaction involved in the formation and decay of H\({}_{2}\), as determined through our simulations. Notably, this figure underscores the pivotal role of reaction (9); its significance becomes increasingly pronounced at higher temperatures, especially in SCW, emphasizing its critical influence under these conditions. There are primarily two reasons why reaction (9) is crucial for the control and management of water chemistry in proposed SCWRs and their SCW-SMR variants.