## 3 Results
### Effectiveness of Hydrogen Addition in Suppressing Production of Stable Oxidants  
Molecular hydrogen is one of the reducing products of water radiolysis, but the amount of radiolytic H\({}_{2}\) is not enough to suppress the formation of stable oxidants. Therefore, to change the oxidising conditions into the reducing ones, hydrogen gas was injected into the coolant. Hydrogen injection promotes reaction (43), converting 'OH into the reducing H' radical atom, which initiates the transformation of O\({}_{2}\) to H\({}_{2}\)O in a sequence of reactions, as illustrated in Scheme 2.  
Figure 3: Simulated changes in the steady state concentrations of oxidants resulting from base addition vs. fraction of \(\gamma\)-rays in the mixed gamma-fast neutron radiation: (**a**) dose rate 1 kGy\(\cdot\)s\({}^{-1}\); (**b**) dose rate 10 kGy\(\cdot\)s\({}^{-1}\). The solid and dashed lines refer to 2\(\cdot\)10\({}^{-4}\) mol kg\({}^{-1}\) LiOH aqueous solution and to neutral water, respectively. The direction of changes is marked by arrows.  
Numerical simulations were performed for fast neutron and gamma radiolysis of neutral water and the alkaline solution (2\(\cdot\)10\({}^{-4}\) mol\(\cdot\)kg\({}^{-1}\) OH\({}^{-}\)). The initial concentration of added H\({}_{2}\) varied from zero (no hydrogen added) to 1.5\(\cdot\)10\({}^{-4}\) mol\(\cdot\)kg\({}^{-1}\). In Figure 4, the calculated steady-state concentration of H\({}_{2}\)O\({}_{2}\) is presented as a function of the amount of added H\({}_{2}\) for the dose rate (a) 1 kGy\(\cdot\)s\({}^{-1}\) and (b) 10 kGy\(\cdot\)s\({}^{-1}\).  
Simulation shows that the addition of H\({}_{2}\) to the alkaline solution results in a very significant decrease in [H\({}_{2}\)O\({}_{2}\)]. This synergic effect of H\({}_{2}\) and OH\({}^{-}\) can be expected even at a small concentration of added hydrogen. The reduction in H\({}_{2}\)O\({}_{2}\) is more effective for high-LET radiolysis and lower dose rate.  
The steady concentration of O\({}_{2}\) calculated vs. the increasing amount of H\({}_{2}\) is presented in Figure 5. Compared with H\({}_{2}\)O\({}_{2}\), the concentration of O\({}_{2}\) responds significantly more to the amount of added H\({}_{2}\). The reduction in O\({}_{2}\) in neutral water is not very sensitive to LET and dose rate. In the case of alkaline solution, the synergic effect of H\({}_{2}\) and OH\({}^{-}\) is more pronounced for gamma radiolysis.  
In Table 3, the effectiveness of hydrogen injection is expressed by a ratio of the steady-state concentration of H\({}_{2}\)O\({}_{2}\) and O\({}_{2}\) in the absence of extra hydrogen and in the presence of 1.5\(\cdot\)10\({}^{-4}\) mol\(\cdot\)kg\({}^{-1}\) H\({}_{2}\). Data are shown as a function of dose rate (1 and 10 kGy\(\cdot\)s\({}^{-1}\)), LET (fast neutron and gamma radiation), and pH (neutral and alkaline water at 300 \({}^{\circ}\)C). An increase in pH enhances the reducing effect of H\({}_{2}\). In the presence of base the effectiveness of H\({}_{2}\) injection increases about one hundred times for H\({}_{2}\)O\({}_{2}\) and by four orders of magnitude for O\({}_{2}\).  
Figure 4: Simulated response of the steady state concentration of H\({}_{2}\)O\({}_{2}\) to hydrogen addition to neutral water at 300 \({}^{\circ}\)C (black) and to alkaline solution containing 2\(\cdot\)10\({}^{-4}\) mol\(\cdot\)kg\({}^{-1}\) OH\({}^{-}\) ions (blue): (**a**) dose rate 1 kGy\(\cdot\)s\({}^{-1}\); (**b**) dose rate 10 kGy\(\cdot\)s\({}^{-1}\). Solid and dashed lines refer to fast neutron and gamma radiolysis, respectively.