## 1 Introduction  
Light-water reactors (LWRs), categorised into boiling-water reactors (BWRs) and pressurised-water reactors (PWRs), are the most widespread type of nuclear power reactors [1]. In LWRs, water plays a role of a neutron moderator and a coolant. The cooling water operates at high pressure and temperature conditions (7-7.5 MPa, 285 \({}^{\circ}\)C in BWRs and ca. 15.5 MPa, 315 \({}^{\circ}\)C in PWRs) and is exposed to intense fluxes of gamma rays and fast neutrons [2] (pp. 960-1023). Exposition to ionising radiation results in the formation of transient radical species: hydrated electrons (\(\mathrm{e_{ad}}^{\bullet-}\)), hydroxyl radicals (OH), hydrogen radical atoms (H\({}^{+}\)), hydroperoxyl radicals (HO\({}_{2}^{-}\)), ions: H\({}_{3}\)O\({}^{+}\), OH\({}^{-}\), O\({}_{2}^{-}\), HO\({}_{2}^{-}\), O\({}^{-}\), and stable molecular products: molecular hydrogen (H\({}_{2}\)), hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)), molecular oxygen (O\({}_{2}\)) [3] (pp. 331-364). The stable oxidants (H\({}_{2}\)O\({}_{2}\) and O\({}_{2}\)) create a corrosive environment contributing significantly to stress corrosion cracking of the structural materials [1, 2, 4, 5, 6]. The harmful effect of oxidants is strengthened by the low acidity (low pH) of water at the operating temperatures. For example, because the ionic product of water (\(K_{\mathrm{w}}\)) abruptly increases with temperature, the pH of liquid neutral water is 5.8 at 300 \({}^{\circ}\)C, compared with pH 7 at 25 \({}^{\circ}\)C. To minimise the concentration of dissolved oxidants and increase pH, the chemicals such as H\({}_{2}\), hydrazine (N\({}_{2}\)H\({}_{4}\)), ammonia (NH\({}_{3}\)), lithium hydroxide (LiOH) are added to the coolant [1]. A quantitative understanding of how these additives change a corrosive environment in the nuclear core is still insufficient but essential to control stress corrosion cracking and maintain the integrity of materials in reactor circuits. In particular, it is important to estimate critical hydrogen concentration (CHC), i.e., the amount of dissolved hydrogen gas needed to suppress the net production of O\({}_{2}\) and H\({}_{2}\)O\({}_{2}\) in the reactor circuit [2] (pp. 960-1023). Direct measurements of the chemical environment at the reactor core are not feasible because of high pressure, high temperature, and intense radiation field. Analysis of the reactor water is possible only at sampling points far away from circulation lines. Therefore, computer simulation is indispensable to model the radiation chemistry of the coolant under different operational conditions and assess the response of the oxidising environment to the additives (H\({}_{2}\), LiOH, N\({}_{2}\)H\({}_{4}\), NH\({}_{3}\)). As knowledge of high-temperature water radiolysis develops the simulations have been widely applied to estimate the chemistry of the coolant, both in the core and in associated pipelines [7; 8; 9; 10; 11; 12; 13; 14]. Usually, simulation of the coolant chemistry is based on the numerical solution of a set of ordinary differential equations describing reactions of the primary and secondary chemical species, which are formed during the exposition of cooling water to ionising radiation. The reliability of such a simulation largely depends on the accuracy of the input data including rate constants for the zero-, first-, and second-order reactions. The former rate constants are directly related to the radiation chemical yields of the primary species formed in ca. \(10^{-7}\) s after the ionising event [3] and expressed in the number of species per 100 eV of energy deposited in the medium. During the past few decades, the input data for the simulation of the high-temperature radiolysis of water have been amassed by several research groups based on the measurements or extrapolation of the data available for lower temperatures [13; 15; 16]. The variations between outputs of simulations using the datasets from different laboratories were examined by [PERSON] et al. [13].