Effects of process-generated hydrogen on RPV walls

4. Sources of hydrogen

the reactions in water due to ionizing radiation is shown in Figure 4.2 [52]. In the first stage, the physical stage, the water molecule gets excited or ionized. In the next stage, these excited or ionized water molecules dissociate or interact with other species in the water to form the end products, which are the radiolytic species. The final stage is called the chemical stage. Here, the radiolytic species will chemically react in the water and species in the environment. Lots of different species are formed, i.e. e – , H, OH, OH – , H 3 O + , H 2 , H 2 O 2 , HO 2 (see Figure 4.2).

Figure 4.2: Different stages in the reactions in water due to ionizing radiation. A time scale is included for the different stages. [52] The total production rate of the different species formed by radiolysis is deter- mined by the amount of energy absorbed in the primary water by every type of radiation: R y i = G γ i Γ γ 100 + G n i Γ n 100 + G α i Γ α 100 ! ˜ Fρ N v (4.10) where R y i is the production rate of specie i formed by radiolysis given in mol cm -3 s -1 , G γ i is the so called “G-value” for the production of specie i due to γ -radiation in number of particles per 100 eV of energy absorbed. Similarly, G n i and G α i are the G- values for specie i due to neutron- and α -radiation, respectively. Γ γ i is the γ -radiation energy dose rate expressed in rad/s. Equivalently, Γ n i and Γ α i are the energy dose rates for neutron- and α -radiation. ˜ F is a conversion factor from rad/s to eV/gs and equals 6.25 10 13 , ρ is the density of water in g/cm 3 and N v is the Avogadro constant, equal to 6.022 10 23 mol -1 . [53] The penetration depth of α -particles in water is very short (order of few tens of µ m). Therefore, the α -particles generated in the fuel will not contribute to the radiolysis of the primary water. However, there is another source of α -particles in the primary water itself. The boron, dissolved in the primary water to control the reactivity, can produce α -particles by an (n, α ) reaction. 10 B 5 + 1 n 0 −−→ 7 Li 3 + 4 He 2 (4.11) 34