Effects of process-generated hydrogen on RPV walls

5.4. Radiolysis in the primary water

The first step in the model is the calculation of the radiolytic yield for each of these species. As mentioned in the previous chapter, this is calculated using the following equation:

i Γ α 100 !

˜ Fρ N v

i Γ γ 100 +

i Γ n 100 +

G γ

G n

G α

i =

(5.12)

R y

with R y i is the production rate of specie i formed by radiolysis given in mol cm -3 s -1 . G γ i , G n i and G α i are the G-values for specie i due to γ -, neutron- and α -radiation, respectively, in number of particles per 100 eV. Γ γ i , Γ n i and Γ α i are the energy dose rates for γ -, neutron- and α -radiation in rad/s. ˜ 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 . In the above equation every value is known, as the G-values for each species can be found in literature, the Γ values are an input parameter for the model and the other values are conversion factors. The G-values used for the following calculations are given in Table 5.1. One can see that the G-values for the very reactive species, i.e. e – , H, OH, H + , are significantly lower for α -radiation compared to γ - and neutron-radiation. This might seem counterintuitive as α -radiation has a much higher Linear Energy Transfer (LET) value. However, this high LET value means that the collisions between the α particle and the environment are much closer to each other. Therefore, the local concentration of radiolytic species will be very high and the reactive species will neutralize each other again before diffusing into the environment. As such the effective generation of these radiolytic species per amount of energy deposited in the water will be much lower compared to the low LET-types of radiation, i.e. γ and neutron. This principle is not valid for the more stable radiolytic species, i.e. H 2 and H 2 O 2 , as these do not react that easily and therefore, can diffuse to the environment. Figure 5.6 gives an impression of the different densities of radiolytic species produced by high or low LET radiations. [67]

Figure 5.6: Density of radiolytic species for low and high LET radiations. For the high LET radiations, the high density of radiolytic species results in a fraction of them that will recombine before diffusing in the environment. [67]

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