Effects of process-generated hydrogen on RPV walls

5.5. Conclusion

research, the only actual interest goes to the different dissolved hydrogen species, i.e. H and H 2 . For these simulations, the B and Li concentrations are taken from the typical operating conditions in Figure 4.1. The H 2 concentration will be taken equal to 35 cm 3 /kg and the O 2 and H 2 O 2 concentrations are 1 ppb. Furthermore, the temperature is taken equal to 300 ◦ C and the γ - and neutron-radiation are 3 10 5 and 5 10 5 G/s, respectively [53]. Figure 5.8 shows the atomic hydrogen and H 2 concentration as a function of the boron concentration in the primary water. As the boron concentration is reduced during the fuel cycle, going from left to right on the x-axis can also be interpreted as increasing time in the fuel cycle. Analysing the graphs in Figure 5.8, clearly shows that there is a very limited decrease of atomic H concentration in the primary water towards the end of the fuel cycle. The concentration is found to be between 8.7 10 -7 and 8.0 10 -7 mol/l. The decrease of H concentration with decreasing boron concentration can be expected. The decreasing boron concentration will result in a decreasing α -radiation in the primary water. Therefore, the generation of radiolytic species, specifically H, will be smaller and as such the steady state concentration will decrease similarly. Also the fact that this decrease is small, is expected as the G-values for α -radiation are smaller compared to those for γ - and neutron-radiation, except for H 2 and H 2 O 2 , as elaborately explained in section 5.4.1. For H 2 no significant change in concentration is detected. The concentration remains constant at 1.102 10 -3 mol/l, which corresponds to the added 35 STP cm 3 /kg for the protection of the primary system against oxidizing conditions. 5.5 Conclusion The three different sources of hydrogen for an in service PWR are analyzed in this chapter. For a dissolved H 2 concentration of 25 and 50 cm 3 /kg in the primary water, it has been found that the equivalent H 2 partial pressure at 300 ◦ C is between 2.29 10 4 and 4.58 10 4 Pa. The second source, which is analyzed, is the corrosion of the RPV wall due to the extreme conditions in the primary system. Despite all the different measures taken to reduce the corrosion rate of the RPV steel, the corrosion is not completely inhibited. It has been found that still a significant amount of hydrogen atoms is generated by the corrosion. Well-thought estimates have lead to a production rate between 50 and 150 mol H per year on the complete RPV surface. Finally, hydrogen generation due to the radiolytic decomposition of water is considered. A model was developed to find the concentration of the radiolytic species in the primary water. This model calculated the radiolytic yield of each specie and subsequently used a set of 60 chemical reactions to find the steady state concentration of these species in the primary water. For the typical primary water chemistry during a fuel cycle, the evolution of the atomic hydrogen and H 2 concentration are calculated. The concentration of atomic H was found to decrease from 8.7 10 -7 to 8.0 10 -7 mol/l from the beginning towards the end of the fuel cycle. No change was noticed in the 55