Effects of process-generated hydrogen on RPV walls

4.4. Radiolysis of the primary water

For a safe operation of a PWR, limits are set for the concentration of O 2 . The dissolved oxygen can not be higher than 0.01 mg/kg in the primary water. [51] Despite all these measures to counteract corrosion of the RPV, it can not be inhibited completely. The anodic dissolution at the inner side of the RPV wall will be iron dominated, as it is a stainless steel: Fe −−→ Fe + 2 + 2 e − (4.8) The following cathodic corrosion reduction reactions can occur at the RPV-water interface [48]: H + + e − −−→ H ads O 2 + 4H + + 4e − −−→ 2H 2 O H 2 O 2 + 2H + + 2e − −−→ 2H 2 O H 2 O + e − −−→ H ads + OH − O 2 + 2H 2 O + 4e − −−→ 4OH − H 2 O 2 + 2e − −−→ 2OH − (4.9) The reactions written next to each other indicate the corresponding reaction in an acidic environment (left) and an alkaline environment (right). As the conditions in the primary water of a PWR are proven to be alkaline, the reactions on the right hand site will predominate. [48] The concentration of dissolved O 2 and H 2 O 2 will be very low due to the presence of H 2 . Therefore, the reactions involving these species will not be very predominant. As a result, the first reaction from 4.9 will be the most important. One of the reaction products is atomic hydrogen, adsorbed on the surface of the steel. This will be one of the important hydrogen sources during operation of the PWR. 4.4 Radiolysis of the primary water Another source of hydrogen to be considered is the radiolysis of the primary water. It is long known that radiolysis of water is a source of corrosive and oxidizing species, e.g. O 2 , H 2 O 2 , OH, . . . . In order to avoid strong oxidizing conditions in the primary water, hydrogen gas is added as mentioned earlier. The hydrogen reacts with the oxidizing species formed by radiolysis and decreases the electrochemical potential in the primary water. The concentration of the species, formed by radiolysis, are given by the radiolytic yield of each of these species due to the absorption of ionizing radiation by water and by the chemical reactions, involving these species and changing their concentration. [48] 4.4.1 Yield of water radiolysis Ionizing radiation in general can result in the formation of radiolytic species in water. However, the amount of species formed by each type of radiation is different and the ratio at which the species are formed also differ for each type of radiation. In the RPV of a PWR, three major types of ionizing radiation can be distinguished: neutrons, γ -photons and α -particles. Each of these will have their own radiolytic yield and the total yield for each specie formed will be the sum of them. The pathways of 33