Effects of process-generated hydrogen on RPV walls

Chapter 4 Sources of hydrogen

4.1 Introduction It has been shown that the presence of hydrogen in steel can have detrimental effects on the mechanical properties of steel. For hydrogen to be present in steel, hydrogen sources must be present. Several different processes acting as hydrogen sources can be distinguished during the operation of a PWR. First of all, there is the corrosion of the internal side of the RPV wall, which is in direct contact with the primary water. Also other hydrogen sources can be found, like transmutation due to fast neutron injection in the RPV steel, entry of recoil protons after elastic neutron-water scattering, radiolysis of the primary water and dissociation of the dissolved hydrogen gas in the primary water. Among these mechanisms, three of them are generally accepted to be the most important hydrogen sources in a PWR: [27, 44] 1. Dissociation of hydrogen gas dissolved in the primary water. 2. Cathodically generated hydrogen resulting from the corrosion of the RPV wall in contact with the primary water at the steel-water interface. 3. Radiolysis of the primary water by interaction with the neutron irradiation. 4.2 Dissolved H 2 in primary water The first important source of hydrogen for the RPV steel is the dissociation of dissolved hydrogen gas in the primary water at the steel-water interface. This hydrogen gas is added as an inhibitor for the radiolysis of the water and for decreasing the electrochemical potential (ECP) for limiting corrosion of the steel. [44] Oxidizing conditions in the primary water can lead to an increased formation of corrosion products and their transport, together with higher irradiation fields, crud deposits and increased susceptibility to stress corrosion cracking (SCC). It was calculated that the addition of 15 STP cm 3 H 2 per kg of primary water is enough to prevent these detrimental oxidizing conditions. In order to have a conservative amount of 29