Effects of process-generated hydrogen on RPV walls

Chapter 6 Hydrogen concentration in PWR wall

6.1 Introduction As calculated in previous chapter, a significant amount of hydrogen is generated when a PWR is in hot service conditions. This hydrogen will partially be absorbed by the RPV wall, increasing the hydrogen concentration in the steel. The hydrogen in the primary water will be differentiated from the adsorbed hydrogen on the RPV steel-water interface as the fraction of absorbed hydrogen differs for both sources. The hydrogen in the primary water originates from the dissolved H 2 and the radiolysis by ionizing radiation. This concentration of hydrogen in the primary water is in equilibrium with an equivalent hydrogen gas pressure, as found in Chapter 5. From this hydrogen pressure in the primary system, an equilibrium H concentration in the RPV steel will be calculated. For the hydrogen generated by corrosion, the approach will be different. As the produced hydrogen is adsorbed on the RPV steel surface, it will have the choice between diffusing in the bulk of the steel or to recombine and desorb to become a part of the primary water. Finally there is the hydrogen, resulting from the production of the RPV steel. This hydrogen results from scrap, dissociation of water molecules and fluxes like CaO added to the steel. 6.2 Hydrogen from RPV production Up to this moment, all focus went to the process generated hydrogen sources, i.e. dissolved H 2 in the primary water, corrosion of the RPV steel and radiolytic decomposition of water. However, there is another source of hydrogen. The hydrogen can enter the steel during the production of the steel melt. This occurs typically by dissolved H in the scrap added to the steel, the dissociation of H 2 O from the atmosphere on the steel surface when it is very hot and similarly the dissociation of H 2 O present in the fluxes. These sources of hydrogen are found to play an important 59