Effects of process-generated hydrogen on RPV walls

5. Hydrogen production in PWR

Figure 5.5: Pourbaix diagram for a) Fe-Cr-H 2 O and b) Fe-H 2 O system at a temperature of 320 ◦ C. [58]

layers passivate the material, resulting in a reduced corrosion rate. The corrosion rate for the RPV wall will be given by the passive current densities of the stainless steel for the primary water conditions. This corrosion rate was measured by different studies and was found to in the range of 0.1 to 1 µ A/cm 2 : 1. 0.1 – 1 µ A/cm 2 [60] 2. 0.3 µ A/cm 2 [61] When it is assumed that the first cathodic reduction reaction from equation 4.9 is the only one that consumes the electrons, H 2 O + e − −−→ H ads + OH − every electron will result in the production of an adsorbed hydrogen atom on the steel surface. Faraday’s law gives a relation between the amount of reaction products formed and the current density of the redox reaction. [62] where ˙ n i is the production rate of the reaction product, i, in mol cm current density of the redox reaction in Ampère/cm 2 , z i is the number of electrons exchanged in the reaction and F is the Faraday constant, equal to 96 485 C/mol. Using the current densities found above, it is found that the production rate of H is equal to: 44 -2 s -1 , I is the ˙ n i = I z i · F (5.7)