Effects of process-generated hydrogen on RPV walls

5.3. Corrosion of the RPV wall

Figure 5.4: Equilibrium pressure of H 2 for a concentration between 25 and 50 STP cm 3 /kg in the primary water as a function of temperature and a pressure of 150 bar. The upper limit of the grey zone gives the pressure corresponding to 50 STP cm 3 /kg, while the lower limit corresponds to 25 STP cm 3 /kg.

5.3 Corrosion of the RPV wall Another main source of hydrogen in the primary system is corrosion. Despite the usage of a stainless steel lining on the inner wall of the RPV and the measures taken concerning the water chemistry, corrosion can not be completely inhibited. A Pourbaix diagram gives a good insight in the stability of phases as a function of the pH and the ECP. Figure 5.5 gives the Pourbaix diagram of the Fe-Cr-H 2 O and Fe-H 2 O system at a temperature of 320 ◦ C [58]. The ECP in PWR primary circuits has been estimated by Urquidi-Macdonald et al. [53]. They have found a ECP varying between -0.35 and -0.8 V SHE through the core of the PWR, with a O 2 concentration of 5 ppb and a H 2 concentration of 25 cm 3 /kg. This value is confirmed by experimental results from Bosch et al. [59], who has found a ECP of -780 mV SHE for 304 SS in PWR conditions. The typical conditions in the primary water of a PWR reactor at full power are indicated in Figure 5.5, pH equals ± 7 and ECP is ± -0.8. The ECP indicated in the figure corresponds to the ECP calculated by Urquidi-Macdonald et al. . The Pourbaix diagram indicates that the stainless steel lining will form a chromium-rich spinel oxidation layer, FeCr 2 O 4 or a magnetite layer, Fe 3 O 4 . More positive ECP results in the formation of hematite, Fe 2 O 3 , at the steel-water interface. These oxidation 43