Effects of process-generated hydrogen on RPV walls

7. Hydrogen pressure in PWR

infinitely fast at the steel water interface. This will result in the lower limit for the concentration of hydrogen in the RPV wall. The diffusion coefficients used for this calculation are taken from equations 6.18 and 6.22. These are given together with the boundary conditions in Table 7.2. Corrosion generated hydrogen The first calculation is performed for a case where only corrosion is considered in its least detrimental conditions, 10% absorption efficiency and a hydrogen production rate of 50 mol H/yr. The solution for the hydrogen concentration profile in the RPV wall is shown in Figure 7.3.

(a) Initial and boundary conditions. Position

Condition

Initial condition

RPV wall

Steady state hot in-service condition (see Fig. 7.3a)

Boundary conditions Outer surface

c=0 c=0

Inner surface

(b) Material properties.

Material

Property

Function

Unit

7 exp( − 53900 RT )

2 /s

SS309

Diffusion coefficient (D)

D H =8 . 9 · 10 −

m

SA508 Cl.3 Diffusion coefficient (D) D H =10 − 2 /s Table 7.2: Data used for the calculation of the time dependent concentration profile in the RPV wall during a cold shutdown. (a) The initial and boundary conditions. (b) The diffusion coefficient of hydrogen atoms in the RPV materials. From Figure 7.3, one can see that the concentration profile in the RPV wall does not decrease a lot during a cold shutdown. Only during the first 3 to 4 hours a significant decrease can be observed close to the steel-water interface, as seen in Fig. 7.3c. Similarly, one can see in Fig. 7.3d, that the concentration in the base material of the RPV wall only decreases with approximately 2% between the moment the cold shutdown is initiated and 50 hours later. The major part of this decrease occurs within the first 4-5 hours after the start of the cold shutdown. This very small decrease in both materials is due to the very low diffusion coefficient of hydrogen at room temperature, 298 K. The highest hydrogen fugacity in the RPV wall will be reached when the temper- ature has just reached its lowest value, 298 K. The hydrogen concentration in the ferritic steel has however not decreased a lot, from 0.076 to 0.0746 mol/m 3 . This corresponds to 0.00967 ppm. Using 6.18, the hydrogen fugacity in the ferritic steel of the RPV wall can be found. Sieverts’ constant room temperature is 7.447 10 -4 82 5 exp( 2320 . 3 T − 1 . 0831 · 10 6 T 2 ) m