Effects of process-generated hydrogen on RPV walls

4. Sources of hydrogen

where

-1 ) 2

K ∗ W = K W / (mol kg

ρ ∗ W = ρ W / (g cm -3 ) with ρ W , the density of water expressed in g/cm

3 , and T , the temperature in Kelvin.

The optimal values for the 7 parameters used in the equations are: A = − 4 . 098 B = − 3245 . 2K

C = +2 . 2362 · 10 5 K 2 D = − 3 . 984 · 10 7 K 3 E = +13 . 957 F = − 1262 . 3K G = +8 . 5641 · 10 5 K 2

The pressure dependence of the ionic product is implemented by the density, ρ W . According to the National Institute for Standards and Technology (NIST), the density of water at a pressure of 150 bar (typical pressure in the PWR) is given by [50]: ρ W = A + B · ( T − 273 . 15)+ C · ( T − 273 . 15) 2 + D · ( T − 273 . 15) 3 E · ( T − 273 . 15) 4 (4.5) with A = +1 . 0061

B = +0 . 07778 · 10 − 3 K − 1 C = − 0 . 00714 · 10 − 3 K − 2 D = +2 . 27281 · 10 − 8 K − 3 E = +4 . 94641 · 10 − 11 K − 4

Using these functions, one can find that the ionic product of water at 300 ◦ C and 150 bar is equal to 10 -11.3 . Therefore, the pH of neutral water under those conditions is 5.65. The primary water in the reactor, with a pH between 6.9 and 7.4 is therefore quite alkaline. The corresponding room temperature pH is around 9 to 10. As mentioned above, hydrogen gas is added to the primary water to have reducing conditions in the primary water. When oxygen is dissolved in the primary water, it will increase the ECP and result in more oxidizing conditions, resulting in stronger corrosion of the materials. Therefore, the concentration of oxygen gas in the primary water must be reduced as far as possible. This is performed by the addition of H 2 during operation, according to the reaction: [45] 2H 2 + O 2 −−→ 2H 2 O (4.6) Similarly the presence of hydrogen peroxide, H 2 O 2 , will increase the ECP. Again, the dissolved hydrogen will react with the H 2 O 2 to limit the oxidizing conditions in the primary water. [45] H 2 + H 2 O 2 −−→ 2H 2 O (4.7) 32