Effects of process-generated hydrogen on RPV walls

4. Sources of hydrogen

dissolved H 2 in the primary water during operation, the concentration is typically in the range 25–50 STP cm 3 /kg. [45] Henry’s law gives the relation between a gas fugacity in equilibrium with the concentration of gas dissolved in a solvent. For the H 2 -water case this relation is: c H 2 = H H 2 · f H 2 (4.1) where c H 2 is the concentration of hydrogen gas dissolved in the water, expressed in STP cm 3 /kg, H H 2 is the Henry constant for hydrogen gas in equilibrium with water, expressed in STP cm 3 kg -1 bar -1 and f H 2 is the fugacity of the hydrogen gas, expressed in bar. This law, of course, can be transformed to different units, of which the most general probably is fugacity in Pa, Henry constant in Pa and the concentration in mole fraction of hydrogen gas in the water. The fugacity of H 2 differs only 3.5% from the pressure, when one is below 100 atm. [46] Therefore, differentiation between fugacity and pressure will only be done in very extreme conditions. 4.3 Corrosion of the RPV wall The corrosion rate in a RPV in operating conditions depends on many variables, e.g. pH, temperature, time, velocity of the water, composition of the material, stresses in the material, heat treatment of the material, water chemistry, presence of gases like oxygen, hydrogen, hydrogen peroxide,. . . . [47] While all of these parameters have their influence on the corrosion rate of the steel, the most important effects result from pH, material sensitivity, water chemistry and the presence of gases. The water chemistry, together with the pH and partial pressures of gases are closely monitored in PWR’s. For the reactivity control in the reactor, boron is added to the primary water as boric acid, H 3 BO 3 . This boron will capture neutrons produced by the fission reactions and as such reduce the number of neutrons able to initiate new fission reactions in the fuel. Therefore, the added boron to the primary water will decrease the reactivity in the reactor and counteract the surplus in reactivity present at the beginning of the fuel cycle in the reactor. As the reactivity of the fuel decreases over time, also the concentration in boron will be decreased correspondingly. Depending on the moment in the fuel cycle, the amount of boron will be decreased from 2000-4000 ppm in the begin-of-cycle to 0 at the end-of-cycle. [48] The addition of this boron will decrease the pH of the primary water. This effect is counteracted by the addition of LiOH. The concentration of Li is adapted to the concentration of B, in order to reach a specific pH in the primary water. At the beginning of cycle, the concentration of boron will be high and therefore also the added lithium concentration will be high. The aimed pH T at the beginning of cycle is 6.9, a lower pH T will result in a significant increase of crud deposits. Over time both the boron and lithium concentration will be decreased, maintaining this pH T . Once a boron concentration of 1200 ppm is reached, the Li concentration will be held constant at 2.2 ppm. This lithium concentration is maintained with decreasing boron concentration, resulting in an increasing pH T . Once the pH T has reached 7.2 to 7.4, the Li concentration will be decreased, once again, corresponding with 30