Effects of process-generated hydrogen on RPV walls

8. Conclusion

mol/l. This calculation is based on the different reactions occurring in the primary water under the influence of ionizing radiation. The hydrogen generated in the primary circuit will get in contact with the inner wall of the RPV and the hydrogen will get absorbed in the steel. If one knows the equilibrium hydrogen pressure at the steel-water interface, one can calculate the hydrogen concentration in the steel by Sieverts’ law. It is found that the hydrogen fugacity for radiolysis generated hydrogen at the steel water interface for a PWR at full power is equal to 4.2 10 27 atm. Using the best possible approximation for the fugacity coefficient of hydrogen, it is found that this corresponds to 1.471 10 5 atm. Sieverts’ law gives the corresponding hydrogen activity at the steel water interface, equal to 3.16 10 15 ppm. Again this value is very high, but as for the hydrogen fugacity, high activity coefficients can be expected. Thus the physical validity of this value is not necessarily undermined. For the corrosion generated hydrogen, Fick’s law allows to calculate the equilibrium hydrogen concentration at the steel water interface due to corrosion. Depending on the assumed conditions concerning the hydrogen production rate and the absorption efficiency of hydrogen in the steel, hydrogen concentrations at the steel-steel water interface are found to range between 0.077 up to 2.084 ppm. Based on these hydrogen concentrations in the RPV wall at the steel-water interface, the equilibrium hydrogen pressures can be calculated. Different operating conditions are considered, hot in-service condition and cold shutdown. But also abnormal or accindental conditions need to be considered. A pressurized thermal shock is one of the most demanding transient conditions for the RPV and therefore this event is also investigated. During hot in-service condition, the hydrogen fugacity in the base material of the RPV wall due to radiolysis generated hydrogen is found to be 4.9 10 32 Pa. This corresponds to a mechanical hydrogen pressure of 1.471 10 5 atm. For corrosion generated hydrogen, a hydrogen fugacity of only 3.62 Pa was found for the worst corrosion assumptions. During a cold shutdown, the temperature is decreased to room temperature over a period of about 25 hours. Therefore, the equilibrium hydrogen fugacity will be higher compared to the hot in-service condition and reach its maximum just after the complete cooling of the RPV wall. For the different corrosion conditions the equilibrium hydrogen fugacity ranges between 166 and 1.30 10 5 Pa. For the radiolysis generated hydrogen, similar calculations were performed. The maximum hydrogen fugacity is found to be 2.79 10 35 Pa. One can again calculate the corresponding mechanical hydrogen pressure, equal to 1.652 10 5 atm. Finally, the event of a PTS is considered for the accidental conditions. Here, the reactor is rapidly cooled by water of 40 ◦ C. For these conditions, the maximum hydrogen fugacity in the RPV base material is reached after 2 hours and equal to 1.02 10 35 Pa. This is lower compared to the maximum hydrogen fugacity for a cold shutdown. One can explain this by the higher temperature of the RPV wall and the important temperature dependency of Sieverts’ constant. The hydrogen fugacity is found to correspond to a mechanical hydrogen pressure of 1.625 10 5 atm. The same calculations for corrosion generated hydrogen resulted in a maximum hydrogen fugacity between 60 and 4.73 10 4 Pa, depending on the hydrogen prodcution rate and the absorption efficiency. 96