Effects of process-generated hydrogen on RPV walls

Chapter 8 Conclusion

A reactor pressure vessel is an important component in the safety system of a pressurized water reactor. As a result of the flaws found in two RPVs in the belgian nuclear fleet, concerns rose about the safety of these nuclear reactors. A lot of time and energy has already been put in the investigation of the structural integrity of the RPV material. However, these studies did not focus on the possibility for these flaws to evolve during operation of the reactor. The distribution of the flaws in the RPVs of Doel 3 and Tihange 2 showed the highest density in the upper and lower core shells. These parts are the closest to the reactor core and, therefore, will have the highest neutron irradiation of the complete RPV. It is known that higher neutron fluence results in a decreased fracture toughness of the material. Therefore this part of the RPV is very critical for the structural integrity of the complete RPV. The size of the flaws in both reactors was measured and found to have an average diameter of 10 to 15 mm, while the largest flaws were found to be up to 179 mm, using a conservative measurement technique. As these flaws are identified as so called hydrogen cracks in the RPV wall, the development of hydrogen gas during operation of the reactor is an important factor to consider when evaluating the stability of these flaws. Also the chemical behavior of hydrogen in steel is important to consider. Literature showed different types of so called hydrogen traps, able to attract the hydrogen atoms in the steel and inhibit or at least delay movement of these trapped hydrogen atoms. For the hydrogen flakes to grow over time, sources of hydrogen must be present. Different sources were identified, being hydrogen gas dissolved in the primary water, hydrogen generation by corrosion at the RPV-water interface and radiolysis of the primary water by the ionizing radiation. The hydrogen generation for these different sources was quantified. It was found that the equilibrium hydrogen pressure in the primary system for the dissolved hydrogen is in the range between a few 10 thousands of Pa at hot in-service temperatures up to a few 100 thousands of Pa when the primary water is cooled down to room temperature. The second important source of hydrogen, corrosion, was calculated to generate between 50 and 150 mol of H per year over the entire inner surface of the RPV. Furthermore, radiolysis was found to result in an atomic hydrogen concentration in the primary water in the order of 10 -7 95