Effects of process-generated hydrogen on RPV walls

1. Reactor pressure vessel

Therefore, it is a very important component for the safety system of a nuclear reactor. Furthermore, the RPV is the major part that limits the lifetime of a nuclear power plant. This is because the RPV is considered to be an irreplaceable component. The replacement of the RPV can not be economically justified by the owner of the plant due to the very high cost to replace it. 1.2 Function of a reactor pressure vessel The main function of a RPV is to assure the containment of the nuclear fuel and the coolant, both during normal operation and possible accidental conditions. These accidental conditions might be initiated by external causes such as seismic activity or internal causes leading to for example pressurized thermal shocks (PTS). The RPV is one of the three safety barriers for the containment of radioactive materials in a nuclear reactor unit, according to the so called “defense in depth” principle. These three barriers consequently are: • Fuel cladding: The cladding is a metallic sheet around the fuel and forms a barrier between the nuclear fuel and the coolant. It is supposed to confine the nuclear fuel and avoid loss of fuel into the coolant and cooling system. [2] • RPV: The RPV contains the fuel and coolant. Its integrity is not only impor- tant when the fuel cladding has failed. It contains the coolant of the reactor. Therefore, a failure of the RPV can result in a loss of coolant which can lead to overheating and possible damage to the fuel. [2] • Containment: This is a concrete wall encapsulating the RPV and most often also the primary cooling circuit. In case of a failure of the RPV or pipes, it should contain all the radioactive material and prevent it from escaping towards the environment. Some reactors have a double containment wall, e.g. Doel 3 and Tihange 2, as shown in Figure 1.1. [2] Besides the forces resulting from the high temperatures and pressures of the coolant, also external forces can act on the RPV, e.g. resulting from earthquakes. These kind of accidents have to be considered to a certain extend when designing a RPV. For a frequency larger than 10 − 4 per reactor year such initiating events have to be taken into account for the safety measures of the NPP. [3] This means that these events must not have any radiological impact at all. Lower frequency events are still limited in the amount of radioactivity that can be released. As the RPV is the most important of the three barriers, these demands impose design requirements and extra safety margins for the mechanical properties of the RPV. 6 This is a clear application of the defense in depth principle, as each barrier encapsulates the other. This means that a failure of one single barrier can not lead to the release of radioactivity to the environment.