Progress in monitoring programs for reactor pressure vessels

Ensuring the long-term safety and reliability of nuclear power plants and sustainably maintaining component integrity under all operating conditions are key principles to be taken into account in design. The reactor pressure vessel is the most important barrier in the primary system of a plant; it must be able at all times to ensure safe confinement of the activity inventory of the fuel elements. The load path in the safety analyses conducted for this purpose also must take into account that the reactor pressure vessel was exposed to radiation in past operating periods, as a result of which changes were induced in the microstructure of materials. To assess the mechanical properties of materials influenced by this exposure, exposure monitoring programs are normally run during plant operation which can be evaluated quantitatively in advance by means of appropriate inserted samples. The existing methods of evaluation were developed further in recent years and are covered in more detail in the article. They key materials indicator used in failure assessment of the reactor pressure vessel is the critical crack toughness, KIc, in the irradiated state. Over the past few years, specific parameter studies were conducted by means of thermal hydraulics analyses for all German reactor pressure vessels during plant operation. The demonstration of RPV safety is performed by fracture mechanics approaches based on the RTNDT concept since the 1960s. Since the mid-1990s, the so-called Master Curve (TO) concept has been developed in parallel. (orig.)